[PATCH v18 0/5] implement getrandom() in vDSO

[PATCH v18 0/5] implement getrandom() in vDSO

[Jason A. Donenfeld]

Changes v14->{present}:
----------------------
This is back after a bit of a hiatus. In the last attempt to do this in
the beginning of 2023, I think we reached consensus on a few things --
the use case, the vDSO implementation and semantics, its integration
with libc, the test code and documentation, and so forth. It was
basically "ready to go". Almost. But there was a lingering issue that
bogged this down, which is that it demanded some new mm semantics that
weren't very popular.

In particular, the series from last year made use of the x86 instruction
decoder to just skip over faulting instructions. I still think this is
nifty, but it's not actually essential for the semantics needed, and I
can understand why this was by far the largest objection. So all of that
is dropped, which simplifies quite a bit.

In another avenue of the mm discussion, Andy had mentioned using
_install_special_mapping() instead of the VM_DROPPABLE work, and I spent
a long while looking into this, and attempted several times to code up a
working implementation that used that. But the semantics really just
weren't possible without adding hooks to lots of other core code, and
duplicating a lot of code that really ought not to be. So I've kept the
VM_DROPPABLE patch here, but because the x86 instruction decoding stuff
has been removed, that patch is actually a lot smaller and simpler and I
don't think should be too controversial. In terms of actual C code, it
only adds around 6 lines, and is compact enough that you can just grep
for VM_DROPPABLE to see the whole thing.

The original cover letter is produced below. I'm eager to finally get
this patchset moving, and sorry for the delay in producing the v+1 from
before.

Assuming this goes well, the plan would be to take this through my
random.git tree for 6.11. And if the mm part looks fine, I'll get this
cooking in linux-next ASAP.

Thanks ahead of time for taking a look at it.

Changes v17->v18:
- Commit message typos.
- Rebase on later 6.10 commit, because some conflicts got introduced that are
  easily resolved this way.
- Add line breaks to cflags in kselftests makefile. There may be more makefile
  fixups in store, but those depend on other patches John posted on LKML that
  haven't landed yet, so that'll have to be fixed later.
- Add "opaque_len" argument to vgetrandom() to address CRIU concerns.
- Use proper smp ordering for generation changes, and document pairing of release/acquires.

Changes v16->v17:
- Generate patchset using --base.
- Rebase on 6.10-rc1, which means bumping the syscall number to 463.
- Adjust documentation and example code to specify proper munmap() deallocation
  calculation.
- Use %xmm0 instead of %xmm8 as temp register for more compact encoding.
- Documentation comment syntax fixups.
- If page-straddling address is passed to vgetrandom, rather than falling back
  to the syscall, return -EFAULT.
- Get rid of vdso_kernel_ulong type and just use u64 uniformly.
- Don't include crypto/chacha.h inside of a vdso header, to keep kernel code
  out of the vdso user code.
- Balance christmas trees.
- Improve comments of vgetrandom() function.
- Get rid of NOWARN|NORETRY memory allocation logic, and also don't clear the
  OOM flag on failure. This is kind of a problem, but Jann and Michal thought
  that it was better to crash than for userspace to keep retrying instructions.
  And the pre-v14 series that tried to skip the instruction instead of retrying
  was marred by controversy. So just table this for later; it's probably not
  essential now.

Changes v15->v16:
- DavidH pointed out a missing swap edge case in 1/5.
- Mostly just a resend because I forgot --cc-cover, and sent it during
  the merge window. 

--------------

Useful links:

- This series:
  - https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git/log/?h=vdso

- Glibc patches by Adhemerval and me against glibc-2.39:
  - https://git.zx2c4.com/glibc/log/?h=vdso

- You may also want these if you're daily driving this as I am on a normal
  desktop system:
  - systemd patch: https://github.com/systemd/systemd/pull/25519
  - libseccomp patch: https://github.com/seccomp/libseccomp/pull/395

- In case you're actually interested in the v≤14 design where faults were
  non-fatal and instructions were skipped (which I think is more coherent, even
  if the implementation is controversial), this lives in my branch here:
  - https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git/log/?h=jd/vdso-skip-insn
  Note that I'm *not* actually proposing this for upstream at this time. But it
  may be of conversational interest.

-------------

Two statements:

  1) Userspace wants faster cryptographically secure random numbers of
     arbitrary size, big or small.

  2) Userspace is currently unable to safely roll its own RNG with the
     same security profile as getrandom().

Statement (1) has been debated for years, with arguments ranging from
"we need faster cryptographically secure card shuffling!" to "the only
things that actually need good randomness are keys, which are few and
far between" to "actually, TLS CBC nonces are frequent" and so on. I
don't intend to wade into that debate substantially, except to note that
recently glibc added arc4random(), whose goal is to return a
cryptographically secure uint32_t, and there are real user reports of it
being too slow. So here we are.

Statement (2) is more interesting. The kernel is the nexus of all
entropic inputs that influence the RNG. It is in the best position, and
probably the only position, to decide anything at all about the current
state of the RNG and of its entropy. One of the things it uniquely knows
about is when reseeding is necessary.

For example, when a virtual machine is forked, restored, or duplicated,
it's imparative that the RNG doesn't generate the same outputs. For this
reason, there's a small protocol between hypervisors and the kernel that
indicates this has happened, alongside some ID, which the RNG uses to
immediately reseed, so as not to return the same numbers. Were userspace
to expand a getrandom() seed from time T1 for the next hour, and at some
point T2 < hour, the virtual machine forked, userspace would continue to
provide the same numbers to two (or more) different virtual machines,
resulting in potential cryptographic catastrophe. Something similar
happens on resuming from hibernation (or even suspend), with various
compromise scenarios there in mind.

There's a more general reason why userspace rolling its own RNG from a
getrandom() seed is fraught. There's a lot of attention paid to this
particular Linuxism we have of the RNG being initialized and thus
non-blocking or uninitialized and thus blocking until it is initialized.
These are our Two Big States that many hold to be the holy
differentiating factor between safe and not safe, between
cryptographically secure and garbage. The fact is, however, that the
distinction between these two states is a hand-wavy wishy-washy inexact
approximation. Outside of a few exceptional cases (e.g. a HW RNG is
available), we actually don't really ever know with any rigor at all
when the RNG is safe and ready (nor when it's compromised). We do the
best we can to "estimate" it, but entropy estimation is fundamentally
impossible in the general case. So really, we're just doing guess work,
and hoping it's good and conservative enough. Let's then assume that
there's always some potential error involved in this differentiator.

In fact, under the surface, the RNG is engineered around a different
principle, and that is trying to *use* new entropic inputs regularly and
at the right specific moments in time. For example, close to boot time,
the RNG reseeds itself more often than later. At certain events, like VM
fork, the RNG reseeds itself immediately. The various heuristics for
when the RNG will use new entropy and how often is really a core aspect
of what the RNG has some potential to do decently enough (and something
that will probably continue to improve in the future from random.c's
present set of algorithms). So in your mind, put away the metal
attachment to the Two Big States, which represent an approximation with
a potential margin of error. Instead keep in mind that the RNG's primary
operating heuristic is how often and exactly when it's going to reseed.

So, if userspace takes a seed from getrandom() at point T1, and uses it
for the next hour (or N megabytes or some other meaningless metric),
during that time, potential errors in the Two Big States approximation
are amplified. During that time potential reseeds are being lost,
forgotten, not reflected in the output stream. That's not good.

The simplest statement you could make is that userspace RNGs that expand
a getrandom() seed at some point T1 are nearly always *worse*, in some
way, than just calling getrandom() every time a random number is
desired.

For those reasons, after some discussion on libc-alpha, glibc's
arc4random() now just calls getrandom() on each invocation. That's
trivially safe, and gives us latitude to then make the safe thing faster
without becoming unsafe at our leasure. Card shuffling isn't
particularly fast, however.

How do we rectify this? By putting a safe implementation of getrandom()
in the vDSO, which has access to whatever information a
particular iteration of random.c is using to make its decisions. I use
that careful language of "particular iteration of random.c", because the
set of things that a vDSO getrandom() implementation might need for making
decisions as good as the kernel's will likely change over time. This
isn't just a matter of exporting certain *data* to userspace. We're not
going to commit to a "data API" where the various heuristics used are
exposed, locking in how the kernel works for decades to come, and then
leave it to various userspaces to roll something on top and shoot
themselves in the foot and have all sorts of complexity disasters.
Rather, vDSO getrandom() is supposed to be the *same exact algorithm*
that runs in the kernel, except it's been hoisted into userspace as
much as possible. And so vDSO getrandom() and kernel getrandom() will
always mirror each other hermetically.

API-wise, the vDSO gains this function:

  ssize_t vgetrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state);

The return value and the first 3 arguments are the same as ordinary
getrandom(), while the last argument is a pointer to some state
allocated with vgetrandom_alloc(), explained below. Were all four
arguments passed to the getrandom syscall, nothing different would
happen, and the functions would have the exact same behavior.

Then, we introduce a new syscall:

  void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each,
                         unsigned long addr, unsigned int flags);

This takes a hinted number of opaque states in `num`, and returns a
pointer to an array of opaque states, the number actually allocated back
in `num`, and the size in bytes of each one in `size_per_each`, enabling
a libc to slice up the returned array into a state per each thread. (The
`flags` and `addr` arguments, as well as the `*size_per_each` input
value, are reserved for the future and are forced to be zero for now.)

Libc is expected to allocate a chunk of these on first use, and then
dole them out to threads as they're created, allocating more when
needed. The returned address of the first state may be passed to
munmap(2) with a length of `num * size_per_each`, in order to deallocate
the memory.

We very intentionally do *not* leave state allocation up to the caller
of vgetrandom, but provide vgetrandom_alloc for that allocation. There
are too many weird things that can go wrong, and it's important that
vDSO does not provide too generic of a mechanism. It's not going to
store its state in just any old memory address. It'll do it only in ones
it allocates.

Right now this means it uses a new mm flag called VM_DROPPABLE, along
with VM_WIPEONFORK. In the future maybe there will be other interesting
page flags or anti-heartbleed measures, or other platform-specific
kernel-specific things that can be set from the syscall. Again, it's
important that the kernel has a say in how this works rather than
agreeing to operate on any old address; memory isn't neutral.

The interesting meat of the implementation is in lib/vdso/getrandom.c,
as generic C code, and it aims to mainly follow random.c's buffered fast
key erasure logic. Before the RNG is initialized, it falls back to the
syscall. Right now it uses a simple generation counter to make its decisions
on reseeding (though this could be made more extensive over time).

The actual place that has the most work to do is in all of the other
files. Most of the vDSO shared page infrastructure is centered around
gettimeofday, and so the main structs are all in arrays for different
timestamp types, and attached to time namespaces, and so forth. I've
done the best I could to add onto this in an unintrusive way.

In my test results, performance is pretty stellar (around 15x for uint32_t
generation), and it seems to be working. There's an extended example in the
second commit of this series, showing how the syscall and the vDSO function
are meant to be used together.

Cc: linux-crypto@vger.kernel.org
Cc: linux-api@vger.kernel.org
Cc: x86@kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Cc: Carlos O'Donell <carlos@redhat.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jann Horn <jannh@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Hildenbrand <dhildenb@redhat.com>

Jason A. Donenfeld (5):
  mm: add VM_DROPPABLE for designating always lazily freeable mappings
  random: add vgetrandom_alloc() syscall
  arch: allocate vgetrandom_alloc() syscall number
  random: introduce generic vDSO getrandom() implementation
  x86: vdso: Wire up getrandom() vDSO implementation

 MAINTAINERS                                   |   2 +
 arch/alpha/kernel/syscalls/syscall.tbl        |   1 +
 arch/arm/tools/syscall.tbl                    |   1 +
 arch/arm64/include/asm/unistd32.h             |   2 +
 arch/m68k/kernel/syscalls/syscall.tbl         |   1 +
 arch/microblaze/kernel/syscalls/syscall.tbl   |   1 +
 arch/mips/kernel/syscalls/syscall_n32.tbl     |   1 +
 arch/mips/kernel/syscalls/syscall_n64.tbl     |   1 +
 arch/mips/kernel/syscalls/syscall_o32.tbl     |   1 +
 arch/parisc/kernel/syscalls/syscall.tbl       |   1 +
 arch/powerpc/kernel/syscalls/syscall.tbl      |   1 +
 arch/s390/kernel/syscalls/syscall.tbl         |   1 +
 arch/sh/kernel/syscalls/syscall.tbl           |   1 +
 arch/sparc/kernel/syscalls/syscall.tbl        |   1 +
 arch/x86/Kconfig                              |   1 +
 arch/x86/entry/syscalls/syscall_32.tbl        |   1 +
 arch/x86/entry/syscalls/syscall_64.tbl        |   1 +
 arch/x86/entry/vdso/Makefile                  |   3 +-
 arch/x86/entry/vdso/vdso.lds.S                |   2 +
 arch/x86/entry/vdso/vgetrandom-chacha.S       | 178 +++++++++++
 arch/x86/entry/vdso/vgetrandom.c              |  17 +
 arch/x86/include/asm/vdso/getrandom.h         |  55 ++++
 arch/x86/include/asm/vdso/vsyscall.h          |   2 +
 arch/x86/include/asm/vvar.h                   |  16 +
 arch/xtensa/kernel/syscalls/syscall.tbl       |   1 +
 drivers/char/random.c                         | 148 ++++++++-
 fs/proc/task_mmu.c                            |   3 +
 include/linux/mm.h                            |   8 +
 include/linux/syscalls.h                      |   3 +
 include/trace/events/mmflags.h                |   7 +
 include/uapi/asm-generic/unistd.h             |   5 +-
 include/vdso/datapage.h                       |  11 +
 include/vdso/getrandom.h                      |  46 +++
 kernel/sys_ni.c                               |   3 +
 lib/vdso/Kconfig                              |   6 +
 lib/vdso/getrandom.c                          | 236 ++++++++++++++
 mm/Kconfig                                    |   3 +
 mm/mprotect.c                                 |   2 +-
 mm/rmap.c                                     |   8 +-
 tools/include/uapi/asm-generic/unistd.h       |   5 +-
 .../arch/mips/entry/syscalls/syscall_n64.tbl  |   1 +
 .../arch/powerpc/entry/syscalls/syscall.tbl   |   1 +
 .../perf/arch/s390/entry/syscalls/syscall.tbl |   1 +
 .../arch/x86/entry/syscalls/syscall_64.tbl    |   1 +
 tools/testing/selftests/vDSO/.gitignore       |   2 +
 tools/testing/selftests/vDSO/Makefile         |  17 +
 .../testing/selftests/vDSO/vdso_test_chacha.c |  43 +++
 .../selftests/vDSO/vdso_test_getrandom.c      | 293 ++++++++++++++++++
 48 files changed, 1138 insertions(+), 8 deletions(-)
 create mode 100644 arch/x86/entry/vdso/vgetrandom-chacha.S
 create mode 100644 arch/x86/entry/vdso/vgetrandom.c
 create mode 100644 arch/x86/include/asm/vdso/getrandom.h
 create mode 100644 include/vdso/getrandom.h
 create mode 100644 lib/vdso/getrandom.c
 create mode 100644 tools/testing/selftests/vDSO/vdso_test_chacha.c
 create mode 100644 tools/testing/selftests/vDSO/vdso_test_getrandom.c


base-commit: c286c21ff94252f778515b21b6bebe749454a852
-- 
2.45.2

[PATCH v18 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings

[Jason A. Donenfeld]

The vDSO getrandom() implementation works with a buffer allocated with a
new system call that has certain requirements:

- It shouldn't be written to core dumps.
  * Easy: VM_DONTDUMP.
- It should be zeroed on fork.
  * Easy: VM_WIPEONFORK.

- It shouldn't be written to swap.
  * Uh-oh: mlock is rlimited.
  * Uh-oh: mlock isn't inherited by forks.

It turns out that the vDSO getrandom() function has three really nice
characteristics that we can exploit to solve this problem:

1) Due to being wiped during fork(), the vDSO code is already robust to
   having the contents of the pages it reads zeroed out midway through
   the function's execution.

2) In the absolute worst case of whatever contingency we're coding for,
   we have the option to fallback to the getrandom() syscall, and
   everything is fine.

3) The buffers the function uses are only ever useful for a maximum of
   60 seconds -- a sort of cache, rather than a long term allocation.

These characteristics mean that we can introduce VM_DROPPABLE, which
has the following semantics:

a) It never is written out to swap.
b) Under memory pressure, mm can just drop the pages (so that they're
   zero when read back again).
c) It is inherited by fork.
d) It doesn't count against the mlock budget, since nothing is locked.

This is fairly simple to implement, with the one snag that we have to
use 64-bit VM_* flags, but this shouldn't be a problem, since the only
consumers will probably be 64-bit anyway.

This way, allocations used by vDSO getrandom() can use:

    VM_DROPPABLE | VM_DONTDUMP | VM_WIPEONFORK | VM_NORESERVE

And there will be no problem with using memory when not in use, not
wiping on fork(), coredumps, or writing out to swap.

Cc: linux-mm@kvack.org
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 fs/proc/task_mmu.c             | 3 +++
 include/linux/mm.h             | 8 ++++++++
 include/trace/events/mmflags.h | 7 +++++++
 mm/Kconfig                     | 3 +++
 mm/mprotect.c                  | 2 +-
 mm/rmap.c                      | 8 +++++---
 6 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index f8d35f993fe5..1883d6462ca8 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -706,6 +706,9 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
 #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
 #ifdef CONFIG_X86_USER_SHADOW_STACK
 		[ilog2(VM_SHADOW_STACK)] = "ss",
+#endif
+#ifdef CONFIG_NEED_VM_DROPPABLE
+		[ilog2(VM_DROPPABLE)]	= "dp",
 #endif
 	};
 	size_t i;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9849dfda44d4..5978cb4cc21c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -321,12 +321,14 @@ extern unsigned int kobjsize(const void *objp);
 #define VM_HIGH_ARCH_BIT_3	35	/* bit only usable on 64-bit architectures */
 #define VM_HIGH_ARCH_BIT_4	36	/* bit only usable on 64-bit architectures */
 #define VM_HIGH_ARCH_BIT_5	37	/* bit only usable on 64-bit architectures */
+#define VM_HIGH_ARCH_BIT_6	38	/* bit only usable on 64-bit architectures */
 #define VM_HIGH_ARCH_0	BIT(VM_HIGH_ARCH_BIT_0)
 #define VM_HIGH_ARCH_1	BIT(VM_HIGH_ARCH_BIT_1)
 #define VM_HIGH_ARCH_2	BIT(VM_HIGH_ARCH_BIT_2)
 #define VM_HIGH_ARCH_3	BIT(VM_HIGH_ARCH_BIT_3)
 #define VM_HIGH_ARCH_4	BIT(VM_HIGH_ARCH_BIT_4)
 #define VM_HIGH_ARCH_5	BIT(VM_HIGH_ARCH_BIT_5)
+#define VM_HIGH_ARCH_6	BIT(VM_HIGH_ARCH_BIT_6)
 #endif /* CONFIG_ARCH_USES_HIGH_VMA_FLAGS */
 
 #ifdef CONFIG_ARCH_HAS_PKEYS
@@ -357,6 +359,12 @@ extern unsigned int kobjsize(const void *objp);
 # define VM_SHADOW_STACK	VM_NONE
 #endif
 
+#ifdef CONFIG_NEED_VM_DROPPABLE
+# define VM_DROPPABLE		VM_HIGH_ARCH_6
+#else
+# define VM_DROPPABLE		VM_NONE
+#endif
+
 #if defined(CONFIG_X86)
 # define VM_PAT		VM_ARCH_1	/* PAT reserves whole VMA at once (x86) */
 #elif defined(CONFIG_PPC)
diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index e46d6e82765e..fab7848df50a 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -165,6 +165,12 @@ IF_HAVE_PG_ARCH_X(arch_3)
 # define IF_HAVE_UFFD_MINOR(flag, name)
 #endif
 
+#ifdef CONFIG_NEED_VM_DROPPABLE
+# define IF_HAVE_VM_DROPPABLE(flag, name) {flag, name},
+#else
+# define IF_HAVE_VM_DROPPABLE(flag, name)
+#endif
+
 #define __def_vmaflag_names						\
 	{VM_READ,			"read"		},		\
 	{VM_WRITE,			"write"		},		\
@@ -197,6 +203,7 @@ IF_HAVE_VM_SOFTDIRTY(VM_SOFTDIRTY,	"softdirty"	)		\
 	{VM_MIXEDMAP,			"mixedmap"	},		\
 	{VM_HUGEPAGE,			"hugepage"	},		\
 	{VM_NOHUGEPAGE,			"nohugepage"	},		\
+IF_HAVE_VM_DROPPABLE(VM_DROPPABLE,	"droppable"	)		\
 	{VM_MERGEABLE,			"mergeable"	}		\
 
 #define show_vma_flags(flags)						\
diff --git a/mm/Kconfig b/mm/Kconfig
index b4cb45255a54..6cd65ea4b3ad 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -1056,6 +1056,9 @@ config ARCH_USES_HIGH_VMA_FLAGS
 	bool
 config ARCH_HAS_PKEYS
 	bool
+config NEED_VM_DROPPABLE
+	select ARCH_USES_HIGH_VMA_FLAGS
+	bool
 
 config ARCH_USES_PG_ARCH_X
 	bool
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 8c6cd8825273..57b8dad9adcc 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -623,7 +623,7 @@ mprotect_fixup(struct vma_iterator *vmi, struct mmu_gather *tlb,
 				may_expand_vm(mm, oldflags, nrpages))
 			return -ENOMEM;
 		if (!(oldflags & (VM_ACCOUNT|VM_WRITE|VM_HUGETLB|
-						VM_SHARED|VM_NORESERVE))) {
+				  VM_SHARED|VM_NORESERVE|VM_DROPPABLE))) {
 			charged = nrpages;
 			if (security_vm_enough_memory_mm(mm, charged))
 				return -ENOMEM;
diff --git a/mm/rmap.c b/mm/rmap.c
index e8fc5ecb59b2..d873a3f06506 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1397,7 +1397,8 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
 	VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
 	VM_BUG_ON_VMA(address < vma->vm_start ||
 			address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
-	__folio_set_swapbacked(folio);
+	if (!(vma->vm_flags & VM_DROPPABLE))
+		__folio_set_swapbacked(folio);
 	__folio_set_anon(folio, vma, address, true);
 
 	if (likely(!folio_test_large(folio))) {
@@ -1841,7 +1842,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				 * plus the rmap(s) (dropped by discard:).
 				 */
 				if (ref_count == 1 + map_count &&
-				    !folio_test_dirty(folio)) {
+				    (!folio_test_dirty(folio) || (vma->vm_flags & VM_DROPPABLE))) {
 					dec_mm_counter(mm, MM_ANONPAGES);
 					goto discard;
 				}
@@ -1851,7 +1852,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				 * discarded. Remap the page to page table.
 				 */
 				set_pte_at(mm, address, pvmw.pte, pteval);
-				folio_set_swapbacked(folio);
+				if (!(vma->vm_flags & VM_DROPPABLE))
+					folio_set_swapbacked(folio);
 				ret = false;
 				page_vma_mapped_walk_done(&pvmw);
 				break;
-- 
2.45.2

[PATCH v18 2/5] random: add vgetrandom_alloc() syscall

[Jason A. Donenfeld]

The vDSO getrandom() works over an opaque per-thread state of an
unexported size, which must be marked VM_WIPEONFORK, VM_DONTDUMP,
VM_NORESERVE, and VM_DROPPABLE for proper operation. Over time, the
nuances of these allocations may change or grow or even differ based on
architectural features.

The syscall has the signature:

  void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each,
                         unsigned long addr, unsigned int flags);

This takes a hinted number of opaque states in `num`, and returns a
pointer to an array of opaque states, the number actually allocated back
in `num`, and the size in bytes of each one in `size_per_each`, enabling
a libc to slice up the returned array into a state per each thread,
while ensuring that no single state straddles a page boundary. (The
`flags` and `addr` arguments, as well as the `*size_per_each` input
value, are reserved for the future and are forced to be zero zero for
now.)

Libc is expected to allocate a chunk of these on first use, and then
dole them out to threads as they're created, allocating more when
needed. The returned address of the first state may be passed to
munmap(2) with a length of `DIV_ROUND_UP(num, PAGE_SIZE / size_per_each)
* PAGE_SIZE`, in order to deallocate the memory.

We very intentionally do *not* leave state allocation for vDSO
getrandom() up to userspace itself, but rather provide this new syscall
for such allocations. vDSO getrandom() must not store its state in just
any old memory address, but rather just ones that the kernel specially
allocates for it, leaving the particularities of those allocations up to
the kernel.

The allocation of states is intended to be integrated into libc's thread
management. As an illustrative example, the following code might be used
to do the same outside of libc. Though, vgetrandom_alloc() is not
expected to be exposed outside of libc, and the pthread usage here is
expected to be elided into libc internals. This allocation scheme is
very naive and does not shrink; other implementations may choose to be
more complex.

  static void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each)
  {
    *size_per_each = 0; /* Must be zero on input. */
    return (void *)syscall(__NR_vgetrandom_alloc, &num, &size_per_each,
                           0 /* reserved @addr */, 0 /* reserved @flags */);
  }

  static struct {
    pthread_mutex_t lock;
    void **states;
    size_t len, cap, size_per_each;
  } grnd_allocator = {
    .lock = PTHREAD_MUTEX_INITIALIZER
  };

  static void *vgetrandom_get_state(void)
  {
    void *state = NULL;

    pthread_mutex_lock(&grnd_allocator.lock);
    if (!grnd_allocator.len) {
      size_t new_cap;
      size_t page_size = getpagesize();
      unsigned int num = sysconf(_SC_NPROCESSORS_ONLN); /* Could be arbitrary, just a hint. */
      unsigned int size_per_each;
      void *new_block = vgetrandom_alloc(&num, &size_per_each);
      void *new_states;

      if (new_block == MAP_FAILED)
        goto out;
      if (grnd_allocator.size_per_each && grnd_allocator.size_per_each != size_per_each)
        goto unmap;
      grnd_allocator.size_per_each = size_per_each;
      new_cap = grnd_allocator.cap + num;
      new_states = reallocarray(grnd_allocator.states, new_cap, sizeof(*grnd_allocator.states));
      if (!new_states)
        goto unmap;
      grnd_allocator.cap = new_cap;
      grnd_allocator.states = new_states;

      for (size_t i = 0; i < num; ++i) {
        grnd_allocator.states[i] = new_block;
        if (((uintptr_t)new_block & (page_size - 1)) + size_per_each > page_size)
          new_block = (void *)(((uintptr_t)new_block + page_size) & (page_size - 1));
        else
          new_block += size_per_each;
      }
      grnd_allocator.len = num;
      goto success;

    unmap:
      munmap(new_block, DIV_ROUND_UP(num, page_size / size_per_each) * page_size);
      goto out;
    }
  success:
    state = grnd_allocator.states[--grnd_allocator.len];

  out:
    pthread_mutex_unlock(&grnd_allocator.lock);
    return state;
  }

  static void vgetrandom_put_state(void *state)
  {
    if (!state)
      return;
    pthread_mutex_lock(&grnd_allocator.lock);
    grnd_allocator.states[grnd_allocator.len++] = state;
    pthread_mutex_unlock(&grnd_allocator.lock);
  }

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 MAINTAINERS              |   1 +
 drivers/char/random.c    | 135 ++++++++++++++++++++++++++++++++++++++-
 include/linux/syscalls.h |   3 +
 include/vdso/getrandom.h |  16 +++++
 kernel/sys_ni.c          |   3 +
 lib/vdso/Kconfig         |   6 ++
 6 files changed, 163 insertions(+), 1 deletion(-)
 create mode 100644 include/vdso/getrandom.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 8aa17e515ef3..8480c4c39915 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -18747,6 +18747,7 @@ T:	git https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
 F:	Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
 F:	drivers/char/random.c
 F:	drivers/virt/vmgenid.c
+F:	include/vdso/getrandom.h
 
 RAPIDIO SUBSYSTEM
 M:	Matt Porter <mporter@kernel.crashing.org>
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 2597cb43f438..ccb35f390c85 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: (GPL-2.0 OR BSD-3-Clause)
 /*
- * Copyright (C) 2017-2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ * Copyright (C) 2017-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
  * Copyright Matt Mackall <mpm@selenic.com>, 2003, 2004, 2005
  * Copyright Theodore Ts'o, 1994, 1995, 1996, 1997, 1998, 1999. All rights reserved.
  *
@@ -8,6 +8,7 @@
  * into roughly six sections, each with a section header:
  *
  *   - Initialization and readiness waiting.
+ *   - vDSO support helpers.
  *   - Fast key erasure RNG, the "crng".
  *   - Entropy accumulation and extraction routines.
  *   - Entropy collection routines.
@@ -39,6 +40,7 @@
 #include <linux/blkdev.h>
 #include <linux/interrupt.h>
 #include <linux/mm.h>
+#include <linux/mman.h>
 #include <linux/nodemask.h>
 #include <linux/spinlock.h>
 #include <linux/kthread.h>
@@ -56,6 +58,9 @@
 #include <linux/sched/isolation.h>
 #include <crypto/chacha.h>
 #include <crypto/blake2s.h>
+#ifdef CONFIG_VDSO_GETRANDOM
+#include <vdso/getrandom.h>
+#endif
 #include <asm/archrandom.h>
 #include <asm/processor.h>
 #include <asm/irq.h>
@@ -169,6 +174,134 @@ int __cold execute_with_initialized_rng(struct notifier_block *nb)
 				__func__, (void *)_RET_IP_, crng_init)
 
 
+
+/********************************************************************
+ *
+ * vDSO support helpers.
+ *
+ * The actual vDSO function is defined over in lib/vdso/getrandom.c,
+ * but this section contains the kernel-mode helpers to support that.
+ *
+ ********************************************************************/
+
+#ifdef CONFIG_VDSO_GETRANDOM
+/**
+ * sys_vgetrandom_alloc - Allocate opaque states for use with vDSO getrandom().
+ *
+ * @num:	   On input, a pointer to a suggested hint of how many states to
+ * 		   allocate, and on return the number of states actually allocated.
+ *
+ * @size_per_each: On input, must be zero. On return, the size of each state allocated,
+ * 		   so that the caller can split up the returned allocation into
+ * 		   individual states.
+ *
+ * @addr:	   Reserved, must be zero.
+ *
+ * @flags:	   Reserved, must be zero.
+ *
+ * The getrandom() vDSO function in userspace requires an opaque state, which
+ * this function allocates by mapping a certain number of special pages into
+ * the calling process. It takes a hint as to the number of opaque states
+ * desired, and provides the caller with the number of opaque states actually
+ * allocated, the size of each one in bytes, and the address of the first
+ * state, which may be split up into @num states of @size_per_each bytes each,
+ * by adding @size_per_each to the returned first state @num times, while
+ * ensuring that no single state straddles a page boundary.
+ *
+ * Returns the address of the first state in the allocation on success, or a
+ * negative error value on failure.
+ *
+ * The returned address of the first state may be passed to munmap(2) with a
+ * length of `DIV_ROUND_UP(num, PAGE_SIZE / size_per_each) * PAGE_SIZE`, in
+ * order to deallocate the memory, after which it is invalid to pass it to vDSO
+ * getrandom().
+ *
+ * States allocated by this function must not be dereferenced, written, read,
+ * or otherwise manipulated. The *only* supported operations are:
+ *   - Splitting up the states in intervals of @size_per_each, no more than
+ *     @num times from the first state, while ensuring that no single state
+ *     straddles a page boundary.
+ *   - Passing a state to the getrandom() vDSO function's @opaque_state
+ *     parameter, but not passing the same state at the same time to two such
+ *     calls.
+ *   - Passing the first state and the total length to munmap(2), as described
+ *     above.
+ * All other uses are undefined behavior, which is subject to change or removal.
+ */
+SYSCALL_DEFINE4(vgetrandom_alloc, unsigned int __user *, num,
+		unsigned int __user *, size_per_each, unsigned long, addr,
+		unsigned int, flags)
+{
+	size_t state_size, alloc_size, num_states;
+	unsigned long pages_addr, populate;
+	unsigned int num_hint;
+	vm_flags_t vm_flags;
+	int ret;
+
+	/*
+	 * @flags and @addr are currently unused, so in order to reserve them
+	 * for the future, force them to be set to zero by current callers.
+	 */
+	if (flags || addr)
+		return -EINVAL;
+
+	/*
+	 * Also enforce that *size_per_each is zero on input, in case this becomes
+	 * useful later on.
+	 */
+	if (get_user(num_hint, size_per_each))
+		return -EFAULT;
+	if (num_hint)
+		return -EINVAL;
+
+	if (get_user(num_hint, num))
+		return -EFAULT;
+
+	state_size = sizeof(struct vgetrandom_state);
+	num_states = clamp_t(size_t, num_hint, 1, (SIZE_MAX & PAGE_MASK) / state_size);
+	alloc_size = PAGE_ALIGN(num_states * state_size);
+	/*
+	 * States cannot straddle page boundaries, so calculate the number of
+	 * states that can fit inside of a page without being split, and then
+	 * multiply that out by the number of pages allocated.
+	 */
+	num_states = (PAGE_SIZE / state_size) * (alloc_size / PAGE_SIZE);
+
+	vm_flags =
+		/*
+		 * Don't allow state to be written to swap, to preserve forward secrecy.
+		 * But also don't mlock it or pre-reserve it, and allow it to
+		 * be discarded under memory pressure. If no memory is available, returns
+		 * zeros rather than segfaulting.
+		 */
+		VM_DROPPABLE | VM_NORESERVE |
+
+		/* Don't allow the state to survive forks, to prevent random number re-use. */
+		VM_WIPEONFORK |
+
+		/* Don't write random state into coredumps. */
+		VM_DONTDUMP;
+
+	if (mmap_write_lock_killable(current->mm))
+		return -EINTR;
+	pages_addr = do_mmap(NULL, 0, alloc_size, PROT_READ | PROT_WRITE,
+			     MAP_PRIVATE | MAP_ANONYMOUS, vm_flags, 0, &populate, NULL);
+	mmap_write_unlock(current->mm);
+	if (IS_ERR_VALUE(pages_addr))
+		return pages_addr;
+
+	ret = -EFAULT;
+	if (put_user(num_states, num) || put_user(state_size, size_per_each))
+		goto err_unmap;
+
+	return pages_addr;
+
+err_unmap:
+	vm_munmap(pages_addr, alloc_size);
+	return ret;
+}
+#endif
+
 /*********************************************************************
  *
  * Fast key erasure RNG, the "crng".
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 9104952d323d..56368ea4f510 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -906,6 +906,9 @@ asmlinkage long sys_seccomp(unsigned int op, unsigned int flags,
 			    void __user *uargs);
 asmlinkage long sys_getrandom(char __user *buf, size_t count,
 			      unsigned int flags);
+asmlinkage long sys_vgetrandom_alloc(unsigned int __user *num,
+				     unsigned int __user *size_per_each,
+				     unsigned long addr, unsigned int flags);
 asmlinkage long sys_memfd_create(const char __user *uname_ptr, unsigned int flags);
 asmlinkage long sys_bpf(int cmd, union bpf_attr *attr, unsigned int size);
 asmlinkage long sys_execveat(int dfd, const char __user *filename,
diff --git a/include/vdso/getrandom.h b/include/vdso/getrandom.h
new file mode 100644
index 000000000000..69037519d20b
--- /dev/null
+++ b/include/vdso/getrandom.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+#ifndef _VDSO_GETRANDOM_H
+#define _VDSO_GETRANDOM_H
+
+/**
+ * struct vgetrandom_state - State used by vDSO getrandom() and allocated by vgetrandom_alloc().
+ *
+ * Currently empty, as the vDSO getrandom() function has not yet been implemented.
+ */
+struct vgetrandom_state { int placeholder; };
+
+#endif /* _VDSO_GETRANDOM_H */
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index d7eee421d4bc..6b17fadb0f59 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -272,6 +272,9 @@ COND_SYSCALL(pkey_free);
 /* memfd_secret */
 COND_SYSCALL(memfd_secret);
 
+/* random */
+COND_SYSCALL(vgetrandom_alloc);
+
 /*
  * Architecture specific weak syscall entries.
  */
diff --git a/lib/vdso/Kconfig b/lib/vdso/Kconfig
index c46c2300517c..99661b731834 100644
--- a/lib/vdso/Kconfig
+++ b/lib/vdso/Kconfig
@@ -38,3 +38,9 @@ config GENERIC_VDSO_OVERFLOW_PROTECT
 	  in the hotpath.
 
 endif
+
+config VDSO_GETRANDOM
+	bool
+	select NEED_VM_DROPPABLE
+	help
+	  Selected by architectures that support vDSO getrandom().
-- 
2.45.2

[PATCH v18 3/5] arch: allocate vgetrandom_alloc() syscall number

[Jason A. Donenfeld]

Add vgetrandom_alloc() as syscall 463 (or 573 on alpha) by adding it to
all of the various syscall.tbl and unistd.h files.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 arch/alpha/kernel/syscalls/syscall.tbl              | 1 +
 arch/arm/tools/syscall.tbl                          | 1 +
 arch/arm64/include/asm/unistd32.h                   | 2 ++
 arch/m68k/kernel/syscalls/syscall.tbl               | 1 +
 arch/microblaze/kernel/syscalls/syscall.tbl         | 1 +
 arch/mips/kernel/syscalls/syscall_n32.tbl           | 1 +
 arch/mips/kernel/syscalls/syscall_n64.tbl           | 1 +
 arch/mips/kernel/syscalls/syscall_o32.tbl           | 1 +
 arch/parisc/kernel/syscalls/syscall.tbl             | 1 +
 arch/powerpc/kernel/syscalls/syscall.tbl            | 1 +
 arch/s390/kernel/syscalls/syscall.tbl               | 1 +
 arch/sh/kernel/syscalls/syscall.tbl                 | 1 +
 arch/sparc/kernel/syscalls/syscall.tbl              | 1 +
 arch/x86/entry/syscalls/syscall_32.tbl              | 1 +
 arch/x86/entry/syscalls/syscall_64.tbl              | 1 +
 arch/xtensa/kernel/syscalls/syscall.tbl             | 1 +
 include/uapi/asm-generic/unistd.h                   | 5 ++++-
 tools/include/uapi/asm-generic/unistd.h             | 5 ++++-
 tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl | 1 +
 tools/perf/arch/powerpc/entry/syscalls/syscall.tbl  | 1 +
 tools/perf/arch/s390/entry/syscalls/syscall.tbl     | 1 +
 tools/perf/arch/x86/entry/syscalls/syscall_64.tbl   | 1 +
 22 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index 74720667fe09..8c38193bf86a 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -502,3 +502,4 @@
 570	common	lsm_set_self_attr		sys_lsm_set_self_attr
 571	common	lsm_list_modules		sys_lsm_list_modules
 572	common  mseal				sys_mseal
+573	common	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index 2ed7d229c8f9..118e41178905 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -476,3 +476,4 @@
 460	common	lsm_set_self_attr		sys_lsm_set_self_attr
 461	common	lsm_list_modules		sys_lsm_list_modules
 462	common	mseal				sys_mseal
+463	common	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
index 266b96acc014..e84fa30ccd30 100644
--- a/arch/arm64/include/asm/unistd32.h
+++ b/arch/arm64/include/asm/unistd32.h
@@ -931,6 +931,8 @@ __SYSCALL(__NR_lsm_set_self_attr, sys_lsm_set_self_attr)
 __SYSCALL(__NR_lsm_list_modules, sys_lsm_list_modules)
 #define __NR_mseal 462
 __SYSCALL(__NR_mseal, sys_mseal)
+#define __NR_vgetrandom_alloc 463
+__SYSCALL(__NR_vgetrandom_alloc, sys_vgetrandom_alloc)
 
 /*
  * Please add new compat syscalls above this comment and update
diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index 22a3cbd4c602..bd919d1a8231 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -462,3 +462,4 @@
 460	common	lsm_set_self_attr		sys_lsm_set_self_attr
 461	common	lsm_list_modules		sys_lsm_list_modules
 462	common	mseal				sys_mseal
+463	common	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index 2b81a6bd78b2..d3d3c017a5bb 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -468,3 +468,4 @@
 460	common	lsm_set_self_attr		sys_lsm_set_self_attr
 461	common	lsm_list_modules		sys_lsm_list_modules
 462	common	mseal				sys_mseal
+463	common	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
index cc869f5d5693..9b1bda3d8383 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
@@ -401,3 +401,4 @@
 460	n32	lsm_set_self_attr		sys_lsm_set_self_attr
 461	n32	lsm_list_modules		sys_lsm_list_modules
 462	n32	mseal				sys_mseal
+463	n32	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index 1464c6be6eb3..33710f855c46 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
@@ -377,3 +377,4 @@
 460	n64	lsm_set_self_attr		sys_lsm_set_self_attr
 461	n64	lsm_list_modules		sys_lsm_list_modules
 462	n64	mseal				sys_mseal
+463	n64	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
index 008ebe60263e..97698956cdce 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
@@ -450,3 +450,4 @@
 460	o32	lsm_set_self_attr		sys_lsm_set_self_attr
 461	o32	lsm_list_modules		sys_lsm_list_modules
 462	o32	mseal				sys_mseal
+463	o32	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index b13c21373974..5fbbb25699b9 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl
@@ -461,3 +461,4 @@
 460	common	lsm_set_self_attr		sys_lsm_set_self_attr
 461	common	lsm_list_modules		sys_lsm_list_modules
 462	common	mseal				sys_mseal
+463	common	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index 3656f1ca7a21..b1c3ae68014c 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -549,3 +549,4 @@
 460	common	lsm_set_self_attr		sys_lsm_set_self_attr
 461	common	lsm_list_modules		sys_lsm_list_modules
 462	common	mseal				sys_mseal
+463	common	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index bd0fee24ad10..1188429316c1 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl
@@ -465,3 +465,4 @@
 460  common	lsm_set_self_attr	sys_lsm_set_self_attr		sys_lsm_set_self_attr
 461  common	lsm_list_modules	sys_lsm_list_modules		sys_lsm_list_modules
 462  common	mseal			sys_mseal			sys_mseal
+463  common	vgetrandom_alloc	sys_vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
index bbf83a2db986..06cec74502f7 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -465,3 +465,4 @@
 460	common	lsm_set_self_attr		sys_lsm_set_self_attr
 461	common	lsm_list_modules		sys_lsm_list_modules
 462	common	mseal				sys_mseal
+463	common	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index ac6c281ccfe0..a3816995d08c 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -508,3 +508,4 @@
 460	common	lsm_set_self_attr		sys_lsm_set_self_attr
 461	common	lsm_list_modules		sys_lsm_list_modules
 462	common	mseal 				sys_mseal
+463	common	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 7fd1f57ad3d3..821990638567 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -467,3 +467,4 @@
 460	i386	lsm_set_self_attr	sys_lsm_set_self_attr
 461	i386	lsm_list_modules	sys_lsm_list_modules
 462	i386	mseal 			sys_mseal
+463	i386	vgetrandom_alloc	sys_vgetrandom_alloc
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index a396f6e6ab5b..441443ba2ae6 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -384,6 +384,7 @@
 460	common	lsm_set_self_attr	sys_lsm_set_self_attr
 461	common	lsm_list_modules	sys_lsm_list_modules
 462 	common  mseal			sys_mseal
+463	common	vgetrandom_alloc	sys_vgetrandom_alloc
 
 #
 # Due to a historical design error, certain syscalls are numbered differently
diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
index 67083fc1b2f5..b9f64edd0b18 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -433,3 +433,4 @@
 460	common	lsm_set_self_attr		sys_lsm_set_self_attr
 461	common	lsm_list_modules		sys_lsm_list_modules
 462	common	mseal 				sys_mseal
+463	common	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index d983c48a3b6a..e94210483f60 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -845,8 +845,11 @@ __SYSCALL(__NR_lsm_list_modules, sys_lsm_list_modules)
 #define __NR_mseal 462
 __SYSCALL(__NR_mseal, sys_mseal)
 
+#define __NR_vgetrandom_alloc 463
+__SYSCALL(__NR_vgetrandom_alloc, sys_vgetrandom_alloc)
+
 #undef __NR_syscalls
-#define __NR_syscalls 463
+#define __NR_syscalls 464
 
 /*
  * 32 bit systems traditionally used different
diff --git a/tools/include/uapi/asm-generic/unistd.h b/tools/include/uapi/asm-generic/unistd.h
index d983c48a3b6a..e94210483f60 100644
--- a/tools/include/uapi/asm-generic/unistd.h
+++ b/tools/include/uapi/asm-generic/unistd.h
@@ -845,8 +845,11 @@ __SYSCALL(__NR_lsm_list_modules, sys_lsm_list_modules)
 #define __NR_mseal 462
 __SYSCALL(__NR_mseal, sys_mseal)
 
+#define __NR_vgetrandom_alloc 463
+__SYSCALL(__NR_vgetrandom_alloc, sys_vgetrandom_alloc)
+
 #undef __NR_syscalls
-#define __NR_syscalls 463
+#define __NR_syscalls 464
 
 /*
  * 32 bit systems traditionally used different
diff --git a/tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl b/tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl
index 1464c6be6eb3..33710f855c46 100644
--- a/tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl
+++ b/tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl
@@ -377,3 +377,4 @@
 460	n64	lsm_set_self_attr		sys_lsm_set_self_attr
 461	n64	lsm_list_modules		sys_lsm_list_modules
 462	n64	mseal				sys_mseal
+463	n64	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/tools/perf/arch/powerpc/entry/syscalls/syscall.tbl b/tools/perf/arch/powerpc/entry/syscalls/syscall.tbl
index 3656f1ca7a21..b1c3ae68014c 100644
--- a/tools/perf/arch/powerpc/entry/syscalls/syscall.tbl
+++ b/tools/perf/arch/powerpc/entry/syscalls/syscall.tbl
@@ -549,3 +549,4 @@
 460	common	lsm_set_self_attr		sys_lsm_set_self_attr
 461	common	lsm_list_modules		sys_lsm_list_modules
 462	common	mseal				sys_mseal
+463	common	vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/tools/perf/arch/s390/entry/syscalls/syscall.tbl b/tools/perf/arch/s390/entry/syscalls/syscall.tbl
index bd0fee24ad10..1188429316c1 100644
--- a/tools/perf/arch/s390/entry/syscalls/syscall.tbl
+++ b/tools/perf/arch/s390/entry/syscalls/syscall.tbl
@@ -465,3 +465,4 @@
 460  common	lsm_set_self_attr	sys_lsm_set_self_attr		sys_lsm_set_self_attr
 461  common	lsm_list_modules	sys_lsm_list_modules		sys_lsm_list_modules
 462  common	mseal			sys_mseal			sys_mseal
+463  common	vgetrandom_alloc	sys_vgetrandom_alloc		sys_vgetrandom_alloc
diff --git a/tools/perf/arch/x86/entry/syscalls/syscall_64.tbl b/tools/perf/arch/x86/entry/syscalls/syscall_64.tbl
index a396f6e6ab5b..441443ba2ae6 100644
--- a/tools/perf/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/tools/perf/arch/x86/entry/syscalls/syscall_64.tbl
@@ -384,6 +384,7 @@
 460	common	lsm_set_self_attr	sys_lsm_set_self_attr
 461	common	lsm_list_modules	sys_lsm_list_modules
 462 	common  mseal			sys_mseal
+463	common	vgetrandom_alloc	sys_vgetrandom_alloc
 
 #
 # Due to a historical design error, certain syscalls are numbered differently
-- 
2.45.2

[PATCH v18 4/5] random: introduce generic vDSO getrandom() implementation

[Jason A. Donenfeld]

Provide a generic C vDSO getrandom() implementation, which operates on
an opaque state returned by vgetrandom_alloc() and produces random bytes
the same way as getrandom(). This has the following API signature:

  ssize_t vgetrandom(void *buffer, size_t len, unsigned int flags,
                     void *opaque_state, size_t opaque_len);

The return value and the first three arguments are the same as ordinary
getrandom(), while the last two arguments are a pointer to the opaque
allocated state and its size. Were all five arguments passed to the
getrandom() syscall, nothing different would happen, and the functions
would have the exact same behavior.

The actual vDSO RNG algorithm implemented is the same one implemented by
drivers/char/random.c, using the same fast-erasure techniques as that.
Should the in-kernel implementation change, so too will the vDSO one.

It requires an implementation of ChaCha20 that does not use any stack,
in order to maintain forward secrecy if a multi-threaded program forks
(though this does not account for a similar issue with SA_SIGINFO
copying registers to the stack), so this is left as an
architecture-specific fill-in. Stack-less ChaCha20 is an easy algorithm
to implement on a variety of architectures, so this shouldn't be too
onerous.

Initially, the state is keyless, and so the first call makes a
getrandom() syscall to generate that key, and then uses it for
subsequent calls. By keeping track of a generation counter, it knows
when its key is invalidated and it should fetch a new one using the
syscall. Later, more than just a generation counter might be used.

Since MADV_WIPEONFORK is set on the opaque state, the key and related
state is wiped during a fork(), so secrets don't roll over into new
processes, and the same state doesn't accidentally generate the same
random stream. The generation counter, as well, is always >0, so that
the 0 counter is a useful indication of a fork() or otherwise
uninitialized state.

If the kernel RNG is not yet initialized, then the vDSO always calls the
syscall, because that behavior cannot be emulated in userspace, but
fortunately that state is short lived and only during early boot. If it
has been initialized, then there is no need to inspect the `flags`
argument, because the behavior does not change post-initialization
regardless of the `flags` value.

Since the opaque state passed to it is mutated, vDSO getrandom() is not
reentrant, when used with the same opaque state, which libc should be
mindful of.

vgetrandom_alloc() and vDSO getrandom() provide the ability for
userspace to generate random bytes quickly and safely, and are intended
to be integrated into libc's thread management. As an illustrative
example, together with the example code from "random: add
vgetrandom_alloc() syscall", the following code might be used to do the
same outside of libc. In a libc, only the non-static vgetrandom()
function at the end would be exported as part of a getrandom()
implementations, and the various pthread-isms are expected to be elided
into libc internals.

  static struct {
    ssize_t(*fn)(void *, size_t, unsigned long, void *, size_t);
    pthread_key_t key;
    pthread_once_t initialized;
  } grnd_ctx = {
    .initialized = PTHREAD_ONCE_INIT
  };

  static void vgetrandom_init(void)
  {
    if (pthread_key_create(&grnd_ctx.key, vgetrandom_put_state) != 0)
      return;
    grnd_ctx.fn = vdso_sym("LINUX_2.6", "__vdso_getrandom");
  }

  ssize_t vgetrandom(void *buf, size_t len, unsigned long flags)
  {
    void *state;

    pthread_once(&grnd_ctx.initialized, vgetrandom_init);
    if (!grnd_ctx.fn)
      return getrandom(buf, len, flags);
    state = pthread_getspecific(grnd_ctx.key);
    if (!state) {
      state = vgetrandom_get_state();
      if (pthread_setspecific(grnd_ctx.key, state) != 0) {
        vgetrandom_put_state(state);
        state = NULL;
      }
      if (!state)
        return getrandom(buf, len, flags);
    }
    return grnd_ctx.fn(buf, len, flags, state, grnd_allocator.size_per_each);
  }

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 MAINTAINERS                                   |   1 +
 drivers/char/random.c                         |  13 +
 include/vdso/datapage.h                       |  11 +
 include/vdso/getrandom.h                      |  34 +-
 lib/vdso/getrandom.c                          | 236 ++++++++++++++
 tools/testing/selftests/vDSO/.gitignore       |   1 +
 tools/testing/selftests/vDSO/Makefile         |   2 +
 .../selftests/vDSO/vdso_test_getrandom.c      | 293 ++++++++++++++++++
 8 files changed, 589 insertions(+), 2 deletions(-)
 create mode 100644 lib/vdso/getrandom.c
 create mode 100644 tools/testing/selftests/vDSO/vdso_test_getrandom.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 8480c4c39915..6fecb837b24b 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -18748,6 +18748,7 @@ F:	Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
 F:	drivers/char/random.c
 F:	drivers/virt/vmgenid.c
 F:	include/vdso/getrandom.h
+F:	lib/vdso/getrandom.c
 
 RAPIDIO SUBSYSTEM
 M:	Matt Porter <mporter@kernel.crashing.org>
diff --git a/drivers/char/random.c b/drivers/char/random.c
index ccb35f390c85..ae458591caed 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -60,6 +60,7 @@
 #include <crypto/blake2s.h>
 #ifdef CONFIG_VDSO_GETRANDOM
 #include <vdso/getrandom.h>
+#include <vdso/datapage.h>
 #endif
 #include <asm/archrandom.h>
 #include <asm/processor.h>
@@ -404,6 +405,15 @@ static void crng_reseed(struct work_struct *work)
 	if (next_gen == ULONG_MAX)
 		++next_gen;
 	WRITE_ONCE(base_crng.generation, next_gen);
+#ifdef CONFIG_VDSO_GETRANDOM
+	/* base_crng.generation's invalid value is ULONG_MAX, while
+	 * _vdso_rng_data.generation's invalid value is 0, so add one to the
+	 * former to arrive at the latter. Use smp_store_release so that this
+	 * is ordered with the write above to base_crng.generation. Pairs with
+	 * the smp_rmb() before the syscall in the vDSO code.
+	 */
+	smp_store_release(&_vdso_rng_data.generation, next_gen + 1);
+#endif
 	if (!static_branch_likely(&crng_is_ready))
 		crng_init = CRNG_READY;
 	spin_unlock_irqrestore(&base_crng.lock, flags);
@@ -854,6 +864,9 @@ static void __cold _credit_init_bits(size_t bits)
 		if (static_key_initialized && system_unbound_wq)
 			queue_work(system_unbound_wq, &set_ready);
 		atomic_notifier_call_chain(&random_ready_notifier, 0, NULL);
+#ifdef CONFIG_VDSO_GETRANDOM
+		WRITE_ONCE(_vdso_rng_data.is_ready, true);
+#endif
 		wake_up_interruptible(&crng_init_wait);
 		kill_fasync(&fasync, SIGIO, POLL_IN);
 		pr_notice("crng init done\n");
diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h
index d04d394db064..05e5787beb73 100644
--- a/include/vdso/datapage.h
+++ b/include/vdso/datapage.h
@@ -113,6 +113,16 @@ struct vdso_data {
 	struct arch_vdso_data	arch_data;
 };
 
+/**
+ * struct vdso_rng_data - vdso RNG state information
+ * @generation:	counter representing the number of RNG reseeds
+ * @is_ready:	boolean signaling whether the RNG is initialized
+ */
+struct vdso_rng_data {
+	u64	generation;
+	u8	is_ready;
+};
+
 /*
  * We use the hidden visibility to prevent the compiler from generating a GOT
  * relocation. Not only is going through a GOT useless (the entry couldn't and
@@ -124,6 +134,7 @@ struct vdso_data {
  */
 extern struct vdso_data _vdso_data[CS_BASES] __attribute__((visibility("hidden")));
 extern struct vdso_data _timens_data[CS_BASES] __attribute__((visibility("hidden")));
+extern struct vdso_rng_data _vdso_rng_data __attribute__((visibility("hidden")));
 
 /**
  * union vdso_data_store - Generic vDSO data page
diff --git a/include/vdso/getrandom.h b/include/vdso/getrandom.h
index 69037519d20b..84f523149d5d 100644
--- a/include/vdso/getrandom.h
+++ b/include/vdso/getrandom.h
@@ -6,11 +6,41 @@
 #ifndef _VDSO_GETRANDOM_H
 #define _VDSO_GETRANDOM_H
 
+#include <linux/types.h>
+
+#define CHACHA_KEY_SIZE         32
+#define CHACHA_BLOCK_SIZE       64
+
 /**
  * struct vgetrandom_state - State used by vDSO getrandom() and allocated by vgetrandom_alloc().
  *
- * Currently empty, as the vDSO getrandom() function has not yet been implemented.
+ * @batch:	One and a half ChaCha20 blocks of buffered RNG output.
+ *
+ * @key:	Key to be used for generating next batch.
+ *
+ * @batch_key:	Union of the prior two members, which is exactly two full
+ * 		ChaCha20 blocks in size, so that @batch and @key can be filled
+ * 		together.
+ *
+ * @generation:	Snapshot of @rng_info->generation in the vDSO data page at
+ *		the time @key was generated.
+ *
+ * @pos:	Offset into @batch of the next available random byte.
+ *
+ * @in_use:	Reentrancy guard for reusing a state within the same thread
+ *		due to signal handlers.
  */
-struct vgetrandom_state { int placeholder; };
+struct vgetrandom_state {
+	union {
+		struct {
+			u8	batch[CHACHA_BLOCK_SIZE * 3 / 2];
+			u32	key[CHACHA_KEY_SIZE / sizeof(u32)];
+		};
+		u8		batch_key[CHACHA_BLOCK_SIZE * 2];
+	};
+	u64			generation;
+	u8			pos;
+	bool 			in_use;
+};
 
 #endif /* _VDSO_GETRANDOM_H */
diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c
new file mode 100644
index 000000000000..5efcd0d0af6b
--- /dev/null
+++ b/lib/vdso/getrandom.c
@@ -0,0 +1,236 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+#include <linux/cache.h>
+#include <linux/kernel.h>
+#include <linux/time64.h>
+#include <vdso/datapage.h>
+#include <vdso/getrandom.h>
+#include <asm/vdso/getrandom.h>
+#include <asm/vdso/vsyscall.h>
+#include <asm/unaligned.h>
+
+#define MEMCPY_AND_ZERO_SRC(type, dst, src, len) do {				\
+	while (len >= sizeof(type)) {						\
+		__put_unaligned_t(type, __get_unaligned_t(type, src), dst);	\
+		__put_unaligned_t(type, 0, src);				\
+		dst += sizeof(type);						\
+		src += sizeof(type);						\
+		len -= sizeof(type);						\
+	}									\
+} while (0)
+
+static void memcpy_and_zero_src(void *dst, void *src, size_t len)
+{
+	if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {
+		if (IS_ENABLED(CONFIG_64BIT))
+			MEMCPY_AND_ZERO_SRC(u64, dst, src, len);
+		MEMCPY_AND_ZERO_SRC(u32, dst, src, len);
+		MEMCPY_AND_ZERO_SRC(u16, dst, src, len);
+	}
+	MEMCPY_AND_ZERO_SRC(u8, dst, src, len);
+}
+
+/**
+ * __cvdso_getrandom_data - Generic vDSO implementation of getrandom() syscall.
+ * @rng_info:		Describes state of kernel RNG, memory shared with kernel.
+ * @buffer:		Destination buffer to fill with random bytes.
+ * @len:		Size of @buffer in bytes.
+ * @flags:		Zero or more GRND_* flags.
+ * @opaque_state:	Pointer to an opaque state area.
+ * @opaque_len:		Length of opaque state area.
+ *
+ * This implements a "fast key erasure" RNG using ChaCha20, in the same way that the kernel's
+ * getrandom() syscall does. It periodically reseeds its key from the kernel's RNG, at the same
+ * schedule that the kernel's RNG is reseeded. If the kernel's RNG is not ready, then this always
+ * calls into the syscall.
+ *
+ * @opaque_state *must* be allocated using the vgetrandom_alloc() syscall.  Unless external locking
+ * is used, one state must be allocated per thread, as it is not safe to call this function
+ * concurrently with the same @opaque_state. However, it is safe to call this using the same
+ * @opaque_state that is shared between main code and signal handling code, within the same thread.
+ *
+ * Returns:	The number of random bytes written to @buffer, or a negative value indicating an error.
+ */
+static __always_inline ssize_t
+__cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_t len,
+		       unsigned int flags, void *opaque_state, size_t opaque_len)
+{
+	ssize_t ret = min_t(size_t, INT_MAX & PAGE_MASK /* = MAX_RW_COUNT */, len);
+	struct vgetrandom_state *state = opaque_state;
+	size_t batch_len, nblocks, orig_len = len;
+	bool in_use, have_retried = false;
+	unsigned long current_generation;
+	void *orig_buffer = buffer;
+	u32 counter[2] = { 0 };
+
+	/* The state must not straddle a page, since pages can be zeroed at any time. */
+	if (unlikely(((unsigned long)opaque_state & ~PAGE_MASK) + sizeof(*state) > PAGE_SIZE))
+		return -EFAULT;
+
+	/* If the caller passes the wrong size, which might happen due to CRIU, fallback. */
+	if (unlikely(opaque_len != sizeof(*state)))
+		goto fallback_syscall;
+
+	/*
+	 * If the kernel's RNG is not yet ready, then it's not possible to provide random bytes from
+	 * userspace, because A) the various @flags require this to block, or not, depending on
+	 * various factors unavailable to userspace, and B) the kernel's behavior before the RNG is
+	 * ready is to reseed from the entropy pool at every invocation.
+	 */
+	if (unlikely(!READ_ONCE(rng_info->is_ready)))
+		goto fallback_syscall;
+
+	/*
+	 * This condition is checked after @rng_info->is_ready, because before the kernel's RNG is
+	 * initialized, the @flags parameter may require this to block or return an error, even when
+	 * len is zero.
+	 */
+	if (unlikely(!len))
+		return 0;
+
+	/*
+	 * @state->in_use is basic reentrancy protection against this running in a signal handler
+	 * with the same @opaque_state, but obviously not atomic wrt multiple CPUs or more than one
+	 * level of reentrancy. If a signal interrupts this after reading @state->in_use, but before
+	 * writing @state->in_use, there is still no race, because the signal handler will run to
+	 * its completion before returning execution.
+	 */
+	in_use = READ_ONCE(state->in_use);
+	if (unlikely(in_use))
+		/* The syscall simply fills the buffer and does not touch @state, so fallback. */
+		goto fallback_syscall;
+	WRITE_ONCE(state->in_use, true);
+
+retry_generation:
+	/*
+	 * @rng_info->generation must always be read here, as it serializes @state->key with the
+	 * kernel's RNG reseeding schedule.
+	 */
+	current_generation = READ_ONCE(rng_info->generation);
+
+	/*
+	 * If @state->generation doesn't match the kernel RNG's generation, then it means the
+	 * kernel's RNG has reseeded, and so @state->key is reseeded as well.
+	 */
+	if (unlikely(state->generation != current_generation)) {
+		/*
+		 * Write the generation before filling the key, in case of fork. If there is a fork
+		 * just after this line, the parent and child will get different random bytes from
+		 * the syscall, which is good. However, were this line to occur after the getrandom
+		 * syscall, then both child and parent could have the same bytes and the same
+		 * generation counter, so the fork would not be detected. Therefore, write
+		 * @state->generation before the call to the getrandom syscall.
+		 */
+		WRITE_ONCE(state->generation, current_generation);
+
+		/*
+		 * Prevent the syscall from being reordered wrt current_generation. Pairs with the
+		 * smp_store_release(&_vdso_rng_data.generation) in random.c.
+		 */
+		smp_rmb();
+
+		/* Reseed @state->key using fresh bytes from the kernel. */
+		if (getrandom_syscall(state->key, sizeof(state->key), 0) != sizeof(state->key)) {
+			/*
+			 * If the syscall failed to refresh the key, then @state->key is now
+			 * invalid, so invalidate the generation so that it is not used again, and
+			 * fallback to using the syscall entirely.
+			 */
+			WRITE_ONCE(state->generation, 0);
+
+			/*
+			 * Set @state->in_use to false only after the last write to @state in the
+			 * line above.
+			 */
+			WRITE_ONCE(state->in_use, false);
+
+			goto fallback_syscall;
+		}
+
+		/*
+		 * Set @state->pos to beyond the end of the batch, so that the batch is refilled
+		 * using the new key.
+		 */
+		state->pos = sizeof(state->batch);
+	}
+
+	/* Set len to the total amount of bytes that this function is allowed to read, ret. */
+	len = ret;
+more_batch:
+	/*
+	 * First use bytes out of @state->batch, which may have been filled by the last call to this
+	 * function.
+	 */
+	batch_len = min_t(size_t, sizeof(state->batch) - state->pos, len);
+	if (batch_len) {
+		/* Zeroing at the same time as memcpying helps preserve forward secrecy. */
+		memcpy_and_zero_src(buffer, state->batch + state->pos, batch_len);
+		state->pos += batch_len;
+		buffer += batch_len;
+		len -= batch_len;
+	}
+
+	if (!len) {
+		/* Prevent the loop from being reordered wrt ->generation. */
+		barrier();
+
+		/*
+		 * Since @rng_info->generation will never be 0, re-read @state->generation, rather
+		 * than using the local current_generation variable, to learn whether a fork
+		 * occurred or if @state was zeroed due to memory pressure. Primarily, though, this
+		 * indicates whether the kernel's RNG has reseeded, in which case generate a new key
+		 * and start over.
+		 */
+		if (unlikely(READ_ONCE(state->generation) != READ_ONCE(rng_info->generation))) {
+			/*
+			 * Prevent this from looping forever in case of low memory or racing with a
+			 * user force-reseeding the kernel's RNG using the ioctl.
+			 */
+			if (have_retried) {
+				WRITE_ONCE(state->in_use, false);
+				goto fallback_syscall;
+			}
+
+			have_retried = true;
+			buffer = orig_buffer;
+			goto retry_generation;
+		}
+
+		/*
+		 * Set @state->in_use to false only when there will be no more reads or writes of
+		 * @state.
+		 */
+		WRITE_ONCE(state->in_use, false);
+		return ret;
+	}
+
+	/* Generate blocks of RNG output directly into @buffer while there's enough room left. */
+	nblocks = len / CHACHA_BLOCK_SIZE;
+	if (nblocks) {
+		__arch_chacha20_blocks_nostack(buffer, state->key, counter, nblocks);
+		buffer += nblocks * CHACHA_BLOCK_SIZE;
+		len -= nblocks * CHACHA_BLOCK_SIZE;
+	}
+
+	BUILD_BUG_ON(sizeof(state->batch_key) % CHACHA_BLOCK_SIZE != 0);
+
+	/* Refill the batch and overwrite the key, in order to preserve forward secrecy. */
+	__arch_chacha20_blocks_nostack(state->batch_key, state->key, counter,
+				       sizeof(state->batch_key) / CHACHA_BLOCK_SIZE);
+
+	/* Since the batch was just refilled, set the position back to 0 to indicate a full batch. */
+	state->pos = 0;
+	goto more_batch;
+
+fallback_syscall:
+	return getrandom_syscall(orig_buffer, orig_len, flags);
+}
+
+static __always_inline ssize_t
+__cvdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state, size_t opaque_len)
+{
+	return __cvdso_getrandom_data(__arch_get_vdso_rng_data(), buffer, len, flags, opaque_state, opaque_len);
+}
diff --git a/tools/testing/selftests/vDSO/.gitignore b/tools/testing/selftests/vDSO/.gitignore
index a8dc51af5a9c..7dbfdec53f3d 100644
--- a/tools/testing/selftests/vDSO/.gitignore
+++ b/tools/testing/selftests/vDSO/.gitignore
@@ -6,3 +6,4 @@ vdso_test_correctness
 vdso_test_gettimeofday
 vdso_test_getcpu
 vdso_standalone_test_x86
+vdso_test_getrandom
diff --git a/tools/testing/selftests/vDSO/Makefile b/tools/testing/selftests/vDSO/Makefile
index d53a4d8008f9..a33b4d200a32 100644
--- a/tools/testing/selftests/vDSO/Makefile
+++ b/tools/testing/selftests/vDSO/Makefile
@@ -11,6 +11,7 @@ ifeq ($(ARCH),$(filter $(ARCH),x86 x86_64))
 TEST_GEN_PROGS += $(OUTPUT)/vdso_standalone_test_x86
 endif
 TEST_GEN_PROGS += $(OUTPUT)/vdso_test_correctness
+TEST_GEN_PROGS += $(OUTPUT)/vdso_test_getrandom
 
 CFLAGS := -std=gnu99
 CFLAGS_vdso_standalone_test_x86 := -nostdlib -fno-asynchronous-unwind-tables -fno-stack-protector
@@ -33,3 +34,4 @@ $(OUTPUT)/vdso_test_correctness: vdso_test_correctness.c
 		vdso_test_correctness.c \
 		-o $@ \
 		$(LDFLAGS_vdso_test_correctness)
+$(OUTPUT)/vdso_test_getrandom: parse_vdso.c
diff --git a/tools/testing/selftests/vDSO/vdso_test_getrandom.c b/tools/testing/selftests/vDSO/vdso_test_getrandom.c
new file mode 100644
index 000000000000..a4315eb4d075
--- /dev/null
+++ b/tools/testing/selftests/vDSO/vdso_test_getrandom.c
@@ -0,0 +1,293 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+#include <assert.h>
+#include <pthread.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include <unistd.h>
+#include <sys/auxv.h>
+#include <sys/mman.h>
+#include <sys/random.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <linux/const.h>
+
+#include "../kselftest.h"
+#include "parse_vdso.h"
+
+#ifndef timespecsub
+#define	timespecsub(tsp, usp, vsp)					\
+	do {								\
+		(vsp)->tv_sec = (tsp)->tv_sec - (usp)->tv_sec;		\
+		(vsp)->tv_nsec = (tsp)->tv_nsec - (usp)->tv_nsec;	\
+		if ((vsp)->tv_nsec < 0) {				\
+			(vsp)->tv_sec--;				\
+			(vsp)->tv_nsec += 1000000000L;			\
+		}							\
+	} while (0)
+#endif
+
+#define DIV_ROUND_UP __KERNEL_DIV_ROUND_UP
+
+static void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each)
+{
+	enum { __NR_vgetrandom_alloc = 463 };
+	*size_per_each = 0;
+	return (void *)syscall(__NR_vgetrandom_alloc, num, size_per_each, 0, 0);
+}
+
+static struct {
+	pthread_mutex_t lock;
+	void **states;
+	size_t len, cap, size_per_each;
+} grnd_allocator = {
+	.lock = PTHREAD_MUTEX_INITIALIZER
+};
+
+static void *vgetrandom_get_state(void)
+{
+	void *state = NULL;
+
+	pthread_mutex_lock(&grnd_allocator.lock);
+	if (!grnd_allocator.len) {
+		size_t new_cap;
+		size_t page_size = getpagesize();
+		unsigned int num = sysconf(_SC_NPROCESSORS_ONLN); /* Just a decent heuristic. */
+		unsigned int size_per_each;
+		void *new_block = vgetrandom_alloc(&num, &size_per_each);
+		void *new_states;
+
+		if (new_block == MAP_FAILED)
+			goto out;
+		if (grnd_allocator.size_per_each && grnd_allocator.size_per_each != size_per_each)
+			goto unmap;
+		grnd_allocator.size_per_each = size_per_each;
+		new_cap = grnd_allocator.cap + num;
+		new_states = reallocarray(grnd_allocator.states, new_cap, sizeof(*grnd_allocator.states));
+		if (!new_states)
+			goto unmap;
+		grnd_allocator.cap = new_cap;
+		grnd_allocator.states = new_states;
+
+		for (size_t i = 0; i < num; ++i) {
+			grnd_allocator.states[i] = new_block;
+			if (((uintptr_t)new_block & (page_size - 1)) + size_per_each > page_size)
+				new_block = (void *)(((uintptr_t)new_block + page_size) & (page_size - 1));
+			else
+				new_block += size_per_each;
+		}
+		grnd_allocator.len = num;
+		goto success;
+
+	unmap:
+		munmap(new_block, DIV_ROUND_UP(num, page_size / size_per_each) * page_size);
+		goto out;
+	}
+success:
+	state = grnd_allocator.states[--grnd_allocator.len];
+
+out:
+	pthread_mutex_unlock(&grnd_allocator.lock);
+	return state;
+}
+
+static void vgetrandom_put_state(void *state)
+{
+	if (!state)
+		return;
+	pthread_mutex_lock(&grnd_allocator.lock);
+	grnd_allocator.states[grnd_allocator.len++] = state;
+	pthread_mutex_unlock(&grnd_allocator.lock);
+}
+
+static struct {
+	ssize_t(*fn)(void *, size_t, unsigned long, void *, size_t);
+	pthread_key_t key;
+	pthread_once_t initialized;
+} grnd_ctx = {
+	.initialized = PTHREAD_ONCE_INIT
+};
+
+static void vgetrandom_init(void)
+{
+	if (pthread_key_create(&grnd_ctx.key, vgetrandom_put_state) != 0)
+		return;
+	unsigned long sysinfo_ehdr = getauxval(AT_SYSINFO_EHDR);
+	if (!sysinfo_ehdr) {
+		printf("AT_SYSINFO_EHDR is not present!\n");
+		exit(KSFT_SKIP);
+	}
+	vdso_init_from_sysinfo_ehdr(sysinfo_ehdr);
+	grnd_ctx.fn = (__typeof__(grnd_ctx.fn))vdso_sym("LINUX_2.6", "__vdso_getrandom");
+	if (!grnd_ctx.fn) {
+		printf("__vdso_getrandom is missing!\n");
+		exit(KSFT_FAIL);
+	}
+}
+
+static ssize_t vgetrandom(void *buf, size_t len, unsigned long flags)
+{
+	void *state;
+
+	pthread_once(&grnd_ctx.initialized, vgetrandom_init);
+	state = pthread_getspecific(grnd_ctx.key);
+	if (!state) {
+		state = vgetrandom_get_state();
+		if (pthread_setspecific(grnd_ctx.key, state) != 0) {
+			vgetrandom_put_state(state);
+			state = NULL;
+		}
+		if (!state) {
+			printf("vgetrandom_get_state failed!\n");
+			exit(KSFT_FAIL);
+		}
+	}
+	return grnd_ctx.fn(buf, len, flags, state, grnd_allocator.size_per_each);
+}
+
+enum { TRIALS = 25000000, THREADS = 256 };
+
+static void *test_vdso_getrandom(void *)
+{
+	for (size_t i = 0; i < TRIALS; ++i) {
+		unsigned int val;
+		ssize_t ret = vgetrandom(&val, sizeof(val), 0);
+		assert(ret == sizeof(val));
+	}
+	return NULL;
+}
+
+static void *test_libc_getrandom(void *)
+{
+	for (size_t i = 0; i < TRIALS; ++i) {
+		unsigned int val;
+		ssize_t ret = getrandom(&val, sizeof(val), 0);
+		assert(ret == sizeof(val));
+	}
+	return NULL;
+}
+
+static void *test_syscall_getrandom(void *)
+{
+	for (size_t i = 0; i < TRIALS; ++i) {
+		unsigned int val;
+		ssize_t ret = syscall(SYS_getrandom, &val, sizeof(val), 0);
+		assert(ret == sizeof(val));
+	}
+	return NULL;
+}
+
+static void bench_single(void)
+{
+	struct timespec start, end, diff;
+
+	clock_gettime(CLOCK_MONOTONIC, &start);
+	test_vdso_getrandom(NULL);
+	clock_gettime(CLOCK_MONOTONIC, &end);
+	timespecsub(&end, &start, &diff);
+	printf("   vdso: %u times in %lu.%09lu seconds\n", TRIALS, diff.tv_sec, diff.tv_nsec);
+
+	clock_gettime(CLOCK_MONOTONIC, &start);
+	test_libc_getrandom(NULL);
+	clock_gettime(CLOCK_MONOTONIC, &end);
+	timespecsub(&end, &start, &diff);
+	printf("   libc: %u times in %lu.%09lu seconds\n", TRIALS, diff.tv_sec, diff.tv_nsec);
+
+	clock_gettime(CLOCK_MONOTONIC, &start);
+	test_syscall_getrandom(NULL);
+	clock_gettime(CLOCK_MONOTONIC, &end);
+	timespecsub(&end, &start, &diff);
+	printf("syscall: %u times in %lu.%09lu seconds\n", TRIALS, diff.tv_sec, diff.tv_nsec);
+}
+
+static void bench_multi(void)
+{
+	struct timespec start, end, diff;
+	pthread_t threads[THREADS];
+
+	clock_gettime(CLOCK_MONOTONIC, &start);
+	for (size_t i = 0; i < THREADS; ++i)
+		assert(pthread_create(&threads[i], NULL, test_vdso_getrandom, NULL) == 0);
+	for (size_t i = 0; i < THREADS; ++i)
+		pthread_join(threads[i], NULL);
+	clock_gettime(CLOCK_MONOTONIC, &end);
+	timespecsub(&end, &start, &diff);
+	printf("   vdso: %u x %u times in %lu.%09lu seconds\n", TRIALS, THREADS, diff.tv_sec, diff.tv_nsec);
+
+	clock_gettime(CLOCK_MONOTONIC, &start);
+	for (size_t i = 0; i < THREADS; ++i)
+		assert(pthread_create(&threads[i], NULL, test_libc_getrandom, NULL) == 0);
+	for (size_t i = 0; i < THREADS; ++i)
+		pthread_join(threads[i], NULL);
+	clock_gettime(CLOCK_MONOTONIC, &end);
+	timespecsub(&end, &start, &diff);
+	printf("   libc: %u x %u times in %lu.%09lu seconds\n", TRIALS, THREADS, diff.tv_sec, diff.tv_nsec);
+
+	clock_gettime(CLOCK_MONOTONIC, &start);
+	for (size_t i = 0; i < THREADS; ++i)
+		assert(pthread_create(&threads[i], NULL, test_syscall_getrandom, NULL) == 0);
+	for (size_t i = 0; i < THREADS; ++i)
+		pthread_join(threads[i], NULL);
+	clock_gettime(CLOCK_MONOTONIC, &end);
+	timespecsub(&end, &start, &diff);
+	printf("   syscall: %u x %u times in %lu.%09lu seconds\n", TRIALS, THREADS, diff.tv_sec, diff.tv_nsec);
+}
+
+static void fill(void)
+{
+	uint8_t weird_size[323929];
+	for (;;)
+		vgetrandom(weird_size, sizeof(weird_size), 0);
+}
+
+static void kselftest(void)
+{
+	uint8_t weird_size[1263];
+
+	ksft_print_header();
+	ksft_set_plan(1);
+
+	for (size_t i = 0; i < 1000; ++i) {
+		ssize_t ret = vgetrandom(weird_size, sizeof(weird_size), 0);
+		if (ret != sizeof(weird_size))
+			exit(KSFT_FAIL);
+	}
+
+	ksft_test_result_pass("getrandom: PASS\n");
+	exit(KSFT_PASS);
+}
+
+static void usage(const char *argv0)
+{
+	fprintf(stderr, "Usage: %s [bench-single|bench-multi|fill]\n", argv0);
+}
+
+int main(int argc, char *argv[])
+{
+	if (argc == 1) {
+		kselftest();
+		return 0;
+	}
+
+	if (argc != 2) {
+		usage(argv[0]);
+		return 1;
+	}
+	if (!strcmp(argv[1], "bench-single"))
+		bench_single();
+	else if (!strcmp(argv[1], "bench-multi"))
+		bench_multi();
+	else if (!strcmp(argv[1], "fill"))
+		fill();
+	else {
+		usage(argv[0]);
+		return 1;
+	}
+	return 0;
+}
-- 
2.45.2

[PATCH v18 5/5] x86: vdso: Wire up getrandom() vDSO implementation

[Jason A. Donenfeld]

Hook up the generic vDSO implementation to the x86 vDSO data page. Since
the existing vDSO infrastructure is heavily based on the timekeeping
functionality, which works over arrays of bases, a new macro is
introduced for vvars that are not arrays.

The vDSO function requires a ChaCha20 implementation that does not write
to the stack, yet can still do an entire ChaCha20 permutation, so
provide this using SSE2, since this is userland code that must work on
all x86-64 processors. There's a simple test for this code as well.

Reviewed-by: Samuel Neves <sneves@dei.uc.pt> # for vgetrandom-chacha.S
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 arch/x86/Kconfig                              |   1 +
 arch/x86/entry/vdso/Makefile                  |   3 +-
 arch/x86/entry/vdso/vdso.lds.S                |   2 +
 arch/x86/entry/vdso/vgetrandom-chacha.S       | 178 ++++++++++++++++++
 arch/x86/entry/vdso/vgetrandom.c              |  17 ++
 arch/x86/include/asm/vdso/getrandom.h         |  55 ++++++
 arch/x86/include/asm/vdso/vsyscall.h          |   2 +
 arch/x86/include/asm/vvar.h                   |  16 ++
 tools/testing/selftests/vDSO/.gitignore       |   1 +
 tools/testing/selftests/vDSO/Makefile         |  15 ++
 .../testing/selftests/vDSO/vdso_test_chacha.c |  43 +++++
 11 files changed, 332 insertions(+), 1 deletion(-)
 create mode 100644 arch/x86/entry/vdso/vgetrandom-chacha.S
 create mode 100644 arch/x86/entry/vdso/vgetrandom.c
 create mode 100644 arch/x86/include/asm/vdso/getrandom.h
 create mode 100644 tools/testing/selftests/vDSO/vdso_test_chacha.c

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1d7122a1883e..9c98b7a88cc2 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -287,6 +287,7 @@ config X86
 	select HAVE_UNSTABLE_SCHED_CLOCK
 	select HAVE_USER_RETURN_NOTIFIER
 	select HAVE_GENERIC_VDSO
+	select VDSO_GETRANDOM			if X86_64
 	select HOTPLUG_PARALLEL			if SMP && X86_64
 	select HOTPLUG_SMT			if SMP
 	select HOTPLUG_SPLIT_STARTUP		if SMP && X86_32
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index 215a1b202a91..c9216ac4fb1e 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -7,7 +7,7 @@
 include $(srctree)/lib/vdso/Makefile
 
 # Files to link into the vDSO:
-vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o
+vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o vgetrandom.o vgetrandom-chacha.o
 vobjs32-y := vdso32/note.o vdso32/system_call.o vdso32/sigreturn.o
 vobjs32-y += vdso32/vclock_gettime.o vdso32/vgetcpu.o
 vobjs-$(CONFIG_X86_SGX)	+= vsgx.o
@@ -73,6 +73,7 @@ CFLAGS_REMOVE_vdso32/vclock_gettime.o = -pg
 CFLAGS_REMOVE_vgetcpu.o = -pg
 CFLAGS_REMOVE_vdso32/vgetcpu.o = -pg
 CFLAGS_REMOVE_vsgx.o = -pg
+CFLAGS_REMOVE_vgetrandom.o = -pg
 
 #
 # X32 processes use x32 vDSO to access 64bit kernel data.
diff --git a/arch/x86/entry/vdso/vdso.lds.S b/arch/x86/entry/vdso/vdso.lds.S
index e8c60ae7a7c8..0bab5f4af6d1 100644
--- a/arch/x86/entry/vdso/vdso.lds.S
+++ b/arch/x86/entry/vdso/vdso.lds.S
@@ -30,6 +30,8 @@ VERSION {
 #ifdef CONFIG_X86_SGX
 		__vdso_sgx_enter_enclave;
 #endif
+		getrandom;
+		__vdso_getrandom;
 	local: *;
 	};
 }
diff --git a/arch/x86/entry/vdso/vgetrandom-chacha.S b/arch/x86/entry/vdso/vgetrandom-chacha.S
new file mode 100644
index 000000000000..bcba5639b8ee
--- /dev/null
+++ b/arch/x86/entry/vdso/vgetrandom-chacha.S
@@ -0,0 +1,178 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+#include <linux/linkage.h>
+#include <asm/frame.h>
+
+.section	.rodata, "a"
+.align 16
+CONSTANTS:	.octa 0x6b20657479622d323320646e61707865
+.text
+
+/*
+ * Very basic SSE2 implementation of ChaCha20. Produces a given positive number
+ * of blocks of output with a nonce of 0, taking an input key and 8-byte
+ * counter. Importantly does not spill to the stack. Its arguments are:
+ *
+ *	rdi: output bytes
+ *	rsi: 32-byte key input
+ *	rdx: 8-byte counter input/output
+ *	rcx: number of 64-byte blocks to write to output
+ */
+SYM_FUNC_START(__arch_chacha20_blocks_nostack)
+
+.set	output,		%rdi
+.set	key,		%rsi
+.set	counter,	%rdx
+.set	nblocks,	%rcx
+.set	i,		%al
+/* xmm registers are *not* callee-save. */
+.set	temp,		%xmm0
+.set	state0,		%xmm1
+.set	state1,		%xmm2
+.set	state2,		%xmm3
+.set	state3,		%xmm4
+.set	copy0,		%xmm5
+.set	copy1,		%xmm6
+.set	copy2,		%xmm7
+.set	copy3,		%xmm8
+.set	one,		%xmm9
+
+	/* copy0 = "expand 32-byte k" */
+	movaps		CONSTANTS(%rip),copy0
+	/* copy1,copy2 = key */
+	movups		0x00(key),copy1
+	movups		0x10(key),copy2
+	/* copy3 = counter || zero nonce */
+	movq		0x00(counter),copy3
+	/* one = 1 || 0 */
+	movq		$1,%rax
+	movq		%rax,one
+
+.Lblock:
+	/* state0,state1,state2,state3 = copy0,copy1,copy2,copy3 */
+	movdqa		copy0,state0
+	movdqa		copy1,state1
+	movdqa		copy2,state2
+	movdqa		copy3,state3
+
+	movb		$10,i
+.Lpermute:
+	/* state0 += state1, state3 = rotl32(state3 ^ state0, 16) */
+	paddd		state1,state0
+	pxor		state0,state3
+	movdqa		state3,temp
+	pslld		$16,temp
+	psrld		$16,state3
+	por		temp,state3
+
+	/* state2 += state3, state1 = rotl32(state1 ^ state2, 12) */
+	paddd		state3,state2
+	pxor		state2,state1
+	movdqa		state1,temp
+	pslld		$12,temp
+	psrld		$20,state1
+	por		temp,state1
+
+	/* state0 += state1, state3 = rotl32(state3 ^ state0, 8) */
+	paddd		state1,state0
+	pxor		state0,state3
+	movdqa		state3,temp
+	pslld		$8,temp
+	psrld		$24,state3
+	por		temp,state3
+
+	/* state2 += state3, state1 = rotl32(state1 ^ state2, 7) */
+	paddd		state3,state2
+	pxor		state2,state1
+	movdqa		state1,temp
+	pslld		$7,temp
+	psrld		$25,state1
+	por		temp,state1
+
+	/* state1[0,1,2,3] = state1[1,2,3,0] */
+	pshufd		$0x39,state1,state1
+	/* state2[0,1,2,3] = state2[2,3,0,1] */
+	pshufd		$0x4e,state2,state2
+	/* state3[0,1,2,3] = state3[3,0,1,2] */
+	pshufd		$0x93,state3,state3
+
+	/* state0 += state1, state3 = rotl32(state3 ^ state0, 16) */
+	paddd		state1,state0
+	pxor		state0,state3
+	movdqa		state3,temp
+	pslld		$16,temp
+	psrld		$16,state3
+	por		temp,state3
+
+	/* state2 += state3, state1 = rotl32(state1 ^ state2, 12) */
+	paddd		state3,state2
+	pxor		state2,state1
+	movdqa		state1,temp
+	pslld		$12,temp
+	psrld		$20,state1
+	por		temp,state1
+
+	/* state0 += state1, state3 = rotl32(state3 ^ state0, 8) */
+	paddd		state1,state0
+	pxor		state0,state3
+	movdqa		state3,temp
+	pslld		$8,temp
+	psrld		$24,state3
+	por		temp,state3
+
+	/* state2 += state3, state1 = rotl32(state1 ^ state2, 7) */
+	paddd		state3,state2
+	pxor		state2,state1
+	movdqa		state1,temp
+	pslld		$7,temp
+	psrld		$25,state1
+	por		temp,state1
+
+	/* state1[0,1,2,3] = state1[3,0,1,2] */
+	pshufd		$0x93,state1,state1
+	/* state2[0,1,2,3] = state2[2,3,0,1] */
+	pshufd		$0x4e,state2,state2
+	/* state3[0,1,2,3] = state3[1,2,3,0] */
+	pshufd		$0x39,state3,state3
+
+	decb		i
+	jnz		.Lpermute
+
+	/* output0 = state0 + copy0 */
+	paddd		copy0,state0
+	movups		state0,0x00(output)
+	/* output1 = state1 + copy1 */
+	paddd		copy1,state1
+	movups		state1,0x10(output)
+	/* output2 = state2 + copy2 */
+	paddd		copy2,state2
+	movups		state2,0x20(output)
+	/* output3 = state3 + copy3 */
+	paddd		copy3,state3
+	movups		state3,0x30(output)
+
+	/* ++copy3.counter */
+	paddq		one,copy3
+
+	/* output += 64, --nblocks */
+	addq		$64,output
+	decq		nblocks
+	jnz		.Lblock
+
+	/* counter = copy3.counter */
+	movq		copy3,0x00(counter)
+
+	/* Zero out the potentially sensitive regs, in case nothing uses these again. */
+	pxor		state0,state0
+	pxor		state1,state1
+	pxor		state2,state2
+	pxor		state3,state3
+	pxor		copy1,copy1
+	pxor		copy2,copy2
+	pxor		temp,temp
+
+	ret
+SYM_FUNC_END(__arch_chacha20_blocks_nostack)
diff --git a/arch/x86/entry/vdso/vgetrandom.c b/arch/x86/entry/vdso/vgetrandom.c
new file mode 100644
index 000000000000..52d3c7faae2e
--- /dev/null
+++ b/arch/x86/entry/vdso/vgetrandom.c
@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+#include <linux/types.h>
+
+#include "../../../../lib/vdso/getrandom.c"
+
+ssize_t __vdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state, size_t opaque_len);
+
+ssize_t __vdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state, size_t opaque_len)
+{
+	return __cvdso_getrandom(buffer, len, flags, opaque_state, opaque_len);
+}
+
+ssize_t getrandom(void *, size_t, unsigned int, void *, size_t)
+	__attribute__((weak, alias("__vdso_getrandom")));
diff --git a/arch/x86/include/asm/vdso/getrandom.h b/arch/x86/include/asm/vdso/getrandom.h
new file mode 100644
index 000000000000..b96e674cafde
--- /dev/null
+++ b/arch/x86/include/asm/vdso/getrandom.h
@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+#ifndef __ASM_VDSO_GETRANDOM_H
+#define __ASM_VDSO_GETRANDOM_H
+
+#ifndef __ASSEMBLY__
+
+#include <asm/unistd.h>
+#include <asm/vvar.h>
+
+/**
+ * getrandom_syscall - Invoke the getrandom() syscall.
+ * @buffer:	Destination buffer to fill with random bytes.
+ * @len:	Size of @buffer in bytes.
+ * @flags:	Zero or more GRND_* flags.
+ * Returns:	The number of random bytes written to @buffer, or a negative value indicating an error.
+ */
+static __always_inline ssize_t getrandom_syscall(void *buffer, size_t len, unsigned int flags)
+{
+	long ret;
+
+	asm ("syscall" : "=a" (ret) :
+	     "0" (__NR_getrandom), "D" (buffer), "S" (len), "d" (flags) :
+	     "rcx", "r11", "memory");
+
+	return ret;
+}
+
+#define __vdso_rng_data (VVAR(_vdso_rng_data))
+
+static __always_inline const struct vdso_rng_data *__arch_get_vdso_rng_data(void)
+{
+	if (IS_ENABLED(CONFIG_TIME_NS) && __vdso_data->clock_mode == VDSO_CLOCKMODE_TIMENS)
+		return (void *)&__vdso_rng_data + ((void *)&__timens_vdso_data - (void *)&__vdso_data);
+	return &__vdso_rng_data;
+}
+
+/**
+ * __arch_chacha20_blocks_nostack - Generate ChaCha20 stream without using the stack.
+ * @dst_bytes:	Destination buffer to hold @nblocks * 64 bytes of output.
+ * @key:	32-byte input key.
+ * @counter:	8-byte counter, read on input and updated on return.
+ * @nblocks:	Number of blocks to generate.
+ *
+ * Generates a given positive number of blocks of ChaCha20 output with nonce=0, and does not write
+ * to any stack or memory outside of the parameters passed to it, in order to mitigate stack data
+ * leaking into forked child processes.
+ */
+extern void __arch_chacha20_blocks_nostack(u8 *dst_bytes, const u32 *key, u32 *counter, size_t nblocks);
+
+#endif /* !__ASSEMBLY__ */
+
+#endif /* __ASM_VDSO_GETRANDOM_H */
diff --git a/arch/x86/include/asm/vdso/vsyscall.h b/arch/x86/include/asm/vdso/vsyscall.h
index be199a9b2676..71c56586a22f 100644
--- a/arch/x86/include/asm/vdso/vsyscall.h
+++ b/arch/x86/include/asm/vdso/vsyscall.h
@@ -11,6 +11,8 @@
 #include <asm/vvar.h>
 
 DEFINE_VVAR(struct vdso_data, _vdso_data);
+DEFINE_VVAR_SINGLE(struct vdso_rng_data, _vdso_rng_data);
+
 /*
  * Update the vDSO data page to keep in sync with kernel timekeeping.
  */
diff --git a/arch/x86/include/asm/vvar.h b/arch/x86/include/asm/vvar.h
index 183e98e49ab9..9d9af37f7cab 100644
--- a/arch/x86/include/asm/vvar.h
+++ b/arch/x86/include/asm/vvar.h
@@ -26,6 +26,8 @@
  */
 #define DECLARE_VVAR(offset, type, name) \
 	EMIT_VVAR(name, offset)
+#define DECLARE_VVAR_SINGLE(offset, type, name) \
+	EMIT_VVAR(name, offset)
 
 #else
 
@@ -37,6 +39,10 @@ extern char __vvar_page;
 	extern type timens_ ## name[CS_BASES]				\
 	__attribute__((visibility("hidden")));				\
 
+#define DECLARE_VVAR_SINGLE(offset, type, name)				\
+	extern type vvar_ ## name					\
+	__attribute__((visibility("hidden")));				\
+
 #define VVAR(name) (vvar_ ## name)
 #define TIMENS(name) (timens_ ## name)
 
@@ -44,12 +50,22 @@ extern char __vvar_page;
 	type name[CS_BASES]						\
 	__attribute__((section(".vvar_" #name), aligned(16))) __visible
 
+#define DEFINE_VVAR_SINGLE(type, name)					\
+	type name							\
+	__attribute__((section(".vvar_" #name), aligned(16))) __visible
+
 #endif
 
 /* DECLARE_VVAR(offset, type, name) */
 
 DECLARE_VVAR(128, struct vdso_data, _vdso_data)
 
+#if !defined(_SINGLE_DATA)
+#define _SINGLE_DATA
+DECLARE_VVAR_SINGLE(640, struct vdso_rng_data, _vdso_rng_data)
+#endif
+
 #undef DECLARE_VVAR
+#undef DECLARE_VVAR_SINGLE
 
 #endif
diff --git a/tools/testing/selftests/vDSO/.gitignore b/tools/testing/selftests/vDSO/.gitignore
index 7dbfdec53f3d..30d5c8f0e5c7 100644
--- a/tools/testing/selftests/vDSO/.gitignore
+++ b/tools/testing/selftests/vDSO/.gitignore
@@ -7,3 +7,4 @@ vdso_test_gettimeofday
 vdso_test_getcpu
 vdso_standalone_test_x86
 vdso_test_getrandom
+vdso_test_chacha
diff --git a/tools/testing/selftests/vDSO/Makefile b/tools/testing/selftests/vDSO/Makefile
index a33b4d200a32..7d59b7bc6ace 100644
--- a/tools/testing/selftests/vDSO/Makefile
+++ b/tools/testing/selftests/vDSO/Makefile
@@ -3,6 +3,7 @@ include ../lib.mk
 
 uname_M := $(shell uname -m 2>/dev/null || echo not)
 ARCH ?= $(shell echo $(uname_M) | sed -e s/i.86/x86/ -e s/x86_64/x86/)
+SODIUM := $(shell pkg-config --libs libsodium 2>/dev/null)
 
 TEST_GEN_PROGS := $(OUTPUT)/vdso_test_gettimeofday $(OUTPUT)/vdso_test_getcpu
 TEST_GEN_PROGS += $(OUTPUT)/vdso_test_abi
@@ -12,9 +13,17 @@ TEST_GEN_PROGS += $(OUTPUT)/vdso_standalone_test_x86
 endif
 TEST_GEN_PROGS += $(OUTPUT)/vdso_test_correctness
 TEST_GEN_PROGS += $(OUTPUT)/vdso_test_getrandom
+ifeq ($(uname_M),x86_64)
+ifneq ($(SODIUM),)
+TEST_GEN_PROGS += $(OUTPUT)/vdso_test_chacha
+endif
+endif
 
 CFLAGS := -std=gnu99
 CFLAGS_vdso_standalone_test_x86 := -nostdlib -fno-asynchronous-unwind-tables -fno-stack-protector
+CFLAGS_vdso_test_chacha := $(SODIUM) -idirafter $(top_srcdir)/include -idirafter \
+			   $(top_srcdir)/arch/$(ARCH)/include -idirafter include \
+			   -D__ASSEMBLY__ -DBULID_VDSO -DCONFIG_FUNCTION_ALIGNMENT=0 -Wa,--noexecstack
 LDFLAGS_vdso_test_correctness := -ldl
 ifeq ($(CONFIG_X86_32),y)
 LDLIBS += -lgcc_s
@@ -35,3 +44,9 @@ $(OUTPUT)/vdso_test_correctness: vdso_test_correctness.c
 		-o $@ \
 		$(LDFLAGS_vdso_test_correctness)
 $(OUTPUT)/vdso_test_getrandom: parse_vdso.c
+$(OUTPUT)/vdso_test_chacha: CFLAGS += $(CFLAGS_vdso_test_chacha)
+$(OUTPUT)/vdso_test_chacha: $(top_srcdir)/arch/$(ARCH)/entry/vdso/vgetrandom-chacha.S
+$(OUTPUT)/vdso_test_chacha: include/asm/rwonce.h
+include/asm/rwonce.h:
+	mkdir -p include/asm
+	touch $@
diff --git a/tools/testing/selftests/vDSO/vdso_test_chacha.c b/tools/testing/selftests/vDSO/vdso_test_chacha.c
new file mode 100644
index 000000000000..e38f44e5f803
--- /dev/null
+++ b/tools/testing/selftests/vDSO/vdso_test_chacha.c
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+#include <sodium/crypto_stream_chacha20.h>
+#include <sys/random.h>
+#include <string.h>
+#include <stdint.h>
+#include "../kselftest.h"
+
+extern void __arch_chacha20_blocks_nostack(uint8_t *dst_bytes, const uint8_t *key, uint32_t *counter, size_t nblocks);
+
+int main(int argc, char *argv[])
+{
+	enum { TRIALS = 1000, BLOCKS = 128, BLOCK_SIZE = 64 };
+	static const uint8_t nonce[8] = { 0 };
+	uint32_t counter[2];
+	uint8_t key[32];
+	uint8_t output1[BLOCK_SIZE * BLOCKS], output2[BLOCK_SIZE * BLOCKS];
+
+	ksft_print_header();
+	ksft_set_plan(1);
+
+	for (unsigned int trial = 0; trial < TRIALS; ++trial) {
+		if (getrandom(key, sizeof(key), 0) != sizeof(key)) {
+			printf("getrandom() failed!\n");
+			return KSFT_SKIP;
+		}
+		crypto_stream_chacha20(output1, sizeof(output1), nonce, key);
+		for (unsigned int split = 0; split < BLOCKS; ++split) {
+			memset(output2, 'X', sizeof(output2));
+			memset(counter, 0, sizeof(counter));
+			if (split)
+				__arch_chacha20_blocks_nostack(output2, key, counter, split);
+			__arch_chacha20_blocks_nostack(output2 + split * BLOCK_SIZE, key, counter, BLOCKS - split);
+			if (memcmp(output1, output2, sizeof(output1)))
+				return KSFT_FAIL;
+		}
+	}
+	ksft_test_result_pass("chacha: PASS\n");
+	return KSFT_PASS;
+}
-- 
2.45.2

Re: [PATCH v18 2/5] random: add vgetrandom_alloc() syscall

[Aleksa Sarai]

[-- Attachment #1: Type: text/plain, Size: 15752 bytes --]

On 2024-06-20, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> The vDSO getrandom() works over an opaque per-thread state of an
> unexported size, which must be marked VM_WIPEONFORK, VM_DONTDUMP,
> VM_NORESERVE, and VM_DROPPABLE for proper operation. Over time, the
> nuances of these allocations may change or grow or even differ based on
> architectural features.
> 
> The syscall has the signature:
> 
>   void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each,
>                          unsigned long addr, unsigned int flags);
> 
> This takes a hinted number of opaque states in `num`, and returns a
> pointer to an array of opaque states, the number actually allocated back
> in `num`, and the size in bytes of each one in `size_per_each`, enabling
> a libc to slice up the returned array into a state per each thread,
> while ensuring that no single state straddles a page boundary. (The
> `flags` and `addr` arguments, as well as the `*size_per_each` input
> value, are reserved for the future and are forced to be zero zero for
> now.)

Given how many flags are going to be reserved at the outset, what about
using an extensible struct (copy_struct_from_user) instead? If you're
absolutely sure you'll never need more arguments that's fine, but it
seems entirely possible to me that you might need an extra argument in a
few years.

Since you need to write to *num in the current syscall, I suspect the
following would be nicer as well.

   struct vgetrandom_args {
           u64 num;
   }

   void *vgetrandom_alloc(struct vgetrandom_args *arg, size_t size);

If you'd prefer to have flags from the outset (even though you could
extend them later without issues), then

   struct vgetrandom_args {
           u64 flags;
           u64 num;
   }

would also work.

Then again, I guess since libc is planned to be the primary user,
creating a new syscall in a decade if necessary is probably not that big
of an issue.

> Libc is expected to allocate a chunk of these on first use, and then
> dole them out to threads as they're created, allocating more when
> needed. The returned address of the first state may be passed to
> munmap(2) with a length of `DIV_ROUND_UP(num, PAGE_SIZE / size_per_each)
> * PAGE_SIZE`, in order to deallocate the memory.
> 
> We very intentionally do *not* leave state allocation for vDSO
> getrandom() up to userspace itself, but rather provide this new syscall
> for such allocations. vDSO getrandom() must not store its state in just
> any old memory address, but rather just ones that the kernel specially
> allocates for it, leaving the particularities of those allocations up to
> the kernel.
> 
> The allocation of states is intended to be integrated into libc's thread
> management. As an illustrative example, the following code might be used
> to do the same outside of libc. Though, vgetrandom_alloc() is not
> expected to be exposed outside of libc, and the pthread usage here is
> expected to be elided into libc internals. This allocation scheme is
> very naive and does not shrink; other implementations may choose to be
> more complex.
> 
>   static void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each)
>   {
>     *size_per_each = 0; /* Must be zero on input. */
>     return (void *)syscall(__NR_vgetrandom_alloc, &num, &size_per_each,
>                            0 /* reserved @addr */, 0 /* reserved @flags */);
>   }
> 
>   static struct {
>     pthread_mutex_t lock;
>     void **states;
>     size_t len, cap, size_per_each;
>   } grnd_allocator = {
>     .lock = PTHREAD_MUTEX_INITIALIZER
>   };
> 
>   static void *vgetrandom_get_state(void)
>   {
>     void *state = NULL;
> 
>     pthread_mutex_lock(&grnd_allocator.lock);
>     if (!grnd_allocator.len) {
>       size_t new_cap;
>       size_t page_size = getpagesize();
>       unsigned int num = sysconf(_SC_NPROCESSORS_ONLN); /* Could be arbitrary, just a hint. */
>       unsigned int size_per_each;
>       void *new_block = vgetrandom_alloc(&num, &size_per_each);
>       void *new_states;
> 
>       if (new_block == MAP_FAILED)
>         goto out;
>       if (grnd_allocator.size_per_each && grnd_allocator.size_per_each != size_per_each)
>         goto unmap;
>       grnd_allocator.size_per_each = size_per_each;
>       new_cap = grnd_allocator.cap + num;
>       new_states = reallocarray(grnd_allocator.states, new_cap, sizeof(*grnd_allocator.states));
>       if (!new_states)
>         goto unmap;
>       grnd_allocator.cap = new_cap;
>       grnd_allocator.states = new_states;
> 
>       for (size_t i = 0; i < num; ++i) {
>         grnd_allocator.states[i] = new_block;
>         if (((uintptr_t)new_block & (page_size - 1)) + size_per_each > page_size)
>           new_block = (void *)(((uintptr_t)new_block + page_size) & (page_size - 1));
>         else
>           new_block += size_per_each;
>       }
>       grnd_allocator.len = num;
>       goto success;
> 
>     unmap:
>       munmap(new_block, DIV_ROUND_UP(num, page_size / size_per_each) * page_size);
>       goto out;
>     }
>   success:
>     state = grnd_allocator.states[--grnd_allocator.len];
> 
>   out:
>     pthread_mutex_unlock(&grnd_allocator.lock);
>     return state;
>   }
> 
>   static void vgetrandom_put_state(void *state)
>   {
>     if (!state)
>       return;
>     pthread_mutex_lock(&grnd_allocator.lock);
>     grnd_allocator.states[grnd_allocator.len++] = state;
>     pthread_mutex_unlock(&grnd_allocator.lock);
>   }
> 
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  MAINTAINERS              |   1 +
>  drivers/char/random.c    | 135 ++++++++++++++++++++++++++++++++++++++-
>  include/linux/syscalls.h |   3 +
>  include/vdso/getrandom.h |  16 +++++
>  kernel/sys_ni.c          |   3 +
>  lib/vdso/Kconfig         |   6 ++
>  6 files changed, 163 insertions(+), 1 deletion(-)
>  create mode 100644 include/vdso/getrandom.h
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 8aa17e515ef3..8480c4c39915 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -18747,6 +18747,7 @@ T:	git https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
>  F:	Documentation/devicetree/bindings/rng/microsoft,vmgenid.yaml
>  F:	drivers/char/random.c
>  F:	drivers/virt/vmgenid.c
> +F:	include/vdso/getrandom.h
>  
>  RAPIDIO SUBSYSTEM
>  M:	Matt Porter <mporter@kernel.crashing.org>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 2597cb43f438..ccb35f390c85 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1,6 +1,6 @@
>  // SPDX-License-Identifier: (GPL-2.0 OR BSD-3-Clause)
>  /*
> - * Copyright (C) 2017-2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
> + * Copyright (C) 2017-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
>   * Copyright Matt Mackall <mpm@selenic.com>, 2003, 2004, 2005
>   * Copyright Theodore Ts'o, 1994, 1995, 1996, 1997, 1998, 1999. All rights reserved.
>   *
> @@ -8,6 +8,7 @@
>   * into roughly six sections, each with a section header:
>   *
>   *   - Initialization and readiness waiting.
> + *   - vDSO support helpers.
>   *   - Fast key erasure RNG, the "crng".
>   *   - Entropy accumulation and extraction routines.
>   *   - Entropy collection routines.
> @@ -39,6 +40,7 @@
>  #include <linux/blkdev.h>
>  #include <linux/interrupt.h>
>  #include <linux/mm.h>
> +#include <linux/mman.h>
>  #include <linux/nodemask.h>
>  #include <linux/spinlock.h>
>  #include <linux/kthread.h>
> @@ -56,6 +58,9 @@
>  #include <linux/sched/isolation.h>
>  #include <crypto/chacha.h>
>  #include <crypto/blake2s.h>
> +#ifdef CONFIG_VDSO_GETRANDOM
> +#include <vdso/getrandom.h>
> +#endif
>  #include <asm/archrandom.h>
>  #include <asm/processor.h>
>  #include <asm/irq.h>
> @@ -169,6 +174,134 @@ int __cold execute_with_initialized_rng(struct notifier_block *nb)
>  				__func__, (void *)_RET_IP_, crng_init)
>  
>  
> +
> +/********************************************************************
> + *
> + * vDSO support helpers.
> + *
> + * The actual vDSO function is defined over in lib/vdso/getrandom.c,
> + * but this section contains the kernel-mode helpers to support that.
> + *
> + ********************************************************************/
> +
> +#ifdef CONFIG_VDSO_GETRANDOM
> +/**
> + * sys_vgetrandom_alloc - Allocate opaque states for use with vDSO getrandom().
> + *
> + * @num:	   On input, a pointer to a suggested hint of how many states to
> + * 		   allocate, and on return the number of states actually allocated.
> + *
> + * @size_per_each: On input, must be zero. On return, the size of each state allocated,
> + * 		   so that the caller can split up the returned allocation into
> + * 		   individual states.
> + *
> + * @addr:	   Reserved, must be zero.
> + *
> + * @flags:	   Reserved, must be zero.
> + *
> + * The getrandom() vDSO function in userspace requires an opaque state, which
> + * this function allocates by mapping a certain number of special pages into
> + * the calling process. It takes a hint as to the number of opaque states
> + * desired, and provides the caller with the number of opaque states actually
> + * allocated, the size of each one in bytes, and the address of the first
> + * state, which may be split up into @num states of @size_per_each bytes each,
> + * by adding @size_per_each to the returned first state @num times, while
> + * ensuring that no single state straddles a page boundary.
> + *
> + * Returns the address of the first state in the allocation on success, or a
> + * negative error value on failure.
> + *
> + * The returned address of the first state may be passed to munmap(2) with a
> + * length of `DIV_ROUND_UP(num, PAGE_SIZE / size_per_each) * PAGE_SIZE`, in
> + * order to deallocate the memory, after which it is invalid to pass it to vDSO
> + * getrandom().
> + *
> + * States allocated by this function must not be dereferenced, written, read,
> + * or otherwise manipulated. The *only* supported operations are:
> + *   - Splitting up the states in intervals of @size_per_each, no more than
> + *     @num times from the first state, while ensuring that no single state
> + *     straddles a page boundary.
> + *   - Passing a state to the getrandom() vDSO function's @opaque_state
> + *     parameter, but not passing the same state at the same time to two such
> + *     calls.
> + *   - Passing the first state and the total length to munmap(2), as described
> + *     above.
> + * All other uses are undefined behavior, which is subject to change or removal.
> + */
> +SYSCALL_DEFINE4(vgetrandom_alloc, unsigned int __user *, num,
> +		unsigned int __user *, size_per_each, unsigned long, addr,
> +		unsigned int, flags)
> +{
> +	size_t state_size, alloc_size, num_states;
> +	unsigned long pages_addr, populate;
> +	unsigned int num_hint;
> +	vm_flags_t vm_flags;
> +	int ret;
> +
> +	/*
> +	 * @flags and @addr are currently unused, so in order to reserve them
> +	 * for the future, force them to be set to zero by current callers.
> +	 */
> +	if (flags || addr)
> +		return -EINVAL;
> +
> +	/*
> +	 * Also enforce that *size_per_each is zero on input, in case this becomes
> +	 * useful later on.
> +	 */
> +	if (get_user(num_hint, size_per_each))
> +		return -EFAULT;
> +	if (num_hint)
> +		return -EINVAL;
> +
> +	if (get_user(num_hint, num))
> +		return -EFAULT;
> +
> +	state_size = sizeof(struct vgetrandom_state);
> +	num_states = clamp_t(size_t, num_hint, 1, (SIZE_MAX & PAGE_MASK) / state_size);
> +	alloc_size = PAGE_ALIGN(num_states * state_size);
> +	/*
> +	 * States cannot straddle page boundaries, so calculate the number of
> +	 * states that can fit inside of a page without being split, and then
> +	 * multiply that out by the number of pages allocated.
> +	 */
> +	num_states = (PAGE_SIZE / state_size) * (alloc_size / PAGE_SIZE);
> +
> +	vm_flags =
> +		/*
> +		 * Don't allow state to be written to swap, to preserve forward secrecy.
> +		 * But also don't mlock it or pre-reserve it, and allow it to
> +		 * be discarded under memory pressure. If no memory is available, returns
> +		 * zeros rather than segfaulting.
> +		 */
> +		VM_DROPPABLE | VM_NORESERVE |
> +
> +		/* Don't allow the state to survive forks, to prevent random number re-use. */
> +		VM_WIPEONFORK |
> +
> +		/* Don't write random state into coredumps. */
> +		VM_DONTDUMP;
> +
> +	if (mmap_write_lock_killable(current->mm))
> +		return -EINTR;
> +	pages_addr = do_mmap(NULL, 0, alloc_size, PROT_READ | PROT_WRITE,
> +			     MAP_PRIVATE | MAP_ANONYMOUS, vm_flags, 0, &populate, NULL);
> +	mmap_write_unlock(current->mm);
> +	if (IS_ERR_VALUE(pages_addr))
> +		return pages_addr;
> +
> +	ret = -EFAULT;
> +	if (put_user(num_states, num) || put_user(state_size, size_per_each))
> +		goto err_unmap;
> +
> +	return pages_addr;
> +
> +err_unmap:
> +	vm_munmap(pages_addr, alloc_size);
> +	return ret;
> +}
> +#endif
> +
>  /*********************************************************************
>   *
>   * Fast key erasure RNG, the "crng".
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 9104952d323d..56368ea4f510 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -906,6 +906,9 @@ asmlinkage long sys_seccomp(unsigned int op, unsigned int flags,
>  			    void __user *uargs);
>  asmlinkage long sys_getrandom(char __user *buf, size_t count,
>  			      unsigned int flags);
> +asmlinkage long sys_vgetrandom_alloc(unsigned int __user *num,
> +				     unsigned int __user *size_per_each,
> +				     unsigned long addr, unsigned int flags);
>  asmlinkage long sys_memfd_create(const char __user *uname_ptr, unsigned int flags);
>  asmlinkage long sys_bpf(int cmd, union bpf_attr *attr, unsigned int size);
>  asmlinkage long sys_execveat(int dfd, const char __user *filename,
> diff --git a/include/vdso/getrandom.h b/include/vdso/getrandom.h
> new file mode 100644
> index 000000000000..69037519d20b
> --- /dev/null
> +++ b/include/vdso/getrandom.h
> @@ -0,0 +1,16 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
> + */
> +
> +#ifndef _VDSO_GETRANDOM_H
> +#define _VDSO_GETRANDOM_H
> +
> +/**
> + * struct vgetrandom_state - State used by vDSO getrandom() and allocated by vgetrandom_alloc().
> + *
> + * Currently empty, as the vDSO getrandom() function has not yet been implemented.
> + */
> +struct vgetrandom_state { int placeholder; };
> +
> +#endif /* _VDSO_GETRANDOM_H */
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index d7eee421d4bc..6b17fadb0f59 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -272,6 +272,9 @@ COND_SYSCALL(pkey_free);
>  /* memfd_secret */
>  COND_SYSCALL(memfd_secret);
>  
> +/* random */
> +COND_SYSCALL(vgetrandom_alloc);
> +
>  /*
>   * Architecture specific weak syscall entries.
>   */
> diff --git a/lib/vdso/Kconfig b/lib/vdso/Kconfig
> index c46c2300517c..99661b731834 100644
> --- a/lib/vdso/Kconfig
> +++ b/lib/vdso/Kconfig
> @@ -38,3 +38,9 @@ config GENERIC_VDSO_OVERFLOW_PROTECT
>  	  in the hotpath.
>  
>  endif
> +
> +config VDSO_GETRANDOM
> +	bool
> +	select NEED_VM_DROPPABLE
> +	help
> +	  Selected by architectures that support vDSO getrandom().
> -- 
> 2.45.2
> 
> 

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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Re: [PATCH v18 2/5] random: add vgetrandom_alloc() syscall

[Jason A. Donenfeld]

Hey Aleksa,

On Wed, Jun 19, 2024 at 07:13:26PM -0700, Aleksa Sarai wrote:
> Then again, I guess since libc is planned to be the primary user,
> creating a new syscall in a decade if necessary is probably not that big
> of an issue.

I'm not sure going the whole big struct thing is really necessary, and
for an additional reason: this is only meant to be used with the vDSO
function, which is also coupled with the kernel. It doesn't return
information that's made to be used (or allowed to be used) anywhere
else. So both the vdso code and the syscall code are part of the same
basic thing that will evolve together. So I'm not convinced extensible
struct really makes sense for this, as neat as it is.

If there's wide consensus that it's desirable, in contrast to what I'm
saying, I'm not vehemently opposed to it and could do it, but it just
seems like massive overkill and not at all necessary. Things are
intentionally as simple and straightforward as can be.

Jason

Re: [PATCH v18 2/5] random: add vgetrandom_alloc() syscall

[Thomas Gleixner]

Jason!

On Thu, Jun 20 2024 at 14:18, Jason A. Donenfeld wrote:
> On Wed, Jun 19, 2024 at 07:13:26PM -0700, Aleksa Sarai wrote:
>> Then again, I guess since libc is planned to be the primary user,
>> creating a new syscall in a decade if necessary is probably not that big
>> of an issue.
>
> I'm not sure going the whole big struct thing is really necessary, and
> for an additional reason: this is only meant to be used with the vDSO
> function, which is also coupled with the kernel. It doesn't return
> information that's made to be used (or allowed to be used) anywhere
> else. So both the vdso code and the syscall code are part of the same
> basic thing that will evolve together. So I'm not convinced extensible
> struct really makes sense for this, as neat as it is.
>
> If there's wide consensus that it's desirable, in contrast to what I'm
> saying, I'm not vehemently opposed to it and could do it, but it just
> seems like massive overkill and not at all necessary. Things are
> intentionally as simple and straightforward as can be.

Right, but the problem is that this is a syscall, so people are free to
explore it even without the vdso part. Now when you want to change it
later then you are caught in the no-regression trap.

So making it extensible with backwards compability in place (add the
unused flag field and check for 0) will allow you to expand without
breaking users.

Thanks,

        tglx

Re: [PATCH v18 4/5] random: introduce generic vDSO getrandom() implementation

[Thomas Gleixner]

On Thu, Jun 20 2024 at 02:53, Jason A. Donenfeld wrote:
>     return grnd_ctx.fn(buf, len, flags, state, grnd_allocator.size_per_each);
>   }
>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Re: [PATCH v18 5/5] x86: vdso: Wire up getrandom() vDSO implementation

[Thomas Gleixner]

On Thu, Jun 20 2024 at 02:53, Jason A. Donenfeld wrote:
> Hook up the generic vDSO implementation to the x86 vDSO data page. Since
> the existing vDSO infrastructure is heavily based on the timekeeping
> functionality, which works over arrays of bases, a new macro is
> introduced for vvars that are not arrays.
>
> The vDSO function requires a ChaCha20 implementation that does not write
> to the stack, yet can still do an entire ChaCha20 permutation, so
> provide this using SSE2, since this is userland code that must work on
> all x86-64 processors. There's a simple test for this code as well.
>
> Reviewed-by: Samuel Neves <sneves@dei.uc.pt> # for vgetrandom-chacha.S
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Re: [PATCH v18 2/5] random: add vgetrandom_alloc() syscall

[Jason A. Donenfeld]

On Fri, Jun 28, 2024 at 03:56:12PM +0200, Thomas Gleixner wrote:
> Jason!
> 
> On Thu, Jun 20 2024 at 14:18, Jason A. Donenfeld wrote:
> > On Wed, Jun 19, 2024 at 07:13:26PM -0700, Aleksa Sarai wrote:
> >> Then again, I guess since libc is planned to be the primary user,
> >> creating a new syscall in a decade if necessary is probably not that big
> >> of an issue.
> >
> > I'm not sure going the whole big struct thing is really necessary, and
> > for an additional reason: this is only meant to be used with the vDSO
> > function, which is also coupled with the kernel. It doesn't return
> > information that's made to be used (or allowed to be used) anywhere
> > else. So both the vdso code and the syscall code are part of the same
> > basic thing that will evolve together. So I'm not convinced extensible
> > struct really makes sense for this, as neat as it is.
> >
> > If there's wide consensus that it's desirable, in contrast to what I'm
> > saying, I'm not vehemently opposed to it and could do it, but it just
> > seems like massive overkill and not at all necessary. Things are
> > intentionally as simple and straightforward as can be.
> 
> Right, but the problem is that this is a syscall, so people are free to
> explore it even without the vdso part. Now when you want to change it
> later then you are caught in the no-regression trap.
> 
> So making it extensible with backwards compability in place (add the
> unused flag field and check for 0) will allow you to expand without
> breaking users.

Okay, so it sounds like you're also in camp-struct. I guess let's do it
then. This opens up a few questions, but I think we can get them sorted.
Right now this version of the patch has this signature:

  void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each,
                         unsigned long addr, unsigned int flags);

The semantics are currently:

- [in] unsigned int num - desired number of states
- [in] unsigned long addr - reserved, nothing
- [in] unsigned int flags - reserved, nothing
- [out] unsigned int num - actual number of states
- [out] unsigned int size_per_each - size of each state
- [out] void* return value - the allocated thing

Following Aleksa's suggestion, we keep the `[out] void* return value` as
a return value, but move all the other into a struct:

    void *vgetrandom_alloc(struct vgetrandom_args *arg, size_t size);

So now the struct can become:

    struct vgetrandom_args {
        [in] u64 flags;
        [in/out] u32 num;
        [out] u32 size_per_each;
    }

Alternatively, this now opens the possibility to incorporate Eric's
suggestion of also returning the number of allocated bytes, which is
perhaps definitely to deal with, but I didn't do because I wanted
symmetry in the argument list. So doing that, now we have:

    struct vgetrandom_args {
        [in] u64 flags;
        [in/out] u32 num;
        [out] u32 size_per_each;
        [out] u64 bytes_allocated;
    }

Does that seem reasonable? There's a little bit of mixing of ins and
outs within the struct, and the return value is still a return value,
rather than a `[out] void *ret` inside of the struct. But maybe that's
fine. Also I used u32 there for the two smaller arguments, but maybe
that's silly and we should go straight to u64?

Anyway, how does that look to you?

Jason

Re: [PATCH v18 2/5] random: add vgetrandom_alloc() syscall

[Jason A. Donenfeld]

On Fri, Jun 28, 2024 at 4:09 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> perhaps definitely to deal with,

What?! I meant "definitely easier to deal with".

Re: [PATCH v18 2/5] random: add vgetrandom_alloc() syscall

[Jason A. Donenfeld]

On Fri, Jun 28, 2024 at 04:09:01PM +0200, Jason A. Donenfeld wrote:
> fine. Also I used u32 there for the two smaller arguments, but maybe
> that's silly and we should go straight to u64?

Judging by `struct clone_args`, it looks like I've got to use
__aligned_u64 for every argument:

    struct clone_args {
        __aligned_u64 flags;
        __aligned_u64 pidfd;
        __aligned_u64 child_tid;
        __aligned_u64 parent_tid;
        __aligned_u64 exit_signal;
        __aligned_u64 stack;
        __aligned_u64 stack_size;
        __aligned_u64 tls;
        __aligned_u64 set_tid;
        __aligned_u64 set_tid_size;
        __aligned_u64 cgroup;
    };
    #define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */
    #define CLONE_ARGS_SIZE_VER1 80 /* sizeof second published struct */
    #define CLONE_ARGS_SIZE_VER2 88 /* sizeof third published struct */
    
So okay, I'll do that, and will have an ARGS_SIZE_VER0 macro too.

Jason

Re: [PATCH v18 2/5] random: add vgetrandom_alloc() syscall

[Jason A. Donenfeld]

On Mon, Jul 1, 2024 at 1:53 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> On Fri, Jun 28, 2024 at 04:09:01PM +0200, Jason A. Donenfeld wrote:
> > fine. Also I used u32 there for the two smaller arguments, but maybe
> > that's silly and we should go straight to u64?
>
> Judging by `struct clone_args`, it looks like I've got to use
> __aligned_u64 for every argument:
>
>     struct clone_args {
>         __aligned_u64 flags;
>         __aligned_u64 pidfd;
>         __aligned_u64 child_tid;
>         __aligned_u64 parent_tid;
>         __aligned_u64 exit_signal;
>         __aligned_u64 stack;
>         __aligned_u64 stack_size;
>         __aligned_u64 tls;
>         __aligned_u64 set_tid;
>         __aligned_u64 set_tid_size;
>         __aligned_u64 cgroup;
>     };
>     #define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */
>     #define CLONE_ARGS_SIZE_VER1 80 /* sizeof second published struct */
>     #define CLONE_ARGS_SIZE_VER2 88 /* sizeof third published struct */
>
> So okay, I'll do that, and will have an ARGS_SIZE_VER0 macro too.

This is now covered by v19 of this patchset:
https://lore.kernel.org/lkml/20240701135801.3698-1-Jason@zx2c4.com/