* [PATCH v15 0/5] implement getrandom() in vDSO
@ 2024-05-21 11:18 Jason A. Donenfeld
2024-05-21 11:18 ` [PATCH v15 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings Jason A. Donenfeld
` (4 more replies)
0 siblings, 5 replies; 11+ messages in thread
From: Jason A. Donenfeld @ 2024-05-21 11:18 UTC (permalink / raw)
To: linux-kernel, patches, tglx
Cc: Jason A. Donenfeld, linux-crypto, linux-api, x86,
Greg Kroah-Hartman, Adhemerval Zanella Netto, Carlos O'Donell,
Florian Weimer, Arnd Bergmann, Jann Horn, Christian Brauner
Changes v14->v15:
----------------
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 8 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.
--------------
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
principal, 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>
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/unistd.h | 2 +-
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 | 143 +++++++++
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 | 12 +
include/vdso/getrandom.h | 44 +++
include/vdso/types.h | 35 +++
kernel/sys_ni.c | 3 +
lib/vdso/Kconfig | 6 +
lib/vdso/getrandom.c | 226 ++++++++++++++
mm/Kconfig | 3 +
mm/memory.c | 4 +
mm/mempolicy.c | 3 +
mm/mprotect.c | 2 +-
mm/rmap.c | 5 +-
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 | 11 +
.../testing/selftests/vDSO/vdso_test_chacha.c | 43 +++
.../selftests/vDSO/vdso_test_getrandom.c | 283 ++++++++++++++++++
52 files changed, 1148 insertions(+), 7 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 include/vdso/types.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
--
2.44.0
^ permalink raw reply [flat|nested] 11+ messages in thread
* [PATCH v15 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings
2024-05-21 11:18 [PATCH v15 0/5] implement getrandom() in vDSO Jason A. Donenfeld
@ 2024-05-21 11:18 ` Jason A. Donenfeld
2024-05-27 13:34 ` David Hildenbrand
2024-05-21 11:18 ` [PATCH v15 2/5] random: add vgetrandom_alloc() syscall Jason A. Donenfeld
` (3 subsequent siblings)
4 siblings, 1 reply; 11+ messages in thread
From: Jason A. Donenfeld @ 2024-05-21 11:18 UTC (permalink / raw)
To: linux-kernel, patches, tglx; +Cc: Jason A. Donenfeld, linux-mm
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 shouldn't reserve actual memory, but it also shouldn't crash when
page faulting in memory if none is available
* Uh-oh: MAP_NORESERVE respects vm.overcommit_memory=2.
* Uh-oh: VM_NORESERVE means segfaults.
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) If there's not enough memory to service a page fault, it's not fatal.
d) It is inherited by fork.
e) 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 OOMing, crashing on overcommitment,
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/memory.c | 4 ++++
mm/mempolicy.c | 3 +++
mm/mprotect.c | 2 +-
mm/rmap.c | 5 +++--
8 files changed, 32 insertions(+), 3 deletions(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index e5a5f015ff03..b5a59e57bde1 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/memory.c b/mm/memory.c
index b5453b86ec4b..57b03fc73159 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5689,6 +5689,10 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
lru_gen_exit_fault();
+ /* If the mapping is droppable, then errors due to OOM aren't fatal. */
+ if (vma->vm_flags & VM_DROPPABLE)
+ ret &= ~VM_FAULT_OOM;
+
if (flags & FAULT_FLAG_USER) {
mem_cgroup_exit_user_fault();
/*
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index aec756ae5637..a66289f1d931 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2300,6 +2300,9 @@ struct folio *vma_alloc_folio_noprof(gfp_t gfp, int order, struct vm_area_struct
pgoff_t ilx;
struct page *page;
+ if (vma->vm_flags & VM_DROPPABLE)
+ gfp |= __GFP_NOWARN | __GFP_NORETRY;
+
pol = get_vma_policy(vma, addr, order, &ilx);
page = alloc_pages_mpol_noprof(gfp | __GFP_COMP, order,
pol, ilx, numa_node_id());
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 94878c39ee32..88ff3ecc08a1 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -622,7 +622,7 @@ 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..f23b9f796aae 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;
}
--
2.44.0
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [PATCH v15 2/5] random: add vgetrandom_alloc() syscall
2024-05-21 11:18 [PATCH v15 0/5] implement getrandom() in vDSO Jason A. Donenfeld
2024-05-21 11:18 ` [PATCH v15 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings Jason A. Donenfeld
@ 2024-05-21 11:18 ` Jason A. Donenfeld
2024-05-21 11:48 ` Florian Weimer
2024-05-21 11:18 ` [PATCH v15 3/5] arch: allocate vgetrandom_alloc() syscall number Jason A. Donenfeld
` (2 subsequent siblings)
4 siblings, 1 reply; 11+ messages in thread
From: Jason A. Donenfeld @ 2024-05-21 11:18 UTC (permalink / raw)
To: linux-kernel, patches, tglx; +Cc: 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 `num * size_per_each`, 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;
} 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;
new_cap = grnd_allocator.cap + num;
new_states = reallocarray(grnd_allocator.states, new_cap, sizeof(*grnd_allocator.states));
if (!new_states) {
munmap(new_block, num * size_per_each);
goto out;
}
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;
}
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 | 132 +++++++++++++++++++++++++++++++++++++++
include/linux/syscalls.h | 3 +
include/vdso/getrandom.h | 16 +++++
kernel/sys_ni.c | 3 +
lib/vdso/Kconfig | 6 ++
6 files changed, 161 insertions(+)
create mode 100644 include/vdso/getrandom.h
diff --git a/MAINTAINERS b/MAINTAINERS
index 82f9fb4c0493..522c88b38550 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -18697,6 +18697,7 @@ 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..b066b8e0bbcb 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -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 +174,133 @@ 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 `(size_t)num * (size_t)size_per_each`, 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 e619ac10cd23..df3f2016f3b4 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -905,6 +905,9 @@ 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..e3ceb1976386
--- /dev/null
+++ b/include/vdso/getrandom.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022 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 faad00cce269..3e94650d28a5 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -271,6 +271,9 @@ 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().
--
2.44.0
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [PATCH v15 3/5] arch: allocate vgetrandom_alloc() syscall number
2024-05-21 11:18 [PATCH v15 0/5] implement getrandom() in vDSO Jason A. Donenfeld
2024-05-21 11:18 ` [PATCH v15 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings Jason A. Donenfeld
2024-05-21 11:18 ` [PATCH v15 2/5] random: add vgetrandom_alloc() syscall Jason A. Donenfeld
@ 2024-05-21 11:18 ` Jason A. Donenfeld
2024-05-22 15:36 ` Geert Uytterhoeven
2024-05-21 11:18 ` [PATCH v15 4/5] random: introduce generic vDSO getrandom() implementation Jason A. Donenfeld
2024-05-21 11:18 ` [PATCH v15 5/5] x86: vdso: Wire up getrandom() vDSO implementation Jason A. Donenfeld
4 siblings, 1 reply; 11+ messages in thread
From: Jason A. Donenfeld @ 2024-05-21 11:18 UTC (permalink / raw)
To: linux-kernel, patches, tglx; +Cc: Jason A. Donenfeld
Add vgetrandom_alloc() as syscall 462 (or 572 on alpha) by adding it to
all of the various syscall.tbl and unistd.h files.
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/unistd.h | 2 +-
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 +
23 files changed, 30 insertions(+), 3 deletions(-)
diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index 26cce7e7f70b..12e80ef2b755 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -501,3 +501,4 @@
569 common lsm_get_self_attr sys_lsm_get_self_attr
570 common lsm_set_self_attr sys_lsm_set_self_attr
571 common lsm_list_modules sys_lsm_list_modules
+572 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index b6c9e01e14f5..07e40ef330af 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -475,3 +475,4 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
+462 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
index 491b2b9bd553..1346579f802f 100644
--- a/arch/arm64/include/asm/unistd.h
+++ b/arch/arm64/include/asm/unistd.h
@@ -39,7 +39,7 @@
#define __ARM_NR_compat_set_tls (__ARM_NR_COMPAT_BASE + 5)
#define __ARM_NR_COMPAT_END (__ARM_NR_COMPAT_BASE + 0x800)
-#define __NR_compat_syscalls 462
+#define __NR_compat_syscalls 463
#endif
#define __ARCH_WANT_SYS_CLONE
diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
index 7118282d1c79..66307d5e751a 100644
--- a/arch/arm64/include/asm/unistd32.h
+++ b/arch/arm64/include/asm/unistd32.h
@@ -929,6 +929,8 @@ __SYSCALL(__NR_lsm_get_self_attr, sys_lsm_get_self_attr)
__SYSCALL(__NR_lsm_set_self_attr, sys_lsm_set_self_attr)
#define __NR_lsm_list_modules 461
__SYSCALL(__NR_lsm_list_modules, sys_lsm_list_modules)
+#define __NR_vgetrandom_alloc 462
+__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 7fd43fd4c9f2..6dbc444dbb24 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -461,3 +461,4 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
+462 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index b00ab2cabab9..9f4869e230b5 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -467,3 +467,4 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
+462 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 83cfc9eb6b88..ad47c1afff45 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
@@ -400,3 +400,4 @@
459 n32 lsm_get_self_attr sys_lsm_get_self_attr
460 n32 lsm_set_self_attr sys_lsm_set_self_attr
461 n32 lsm_list_modules sys_lsm_list_modules
+462 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 532b855df589..92aa52c6b5f8 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
@@ -376,3 +376,4 @@
459 n64 lsm_get_self_attr sys_lsm_get_self_attr
460 n64 lsm_set_self_attr sys_lsm_set_self_attr
461 n64 lsm_list_modules sys_lsm_list_modules
+462 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 f45c9530ea93..a2c4d1624b3b 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
@@ -449,3 +449,4 @@
459 o32 lsm_get_self_attr sys_lsm_get_self_attr
460 o32 lsm_set_self_attr sys_lsm_set_self_attr
461 o32 lsm_list_modules sys_lsm_list_modules
+462 o32 vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index b236a84c4e12..62296f1f7465 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl
@@ -460,3 +460,4 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
+462 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index 17173b82ca21..425604735056 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -548,3 +548,4 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
+462 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index 095bb86339a7..81b995ef7b20 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl
@@ -464,3 +464,4 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr sys_lsm_get_self_attr
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 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 86fe269f0220..870fa18c7b34 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -464,3 +464,4 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
+462 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index b23d59313589..5514b14fadcc 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -507,3 +507,4 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
+462 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 5f8591ce7f25..8509e7cbe77c 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -466,3 +466,4 @@
459 i386 lsm_get_self_attr sys_lsm_get_self_attr
460 i386 lsm_set_self_attr sys_lsm_set_self_attr
461 i386 lsm_list_modules sys_lsm_list_modules
+462 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 cc78226ffc35..f7592efebf64 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -383,6 +383,7 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
+462 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 dd116598fb25..4298d84d4cb9 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -432,3 +432,4 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
+462 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 75f00965ab15..abfb32300836 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -842,8 +842,11 @@ __SYSCALL(__NR_lsm_set_self_attr, sys_lsm_set_self_attr)
#define __NR_lsm_list_modules 461
__SYSCALL(__NR_lsm_list_modules, sys_lsm_list_modules)
+#define __NR_vgetrandom_alloc 462
+__SYSCALL(__NR_vgetrandom_alloc, sys_vgetrandom_alloc)
+
#undef __NR_syscalls
-#define __NR_syscalls 462
+#define __NR_syscalls 463
/*
* 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 75f00965ab15..abfb32300836 100644
--- a/tools/include/uapi/asm-generic/unistd.h
+++ b/tools/include/uapi/asm-generic/unistd.h
@@ -842,8 +842,11 @@ __SYSCALL(__NR_lsm_set_self_attr, sys_lsm_set_self_attr)
#define __NR_lsm_list_modules 461
__SYSCALL(__NR_lsm_list_modules, sys_lsm_list_modules)
+#define __NR_vgetrandom_alloc 462
+__SYSCALL(__NR_vgetrandom_alloc, sys_vgetrandom_alloc)
+
#undef __NR_syscalls
-#define __NR_syscalls 462
+#define __NR_syscalls 463
/*
* 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 532b855df589..92aa52c6b5f8 100644
--- a/tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl
+++ b/tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl
@@ -376,3 +376,4 @@
459 n64 lsm_get_self_attr sys_lsm_get_self_attr
460 n64 lsm_set_self_attr sys_lsm_set_self_attr
461 n64 lsm_list_modules sys_lsm_list_modules
+462 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 17173b82ca21..425604735056 100644
--- a/tools/perf/arch/powerpc/entry/syscalls/syscall.tbl
+++ b/tools/perf/arch/powerpc/entry/syscalls/syscall.tbl
@@ -548,3 +548,4 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
+462 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 095bb86339a7..81b995ef7b20 100644
--- a/tools/perf/arch/s390/entry/syscalls/syscall.tbl
+++ b/tools/perf/arch/s390/entry/syscalls/syscall.tbl
@@ -464,3 +464,4 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr sys_lsm_get_self_attr
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 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 7e8d46f4147f..e0f21b6e92fc 100644
--- a/tools/perf/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/tools/perf/arch/x86/entry/syscalls/syscall_64.tbl
@@ -383,6 +383,7 @@
459 common lsm_get_self_attr sys_lsm_get_self_attr
460 common lsm_set_self_attr sys_lsm_set_self_attr
461 common lsm_list_modules sys_lsm_list_modules
+462 common vgetrandom_alloc sys_vgetrandom_alloc
#
# Due to a historical design error, certain syscalls are numbered differently
--
2.44.0
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [PATCH v15 4/5] random: introduce generic vDSO getrandom() implementation
2024-05-21 11:18 [PATCH v15 0/5] implement getrandom() in vDSO Jason A. Donenfeld
` (2 preceding siblings ...)
2024-05-21 11:18 ` [PATCH v15 3/5] arch: allocate vgetrandom_alloc() syscall number Jason A. Donenfeld
@ 2024-05-21 11:18 ` Jason A. Donenfeld
2024-05-21 11:18 ` [PATCH v15 5/5] x86: vdso: Wire up getrandom() vDSO implementation Jason A. Donenfeld
4 siblings, 0 replies; 11+ messages in thread
From: Jason A. Donenfeld @ 2024-05-21 11:18 UTC (permalink / raw)
To: linux-kernel, patches, tglx; +Cc: 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 a the API signature:
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 the opaque
allocated state. Were all four 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 *buf, size_t len, unsigned long flags, void *state);
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);
}
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
MAINTAINERS | 1 +
drivers/char/random.c | 11 +
include/vdso/datapage.h | 12 +
include/vdso/getrandom.h | 32 +-
include/vdso/types.h | 35 +++
lib/vdso/getrandom.c | 226 ++++++++++++++
tools/testing/selftests/vDSO/.gitignore | 1 +
tools/testing/selftests/vDSO/Makefile | 2 +
.../selftests/vDSO/vdso_test_getrandom.c | 283 ++++++++++++++++++
9 files changed, 601 insertions(+), 2 deletions(-)
create mode 100644 include/vdso/types.h
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 522c88b38550..55ffa68426bc 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -18698,6 +18698,7 @@ 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 b066b8e0bbcb..3e66827f06ea 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>
@@ -403,6 +404,13 @@ 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.
+ */
+ 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);
@@ -853,6 +861,9 @@ 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
+ smp_store_release(&_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..3ca19296ce28 100644
--- a/include/vdso/datapage.h
+++ b/include/vdso/datapage.h
@@ -18,6 +18,7 @@
#include <vdso/time.h>
#include <vdso/time32.h>
#include <vdso/time64.h>
+#include <vdso/types.h>
#ifdef CONFIG_ARCH_HAS_VDSO_DATA
#include <asm/vdso/data.h>
@@ -113,6 +114,16 @@ 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 {
+ vdso_kernel_ulong 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 +135,7 @@ 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 e3ceb1976386..7dc93d5f72dc 100644
--- a/include/vdso/getrandom.h
+++ b/include/vdso/getrandom.h
@@ -6,11 +6,39 @@
#ifndef _VDSO_GETRANDOM_H
#define _VDSO_GETRANDOM_H
+#include <crypto/chacha.h>
+#include <vdso/types.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.
+ * @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];
+ };
+ vdso_kernel_ulong generation;
+ u8 pos;
+ bool in_use;
+};
#endif /* _VDSO_GETRANDOM_H */
diff --git a/include/vdso/types.h b/include/vdso/types.h
new file mode 100644
index 000000000000..ce131463aeff
--- /dev/null
+++ b/include/vdso/types.h
@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+#ifndef __VDSO_TYPES_H
+#define __VDSO_TYPES_H
+
+#include <linux/types.h>
+
+/**
+ * type vdso_kernel_ulong - unsigned long type that matches kernel's unsigned long
+ *
+ * Data shared between userspace and the kernel must operate the same way in both 64-bit code and in
+ * 32-bit compat code, over the same potentially 64-bit kernel. This type represents the size of an
+ * unsigned long as used by kernel code. This isn't necessarily the same as an unsigned long as used
+ * by userspace, however.
+ *
+ * +-------------------+-------------------+------------------+-------------------+
+ * | 32-bit userspace | 32-bit userspace | 64-bit userspace | 64-bit userspace |
+ * | unsigned long | vdso_kernel_ulong | unsigned long | vdso_kernel_ulong |
+ * +---------------+-------------------+-------------------+------------------+-------------------+
+ * | 32-bit kernel | ✓ same size | ✓ same size |
+ * | unsigned long | | |
+ * +---------------+-------------------+-------------------+------------------+-------------------+
+ * | 64-bit kernel | ✘ different size! | ✓ same size | ✓ same size | ✓ same size |
+ * | unsigned long | | | | |
+ * +---------------+-------------------+-------------------+------------------+-------------------+
+ */
+#ifdef CONFIG_64BIT
+typedef u64 vdso_kernel_ulong;
+#else
+typedef u32 vdso_kernel_ulong;
+#endif
+
+#endif /* __VDSO_TYPES_H */
diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c
new file mode 100644
index 000000000000..4d9bb59985f8
--- /dev/null
+++ b/lib/vdso/getrandom.c
@@ -0,0 +1,226 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 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>
+
+#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.
+ *
+ * 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)
+{
+ 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;
+ unsigned long current_generation;
+ void *orig_buffer = buffer;
+ u32 counter[2] = { 0 };
+ bool in_use, have_retried = false;
+
+ /* 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))
+ 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))
+ 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 two forks 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. */
+ barrier();
+
+ /* 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 then 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)
+{
+ return __cvdso_getrandom_data(__arch_get_vdso_rng_data(), buffer, len, flags, opaque_state);
+}
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..80b9849fe911
--- /dev/null
+++ b/tools/testing/selftests/vDSO/vdso_test_getrandom.c
@@ -0,0 +1,283 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 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 "../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
+
+static void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each)
+{
+ enum { __NR_vgetrandom_alloc = 462 };
+ *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;
+} 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;
+ new_cap = grnd_allocator.cap + num;
+ new_states = reallocarray(grnd_allocator.states, new_cap, sizeof(*grnd_allocator.states));
+ if (!new_states) {
+ munmap(new_block, num * size_per_each);
+ goto out;
+ }
+ 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;
+ }
+ 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 *buf, size_t len, unsigned long flags, void *state);
+ 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);
+}
+
+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;
+}
--
2.44.0
^ permalink raw reply related [flat|nested] 11+ messages in thread
* [PATCH v15 5/5] x86: vdso: Wire up getrandom() vDSO implementation
2024-05-21 11:18 [PATCH v15 0/5] implement getrandom() in vDSO Jason A. Donenfeld
` (3 preceding siblings ...)
2024-05-21 11:18 ` [PATCH v15 4/5] random: introduce generic vDSO getrandom() implementation Jason A. Donenfeld
@ 2024-05-21 11:18 ` Jason A. Donenfeld
4 siblings, 0 replies; 11+ messages in thread
From: Jason A. Donenfeld @ 2024-05-21 11:18 UTC (permalink / raw)
To: linux-kernel, patches, tglx; +Cc: Jason A. Donenfeld, Samuel Neves
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 | 9 +
.../testing/selftests/vDSO/vdso_test_chacha.c | 43 +++++
11 files changed, 326 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 671bc6fd557c..94feb2db64c8 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -286,6 +286,7 @@ 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..d79e2bd97598
--- /dev/null
+++ b/arch/x86/entry/vdso/vgetrandom-chacha.S
@@ -0,0 +1,178 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 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 state0, %xmm0
+.set state1, %xmm1
+.set state2, %xmm2
+.set state3, %xmm3
+.set copy0, %xmm4
+.set copy1, %xmm5
+.set copy2, %xmm6
+.set copy3, %xmm7
+.set temp, %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..6045ded5da90
--- /dev/null
+++ b/arch/x86/entry/vdso/vgetrandom.c
@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2022 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 *state);
+
+ssize_t __vdso_getrandom(void *buffer, size_t len, unsigned int flags, void *state)
+{
+ return __cvdso_getrandom(buffer, len, flags, state);
+}
+
+ssize_t getrandom(void *, size_t, unsigned int, void *)
+ __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..46f99d735ae6
--- /dev/null
+++ b/arch/x86/include/asm/vdso/getrandom.h
@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022 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 (__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..54a015afe60c 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 +13,15 @@ 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 -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 +42,5 @@ $(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
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..bce7a7752b11
--- /dev/null
+++ b/tools/testing/selftests/vDSO/vdso_test_chacha.c
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 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;
+}
--
2.44.0
^ permalink raw reply related [flat|nested] 11+ messages in thread
* Re: [PATCH v15 2/5] random: add vgetrandom_alloc() syscall
2024-05-21 11:18 ` [PATCH v15 2/5] random: add vgetrandom_alloc() syscall Jason A. Donenfeld
@ 2024-05-21 11:48 ` Florian Weimer
2024-05-21 12:32 ` Jason A. Donenfeld
0 siblings, 1 reply; 11+ messages in thread
From: Florian Weimer @ 2024-05-21 11:48 UTC (permalink / raw)
To: Jason A. Donenfeld; +Cc: linux-kernel, patches, tglx
* Jason A. Donenfeld:
> 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.
I think “first use” actually means “processs start” here? We don't have
a way to report failure from arc4random and similar interfaces.
Thanks,
Florian
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH v15 2/5] random: add vgetrandom_alloc() syscall
2024-05-21 11:48 ` Florian Weimer
@ 2024-05-21 12:32 ` Jason A. Donenfeld
0 siblings, 0 replies; 11+ messages in thread
From: Jason A. Donenfeld @ 2024-05-21 12:32 UTC (permalink / raw)
To: Florian Weimer; +Cc: linux-kernel, patches, tglx, adhemerval.zanella
On Tue, May 21, 2024 at 01:48:10PM +0200, Florian Weimer wrote:
> * Jason A. Donenfeld:
>
> > 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.
>
> I think “first use” actually means “processs start” here? We don't have
> a way to report failure from arc4random and similar interfaces.
First use, actually. On failure this just falls back to slow syscall
getrandom() with identical semantics.
Adhemerval has a branch of libc where he implemented this, if you're
interested. He's in CC to send the link.
Jason
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH v15 3/5] arch: allocate vgetrandom_alloc() syscall number
2024-05-21 11:18 ` [PATCH v15 3/5] arch: allocate vgetrandom_alloc() syscall number Jason A. Donenfeld
@ 2024-05-22 15:36 ` Geert Uytterhoeven
0 siblings, 0 replies; 11+ messages in thread
From: Geert Uytterhoeven @ 2024-05-22 15:36 UTC (permalink / raw)
To: Jason A. Donenfeld; +Cc: linux-kernel, patches, tglx
On Tue, May 21, 2024 at 1:21 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> Add vgetrandom_alloc() as syscall 462 (or 572 on alpha) by adding it to
> all of the various syscall.tbl and unistd.h files.
>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> arch/m68k/kernel/syscalls/syscall.tbl | 1 +
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH v15 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings
2024-05-21 11:18 ` [PATCH v15 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings Jason A. Donenfeld
@ 2024-05-27 13:34 ` David Hildenbrand
2024-05-28 11:58 ` Jason A. Donenfeld
0 siblings, 1 reply; 11+ messages in thread
From: David Hildenbrand @ 2024-05-27 13:34 UTC (permalink / raw)
To: Jason A. Donenfeld, linux-kernel, patches, tglx; +Cc: linux-mm
Am 21.05.24 um 13:18 schrieb 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 shouldn't reserve actual memory, but it also shouldn't crash when
> page faulting in memory if none is available
> * Uh-oh: MAP_NORESERVE respects vm.overcommit_memory=2.
> * Uh-oh: VM_NORESERVE means segfaults.
>
> 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) If there's not enough memory to service a page fault, it's not fatal.
> d) It is inherited by fork.
> e) 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 OOMing, crashing on overcommitment,
> 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/memory.c | 4 ++++
> mm/mempolicy.c | 3 +++
> mm/mprotect.c | 2 +-
> mm/rmap.c | 5 +++--
> 8 files changed, 32 insertions(+), 3 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index e5a5f015ff03..b5a59e57bde1 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/memory.c b/mm/memory.c
> index b5453b86ec4b..57b03fc73159 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5689,6 +5689,10 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>
> lru_gen_exit_fault();
>
> + /* If the mapping is droppable, then errors due to OOM aren't fatal. */
> + if (vma->vm_flags & VM_DROPPABLE)
> + ret &= ~VM_FAULT_OOM;
> +
> if (flags & FAULT_FLAG_USER) {
> mem_cgroup_exit_user_fault();
> /*
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index aec756ae5637..a66289f1d931 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2300,6 +2300,9 @@ struct folio *vma_alloc_folio_noprof(gfp_t gfp, int order, struct vm_area_struct
> pgoff_t ilx;
> struct page *page;
>
> + if (vma->vm_flags & VM_DROPPABLE)
> + gfp |= __GFP_NOWARN | __GFP_NORETRY;
> +
> pol = get_vma_policy(vma, addr, order, &ilx);
> page = alloc_pages_mpol_noprof(gfp | __GFP_COMP, order,
> pol, ilx, numa_node_id());
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 94878c39ee32..88ff3ecc08a1 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -622,7 +622,7 @@ 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..f23b9f796aae 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;
> }
If there is any unexpected temporary folio reference (ref_count != 1 +
map_count), you'll end up calling folio_set_swapbacked() and the next swapout
round will write that folio to swap.
--
Thanks,
David / dhildenb
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [PATCH v15 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings
2024-05-27 13:34 ` David Hildenbrand
@ 2024-05-28 11:58 ` Jason A. Donenfeld
0 siblings, 0 replies; 11+ messages in thread
From: Jason A. Donenfeld @ 2024-05-28 11:58 UTC (permalink / raw)
To: David Hildenbrand; +Cc: linux-kernel, patches, tglx, linux-mm
On Mon, May 27, 2024 at 03:34:33PM +0200, David Hildenbrand wrote:
>
>
> Am 21.05.24 um 13:18 schrieb 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 shouldn't reserve actual memory, but it also shouldn't crash when
> > page faulting in memory if none is available
> > * Uh-oh: MAP_NORESERVE respects vm.overcommit_memory=2.
> > * Uh-oh: VM_NORESERVE means segfaults.
> >
> > 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) If there's not enough memory to service a page fault, it's not fatal.
> > d) It is inherited by fork.
> > e) 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 OOMing, crashing on overcommitment,
> > 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/memory.c | 4 ++++
> > mm/mempolicy.c | 3 +++
> > mm/mprotect.c | 2 +-
> > mm/rmap.c | 5 +++--
> > 8 files changed, 32 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index e5a5f015ff03..b5a59e57bde1 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/memory.c b/mm/memory.c
> > index b5453b86ec4b..57b03fc73159 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5689,6 +5689,10 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> >
> > lru_gen_exit_fault();
> >
> > + /* If the mapping is droppable, then errors due to OOM aren't fatal. */
> > + if (vma->vm_flags & VM_DROPPABLE)
> > + ret &= ~VM_FAULT_OOM;
> > +
> > if (flags & FAULT_FLAG_USER) {
> > mem_cgroup_exit_user_fault();
> > /*
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index aec756ae5637..a66289f1d931 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -2300,6 +2300,9 @@ struct folio *vma_alloc_folio_noprof(gfp_t gfp, int order, struct vm_area_struct
> > pgoff_t ilx;
> > struct page *page;
> >
> > + if (vma->vm_flags & VM_DROPPABLE)
> > + gfp |= __GFP_NOWARN | __GFP_NORETRY;
> > +
> > pol = get_vma_policy(vma, addr, order, &ilx);
> > page = alloc_pages_mpol_noprof(gfp | __GFP_COMP, order,
> > pol, ilx, numa_node_id());
> > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > index 94878c39ee32..88ff3ecc08a1 100644
> > --- a/mm/mprotect.c
> > +++ b/mm/mprotect.c
> > @@ -622,7 +622,7 @@ 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..f23b9f796aae 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;
> > }
>
> If there is any unexpected temporary folio reference (ref_count != 1 +
> map_count), you'll end up calling folio_set_swapbacked() and the next swapout
> round will write that folio to swap.
Thanks, nice call. I'll guard the re-setting of folio_set_swapbacked()
in that case.
Jason
^ permalink raw reply [flat|nested] 11+ messages in thread
end of thread, other threads:[~2024-05-28 11:58 UTC | newest]
Thread overview: 11+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2024-05-21 11:18 [PATCH v15 0/5] implement getrandom() in vDSO Jason A. Donenfeld
2024-05-21 11:18 ` [PATCH v15 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings Jason A. Donenfeld
2024-05-27 13:34 ` David Hildenbrand
2024-05-28 11:58 ` Jason A. Donenfeld
2024-05-21 11:18 ` [PATCH v15 2/5] random: add vgetrandom_alloc() syscall Jason A. Donenfeld
2024-05-21 11:48 ` Florian Weimer
2024-05-21 12:32 ` Jason A. Donenfeld
2024-05-21 11:18 ` [PATCH v15 3/5] arch: allocate vgetrandom_alloc() syscall number Jason A. Donenfeld
2024-05-22 15:36 ` Geert Uytterhoeven
2024-05-21 11:18 ` [PATCH v15 4/5] random: introduce generic vDSO getrandom() implementation Jason A. Donenfeld
2024-05-21 11:18 ` [PATCH v15 5/5] x86: vdso: Wire up getrandom() vDSO implementation Jason A. Donenfeld
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