Re: [PATCH v5 19/39] mm: Fixup places that call pte_mkwrite() directly

Deepak Gupta
2023-02-14T00:09:54Z

Since I've a general question on outcome of discussion of how to handle
`pte_mkwrite`, so I am top posting.

I have posted patches yesterday targeting riscv zisslpcfi extension.
https://lore.kernel.org/lkml/20230213045351.3945824-1-debug@rivosinc.com/

Since there're similarities in extension(s), patches have similarity too.
One of the similarity was updating `maybe_mkwrite`. I was asked (by dhildenb
on my patch #11) to look at x86 approach on how to approach this so that
core-mm approach fits multiple architectures along with the need to
update `pte_mkwrite` to consume vma flags.
In x86 CET patch series, I see that locations where `pte_mkwrite` is
invoked are updated to check for shadow stack vma and not necessarily
`pte_mkwrite` itself is updated to consume vma flags. Let me know if my
understanding is correct and that's the current direction (to update
call sites for vma check where `pte_mkwrite` is invoked)

Being said that as I've mentioned in my patch series that there're
similarities between x86, arm and now riscv for implementing shadow stack
and indirect branch tracking, overall it'll be a good thing if we can
collaborate and come up with common bits.

Rest inline.

On Thu, Jan 19, 2023 at 01:22:57PM -0800, Rick Edgecombe wrote:
>From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
>The x86 Control-flow Enforcement Technology (CET) feature includes a new
>type of memory called shadow stack. This shadow stack memory has some
>unusual properties, which requires some core mm changes to function
>properly.
>
>With the introduction of shadow stack memory there are two ways a pte can
>be writable: regular writable memory and shadow stack memory.
>
>In past patches, maybe_mkwrite() has been updated to apply pte_mkwrite()
>or pte_mkwrite_shstk() depending on the VMA flag. This covers most cases
>where a PTE is made writable. However, there are places where pte_mkwrite()
>is called directly and the logic should now also create a shadow stack PTE
>in the case of a shadow stack VMA.
>
>- do_anonymous_page() and migrate_vma_insert_page() check VM_WRITE
> directly and call pte_mkwrite(). Teach it about pte_mkwrite_shstk()
>
>- When userfaultfd is creating a PTE after userspace handles the fault
> it calls pte_mkwrite() directly. Teach it about pte_mkwrite_shstk()
>
>To make the code cleaner, introduce is_shstk_write() which simplifies
>checking for VM_WRITE | VM_SHADOW_STACK together.
>
>In other cases where pte_mkwrite() is called directly, the VMA will not
>be VM_SHADOW_STACK, and so shadow stack memory should not be created.
> - In the case of pte_savedwrite(), shadow stack VMA's are excluded.
> - In the case of the "dirty_accountable" optimization in mprotect(),
> shadow stack VMA's won't be VM_SHARED, so it is not necessary.
>
>Tested-by: Pengfei Xu <pengfei.xu@intel.com>
>Tested-by: John Allen <john.allen@amd.com>
>Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
>Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>Cc: Kees Cook <keescook@chromium.org>
>---
>
>v5:
> - Fix typo in commit log
>
>v3:
> - Restore do_anonymous_page() that accidetally moved commits (Kirill)
> - Open code maybe_mkwrite() cases from v2, so the behavior doesn't change
> to mark that non-writable PTEs dirty. (Nadav)
>
>v2:
> - Updated commit log with comment's from Dave Hansen
> - Dave also suggested (I understood) to maybe tweak vm_get_page_prot()
> to avoid having to call maybe_mkwrite(). After playing around with
> this I opted to *not* do this. Shadow stack memory memory is
> effectively writable, so having the default permissions be writable
> ended up mapping the zero page as writable and other surprises. So
> creating shadow stack memory needs to be done with manual logic
> like pte_mkwrite().
> - Drop change in change_pte_range() because it couldn't actually trigger
> for shadow stack VMAs.
> - Clarify reasoning for skipped cases of pte_mkwrite().
>
>Yu-cheng v25:
> - Apply same changes to do_huge_pmd_numa_page() as to do_numa_page().
>
> arch/x86/include/asm/pgtable.h | 3 +++
> arch/x86/mm/pgtable.c | 6 ++++++
> include/linux/pgtable.h | 7 +++++++
> mm/memory.c | 5 ++++-
> mm/migrate_device.c | 4 +++-
> mm/userfaultfd.c | 10 +++++++---
> 6 files changed, 30 insertions(+), 5 deletions(-)
>
>diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>index 45b1a8f058fe..87d3068734ec 100644
>--- a/arch/x86/include/asm/pgtable.h
>+++ b/arch/x86/include/asm/pgtable.h
>@@ -951,6 +951,9 @@ static inline pgd_t pti_set_user_pgtbl(pgd_t *pgdp, pgd_t pgd)
> }
> #endif /* CONFIG_PAGE_TABLE_ISOLATION */
>
>+#define is_shstk_write is_shstk_write
>+extern bool is_shstk_write(unsigned long vm_flags);
>+
> #endif /* __ASSEMBLY__ */
>
>
>diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
>index e4f499eb0f29..d103945ba502 100644
>--- a/arch/x86/mm/pgtable.c
>+++ b/arch/x86/mm/pgtable.c
>@@ -880,3 +880,9 @@ int pmd_free_pte_page(pmd_t *pmd, unsigned long addr)
>
> #endif /* CONFIG_X86_64 */
> #endif /* CONFIG_HAVE_ARCH_HUGE_VMAP */
>+
>+bool is_shstk_write(unsigned long vm_flags)
>+{
>+ return (vm_flags & (VM_SHADOW_STACK | VM_WRITE)) ==
>+ (VM_SHADOW_STACK | VM_WRITE);
>+}

Can we call this function something along the lines `is_shadow_stack_vma`?
Reason being, we're actually checking for vma property here.

Also can we move this into common code? Common code can then further call
`arch_is_shadow_stack_vma`. Respective arch can implement their own shadow
stack encoding. I see that x86 is using one of the arch bit. Current riscv
implementation uses presence of only `VM_WRITE` as shadow stack encoding.

Please see patch #11 and #12 in the series I posted (URL at the top of
this e-mail).

>diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>index 14a820a45a37..49ce1f055242 100644
>--- a/include/linux/pgtable.h
>+++ b/include/linux/pgtable.h
>@@ -1578,6 +1578,13 @@ static inline bool arch_has_pfn_modify_check(void)
> }
> #endif /* !_HAVE_ARCH_PFN_MODIFY_ALLOWED */
>
>+#ifndef is_shstk_write
>+static inline bool is_shstk_write(unsigned long vm_flags)
>+{
>+ return false;
>+}
>+#endif
>+
> /*
> * Architecture PAGE_KERNEL_* fallbacks
> *
>diff --git a/mm/memory.c b/mm/memory.c
>index aad226daf41b..5e5107232a26 100644
>--- a/mm/memory.c
>+++ b/mm/memory.c
>@@ -4088,7 +4088,10 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>
> entry = mk_pte(page, vma->vm_page_prot);
> entry = pte_sw_mkyoung(entry);
>- if (vma->vm_flags & VM_WRITE)
>+
>+ if (is_shstk_write(vma->vm_flags))
>+ entry = pte_mkwrite_shstk(pte_mkdirty(entry));
>+ else if (vma->vm_flags & VM_WRITE)
> entry = pte_mkwrite(pte_mkdirty(entry));
>
> vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
>diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>index 721b2365dbca..53d417683e01 100644
>--- a/mm/migrate_device.c
>+++ b/mm/migrate_device.c
>@@ -645,7 +645,9 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
> goto abort;
> }
> entry = mk_pte(page, vma->vm_page_prot);
>- if (vma->vm_flags & VM_WRITE)
>+ if (is_shstk_write(vma->vm_flags))
>+ entry = pte_mkwrite_shstk(pte_mkdirty(entry));
>+ else if (vma->vm_flags & VM_WRITE)
> entry = pte_mkwrite(pte_mkdirty(entry));
> }
>
>diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
>index 0499907b6f1a..832f0250ca61 100644
>--- a/mm/userfaultfd.c
>+++ b/mm/userfaultfd.c
>@@ -63,6 +63,7 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> int ret;
> pte_t _dst_pte, *dst_pte;
> bool writable = dst_vma->vm_flags & VM_WRITE;
>+ bool shstk = dst_vma->vm_flags & VM_SHADOW_STACK;
> bool vm_shared = dst_vma->vm_flags & VM_SHARED;
> bool page_in_cache = page_mapping(page);
> spinlock_t *ptl;
>@@ -84,9 +85,12 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> writable = false;
> }
>
>- if (writable)
>- _dst_pte = pte_mkwrite(_dst_pte);
>- else
>+ if (writable) {
>+ if (shstk)
>+ _dst_pte = pte_mkwrite_shstk(_dst_pte);
>+ else
>+ _dst_pte = pte_mkwrite(_dst_pte);
>+ } else
> /*
> * We need this to make sure write bit removed; as mk_pte()
> * could return a pte with write bit set.
>--
>2.17.1
>