Re: [PATCH v19 18/25] mm: Update can_follow_write_pte() for shadow stack

Kees Cook
2021-02-04T20:27:40Z

On Wed, Feb 03, 2021 at 02:55:40PM -0800, Yu-cheng Yu wrote:
> Can_follow_write_pte() ensures a read-only page is COWed by checking the
> FOLL_COW flag, and uses pte_dirty() to validate the flag is still valid.
>
> Like a writable data page, a shadow stack page is writable, and becomes
> read-only during copy-on-write, but it is always dirty. Thus, in the
> can_follow_write_pte() check, it belongs to the writable page case and
> should be excluded from the read-only page pte_dirty() check. Apply
> the same changes to can_follow_write_pmd().

Does this need the vma passed down? Should it just pass vm_flags? I
suppose it doesn't really matter, though.

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
> mm/gup.c | 8 +++++---
> mm/huge_memory.c | 8 +++++---
> 2 files changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index e4c224cd9661..66ab67626f57 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -357,10 +357,12 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
> * FOLL_FORCE can write to even unwritable pte's, but only
> * after we've gone through a COW cycle and they are dirty.
> */
> -static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
> +static inline bool can_follow_write_pte(pte_t pte, unsigned int flags,
> + struct vm_area_struct *vma)
> {
> return pte_write(pte) ||
> - ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
> + ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte) &&
> + !arch_shadow_stack_mapping(vma->vm_flags));
> }
>
> static struct page *follow_page_pte(struct vm_area_struct *vma,
> @@ -403,7 +405,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> }
> if ((flags & FOLL_NUMA) && pte_protnone(pte))
> goto no_page;
> - if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
> + if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags, vma)) {
> pte_unmap_unlock(ptep, ptl);
> return NULL;
> }
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bfec65c9308b..eb64e2b56bc9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1337,10 +1337,12 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd)
> * FOLL_FORCE can write to even unwritable pmd's, but only
> * after we've gone through a COW cycle and they are dirty.
> */
> -static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
> +static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags,
> + struct vm_area_struct *vma)
> {
> return pmd_write(pmd) ||
> - ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
> + ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd) &&
> + !arch_shadow_stack_mapping(vma->vm_flags));
> }
>
> struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
> @@ -1353,7 +1355,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>
> assert_spin_locked(pmd_lockptr(mm, pmd));
>
> - if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags))
> + if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags, vma))
> goto out;
>
> /* Avoid dumping huge zero page */
> --
> 2.21.0
>
>

--
Kees Cook