Re: [PATCH 20/35] mm: Update can_follow_write_pte() for shadow stack

Dave Hansen
2022-02-09T22:50:34Z

On 1/30/22 13:18, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> 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,

I thought we could not have read-only shadow stack pages. What does a
read-only shadow stack PTE look like? ;)

> 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().
>
> While at it, also split the long line into smaller ones.

FWIW, I probably would have had a preparatory patch for this part. The
advantage is that if you break existing code, it's a lot easier to
figure it out if you have a separate refactoring patch. Also, for a
patch like this, the refactoring might result in the same exact binary.
It's a pretty good sign that your patch won't cause regressions if it
results in the same binary.

> diff --git a/mm/gup.c b/mm/gup.c
> index f0af462ac1e2..95b7d1084c44 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -464,10 +464,18 @@ 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));
> + if (pte_write(pte))
> + return true;
> + if ((flags & (FOLL_FORCE | FOLL_COW)) != (FOLL_FORCE | FOLL_COW))
> + return false;
> + if (!pte_dirty(pte))
> + return false;
> + if (is_shadow_stack_mapping(vma->vm_flags))
> + return false;

You had me up until this is_shadow_stack_mapping(). It wasn't mentioned
at all in the changelog. Logically, I think it's trying to say that a
shadow stack VMA never allows a FOLL_FORCE override.

That makes some sense, but it's a pretty big point not to mention in the
changelog.

> + return true;
> }
>
> static struct page *follow_page_pte(struct vm_area_struct *vma,
> @@ -510,7 +518,7 @@ 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;
> }