Re: [PATCH 1/3] mm/pagemap: Fix ioctl(PAGEMAP_SCAN) on vma check

[Andrei Vagin]

On Thu, Nov 16, 2023 at 12:15 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> The new ioctl(PAGEMAP_SCAN) relies on vma wr-protect capability provided by
> userfault, however in the vma test it didn't explicitly require the vma to
> have wr-protect function enabled, even if PM_SCAN_WP_MATCHING flag is set.
>
> It means the pagemap code can now apply uffd-wp bit to a page in the vma
> even if not registered to userfaultfd at all.
>
> Then in whatever way as long as the pte got written and page fault
> resolved, we'll apply the write bit even if uffd-wp bit is set.  We'll see
> a pte that has both UFFD_WP and WRITE bit set.  Anything later that looks
> up the pte for uffd-wp bit will trigger the warning:
>
> WARNING: CPU: 1 PID: 5071 at arch/x86/include/asm/pgtable.h:403 pte_uffd_wp arch/x86/include/asm/pgtable.h:403 [inline]
>
> Fix it by doing proper check over the vma attributes when
> PM_SCAN_WP_MATCHING is specified.
>
> Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
> Reported-by: syzbot+e94c5aaf7890901ebf9b@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>

Reviewed-by: Andrei Vagin <avagin@xxxxxxxxx>

> ---
>  fs/proc/task_mmu.c | 24 ++++++++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 51e0ec658457..e91085d79926 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1994,15 +1994,31 @@ static int pagemap_scan_test_walk(unsigned long start, unsigned long end,
>         struct pagemap_scan_private *p = walk->private;
>         struct vm_area_struct *vma = walk->vma;
>         unsigned long vma_category = 0;
> +       bool wp_allowed = userfaultfd_wp_async(vma) &&
> +           userfaultfd_wp_use_markers(vma);
>
> -       if (userfaultfd_wp_async(vma) && userfaultfd_wp_use_markers(vma))
> -               vma_category |= PAGE_IS_WPALLOWED;
> -       else if (p->arg.flags & PM_SCAN_CHECK_WPASYNC)
> -               return -EPERM;
> +       if (!wp_allowed) {
> +               /* User requested explicit failure over wp-async capability */
> +               if (p->arg.flags & PM_SCAN_CHECK_WPASYNC)
> +                       return -EPERM;
> +               /*
> +                * User requires wr-protect, and allows silently skipping
> +                * unsupported vmas.
> +                */
> +               if (p->arg.flags & PM_SCAN_WP_MATCHING)
> +                       return 1;
> +               /*
> +                * Then the request doesn't involve wr-protects at all,
> +                * fall through to the rest checks, and allow vma walk.
> +                */
> +       }
>
>         if (vma->vm_flags & VM_PFNMAP)
>                 return 1;
>
> +       if (wp_allowed)
> +               vma_category |= PAGE_IS_WPALLOWED;
> +
>         if (vma->vm_flags & VM_SOFTDIRTY)
>                 vma_category |= PAGE_IS_SOFT_DIRTY;
>
> --
> 2.41.0
>