Re: [PATCH v2 1/1] userfaultfd: fix a crash when UFFDIO_MOVE handles a THP hole

[Lokesh Gidra <lokeshgidra@google.com>]

On Thu, Jul 31, 2025 at 8:44 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it
> encounters a non-present THP, it fails to properly recognize an unmapped
> hole and tries to access a non-existent folio, resulting in
> a crash. Add a check to skip non-present THPs.
>
> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Reported-by: syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

Reviewed-by: Lokesh Gidra <lokeshgidra@google.com>
> Cc: stable@vger.kernel.org
> ---
> Changes since v1 [1]
> - Fixed step size calculation, per Lokesh Gidra
> - Added missing check for UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES, per Lokesh Gidra
>
> [1] https://lore.kernel.org/all/20250730170733.3829267-1-surenb@google.com/
>
>  mm/userfaultfd.c | 45 +++++++++++++++++++++++++++++----------------
>  1 file changed, 29 insertions(+), 16 deletions(-)
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index cbed91b09640..b5af31c22731 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1818,28 +1818,41 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
>
>                 ptl = pmd_trans_huge_lock(src_pmd, src_vma);
>                 if (ptl) {
> -                       /* Check if we can move the pmd without splitting it. */
> -                       if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
> -                           !pmd_none(dst_pmdval)) {
> -                               struct folio *folio = pmd_folio(*src_pmd);
> +                       if (pmd_present(*src_pmd) || is_pmd_migration_entry(*src_pmd)) {
> +                               /* Check if we can move the pmd without splitting it. */
> +                               if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
> +                                   !pmd_none(dst_pmdval)) {
> +                                       if (pmd_present(*src_pmd)) {
> +                                               struct folio *folio = pmd_folio(*src_pmd);
> +
> +                                               if (!folio || (!is_huge_zero_folio(folio) &&
> +                                                              !PageAnonExclusive(&folio->page))) {
> +                                                       spin_unlock(ptl);
> +                                                       err = -EBUSY;
> +                                                       break;
> +                                               }
> +                                       }
>
> -                               if (!folio || (!is_huge_zero_folio(folio) &&
> -                                              !PageAnonExclusive(&folio->page))) {
>                                         spin_unlock(ptl);
> -                                       err = -EBUSY;
> -                                       break;
> +                                       split_huge_pmd(src_vma, src_pmd, src_addr);
> +                                       /* The folio will be split by move_pages_pte() */
> +                                       continue;
>                                 }
>
> +                               err = move_pages_huge_pmd(mm, dst_pmd, src_pmd,
> +                                                         dst_pmdval, dst_vma, src_vma,
> +                                                         dst_addr, src_addr);
> +                               step_size = HPAGE_PMD_SIZE;
> +                       } else {
>                                 spin_unlock(ptl);
> -                               split_huge_pmd(src_vma, src_pmd, src_addr);
> -                               /* The folio will be split by move_pages_pte() */
> -                               continue;
> +                               if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) {
> +                                       err = -ENOENT;
> +                                       break;
> +                               }
> +                               /* nothing to do to move a hole */
> +                               err = 0;
> +                               step_size = min(HPAGE_PMD_SIZE, src_start + len - src_addr);
>                         }
> -
> -                       err = move_pages_huge_pmd(mm, dst_pmd, src_pmd,
> -                                                 dst_pmdval, dst_vma, src_vma,
> -                                                 dst_addr, src_addr);
> -                       step_size = HPAGE_PMD_SIZE;
>                 } else {
>                         if (pmd_none(*src_pmd)) {
>                                 if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) {
>
> base-commit: 01da54f10fddf3b01c5a3b80f6b16bbad390c302
> --
> 2.50.1.552.g942d659e1b-goog
>