linux-kernel - Re: [PATCH 1/1] userfaultfd: fix a crash when UFFDIO

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Message-ID: <CA+EESO4mkiedqVMCV3fEnB-xeBMKyct1_WA=YDFVbqSGU4F+6A@mail.gmail.com>
Date: Thu, 31 Jul 2025 00:35:25 -0700
From: Lokesh Gidra <lokeshgidra@...gle.com>
To: Hillf Danton <hdanton@...a.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org, peterx@...hat.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	syzkaller-bugs@...glegroups.com, 
	syzbot <syzbot+b446dbe27035ef6bd6c2@...kaller.appspotmail.com>, 
	stable@...r.kernel.org
Subject: Re: [PATCH 1/1] userfaultfd: fix a crash when UFFDIO_MOVE handles a
 THP hole

On Wed, Jul 30, 2025 at 6:58 PM Hillf Danton <hdanton@...a.com> wrote:
>
> #syz test
>
> 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.
>
Thanks Suren for promptly addressing this issue.

> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Reported-by: syzbot+b446dbe27035ef6bd6c2@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> Cc: stable@...r.kernel.org
> ---
>  mm/userfaultfd.c | 38 +++++++++++++++++++++++---------------
>  1 file changed, 23 insertions(+), 15 deletions(-)
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index cbed91b09640..60be8080ddd0 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1818,27 +1818,35 @@ 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);
> +                       } else {
> +                               /* nothing to do to move a hole */
>                                 spin_unlock(ptl);
> -                               split_huge_pmd(src_vma, src_pmd, src_addr);
> -                               /* The folio will be split by move_pages_pte() */
> -                               continue;
> +                               err = 0;
I think we need to act here depending on whether
UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES is set or not.

           err = (mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES) ? 0 : -ENOENT;

Also, IMO, the step_size in this case should be the minimum of
remaining length and HPAGE_PMD_SIZE.
>                         }
> -
> -                       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)) {
I have a related question/doubt: why do we populate the page-table
hierarchy on the src side [1] (and then also at line 1857) when a hole
is found? IMHO, it shouldn't be needed. Depending on whether
UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES is set or not, it should either
return -ENOENT, or continue past the hole. Please correct me if I'm
wrong.

[1] https://elixir.bootlin.com/linux/v6.16/source/mm/userfaultfd.c#L1797

>
> base-commit: 01da54f10fddf3b01c5a3b80f6b16bbad390c302
> --
> 2.50.1.552.g942d659e1b-goog