linux-kernel - Re: [PATCH v3 1/1] userfaultfd: fix a crash in UFFDIO

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Message-ID: <aJOJI-YZ0TTxEzV9@x1.local>
Date: Wed, 6 Aug 2025 12:56:03 -0400
From: Peter Xu <peterx@...hat.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, aarcange@...hat.com,
	lokeshgidra@...gle.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	syzbot+b446dbe27035ef6bd6c2@...kaller.appspotmail.com,
	stable@...r.kernel.org
Subject: Re: [PATCH v3 1/1] userfaultfd: fix a crash in UFFDIO_MOVE with some
 non-present PMDs

On Wed, Aug 06, 2025 at 08:40:15AM -0700, Suren Baghdasaryan wrote:
> When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it

The migration entry can appear with/without ALLOW_SRC_HOLES, right?  Maybe
drop this line?

If we need another repost, the subject can further be tailored to mention
migration entry too rather than non-present.  IMHO that's clearer on
explaining the issue this patch is fixing (e.g. a valid transhuge THP can
also have present bit cleared).

> encounters a non-present PMD (migration entry), it proceeds with folio
> access even though the folio is not present. Add the missing check and

IMHO "... even though folio is not present" is pretty vague.  Maybe
"... even though it's a swap entry"?  Fundamentally it's because of the
different layouts of normal THP v.s. a swap entry, hence pmd_folio() should
not be used on top of swap entries.

> let split_huge_pmd() handle migration entries.
> 
> 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
> ---
> Changes since v2 [1]
> - Updated the title and changelog, per David Hildenbrand
> - Removed extra checks for non-present not-migration PMD entries,
> per Peter Xu
> 
> [1] https://lore.kernel.org/all/20250731154442.319568-1-surenb@google.com/
> 
>  mm/userfaultfd.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 5431c9dd7fd7..116481606be8 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1826,13 +1826,16 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
>  			/* 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 (!folio || (!is_huge_zero_folio(folio) &&
> -					       !PageAnonExclusive(&folio->page))) {
> -					spin_unlock(ptl);
> -					err = -EBUSY;
> -					break;
> +				/* Can be a migration entry */
> +				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;
> +					}
>  				}

The change itself looks all correct, thanks.  If you agree with above
commit message / subject updates, feel free to take this after some
amendment of the commit message:

Reviewed-by: Peter Xu <peterx@...hat.com>

>  
>  				spin_unlock(ptl);
> 
> base-commit: 8e7e0c6d09502e44aa7a8fce0821e042a6ec03d1
> -- 
> 2.50.1.565.gc32cd1483b-goog
> 

-- 
Peter Xu