Re: [PATCH v5 1/1] userfaultfd: fix a crash in UFFDIO_MOVE when PMD is a migration entry

[David Hildenbrand]

On 07.08.25 22:04, Suren Baghdasaryan wrote:
> When UFFDIO_MOVE encounters a migration PMD entry, it proceeds with
> obtaining a folio and accessing it even though the entry is swp_entry_t.
> Add the missing check and let split_huge_pmd() handle migration entries.
> While at it also remove unnecessary folio check.
>
> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Reported-by: syzbot+b446dbe27035ef6bd6c2@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@xxxxxxxxxx/
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> Applies to mm-unstable after reverting older v4 [1] version.
>
> Changes since v4 [1]
> - Removed extra folio check, per David Hildenbrand
>
> [1] https://lore.kernel.org/all/20250806220022.926763-1-surenb@xxxxxxxxxx/
>
>   mm/userfaultfd.c | 17 ++++++++++-------
>   1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 5431c9dd7fd7..aefdf3a812a1 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 (!is_huge_zero_folio(folio) &&
> +					    !PageAnonExclusive(&folio->page)) {
> +						spin_unlock(ptl);
> +						err = -EBUSY;
> +						break;
> +					}

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb