linux-kernel - Re: [PATCH v4 1/1] userfaultfd: fix a crash in UFFDIO

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Message-ID: <3eba855a-740c-4423-b2ed-24d622af29a5@redhat.com>
Date: Thu, 7 Aug 2025 12:30:59 +0200
From: David Hildenbrand <david@...hat.com>
To: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
Cc: peterx@...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 v4 1/1] userfaultfd: fix a crash in UFFDIO_MOVE when PMD
 is a migration entry

On 07.08.25 00:00, 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.
> 
> 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>
> Reviewed-by: Peter Xu <peterx@...hat.com>
> Cc: stable@...r.kernel.org
> ---
> Changes since v3 [1]
> - Updated the title and changelog, per Peter Xu
> - Added Reviewed-by: per Peter Xu
> 
> [1] https://lore.kernel.org/all/20250806154015.769024-1-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


How could you get !folio here? That only makes sense when calling 
vm_normal_folio_pmd(), no?


-- 
Cheers,

David / dhildenb