linux-kernel - Re: [PATCH v2 1/1] userfaultfd: fix a crash when UFFDIO

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Message-ID: <d2b6be85-44d5-4a87-bfe5-4a9e80f95bb8@redhat.com>
Date: Fri, 1 Aug 2025 09:21:30 +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 v2 1/1] userfaultfd: fix a crash when UFFDIO_MOVE handles
 a THP hole

On 31.07.25 17:44, Suren Baghdasaryan wrote:

Hi!

Did you mean in you patch description:

"userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs"

Talking about THP holes is very very confusing.

> 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

You mean a "non-present PMD that is not a migration entry".

> hole and tries to access a non-existent folio, resulting in
> a crash. Add a check to skip non-present THPs.

That makes sense. The code we have after this patch is rather 
complicated and hard to read.

> 
> 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 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;
> +						}
> +					}

... in particular that. Is there some way to make this code simpler / 
easier to read? Like moving that whole last folio-check thingy into a 
helper?


-- 
Cheers,

David / dhildenb