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

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Message-ID: <aIzMGlrR1SL5Y_Gp@x1.local>
Date: Fri, 1 Aug 2025 10:15:54 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
	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 Fri, Aug 01, 2025 at 09:21:30AM +0200, David Hildenbrand wrote:
> 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)) {

[1]

> > +				/* 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?

One question might be relevant is, whether the check above [1] can be
dropped.

The thing is __pmd_trans_huge_lock() does double check the pmd to be !none
before returning the ptl.  I didn't follow closely on the recent changes on
mm side on possible new pmd swap entries, if migration is the only possible
one then it looks like [1] can be avoided.

And it also looks applicable to also drop the "else" later, because in "if
(ptl)" it cannot hit pmd_none().

Thanks,

-- 
Peter Xu