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Message-ID: <CAJuCfpFSY3fDH36dabS=nGzasZJ6FtQ_jv79eFWVZrEWRMMTiQ@mail.gmail.com>
Date: Fri, 1 Aug 2025 19:30:02 +0000
From: Suren Baghdasaryan <surenb@...gle.com>
To: Peter Xu <peterx@...hat.com>
Cc: David Hildenbrand <david@...hat.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 1, 2025 at 6:21 PM Peter Xu <peterx@...hat.com> wrote:
>
> On Fri, Aug 01, 2025 at 05:45:10PM +0000, Suren Baghdasaryan wrote:
> > On Fri, Aug 1, 2025 at 5:13 PM Peter Xu <peterx@...hat.com> wrote:
> > >
> > > On Fri, Aug 01, 2025 at 09:41:31AM -0700, Suren Baghdasaryan wrote:
> > > > On Fri, Aug 1, 2025 at 9:23 AM Peter Xu <peterx@...hat.com> wrote:
> > > > >
> > > > > On Fri, Aug 01, 2025 at 08:28:38AM -0700, Suren Baghdasaryan wrote:
> > > > > > On Fri, Aug 1, 2025 at 7:16 AM Peter Xu <peterx@...hat.com> wrote:
> > > > > > >
> > > > > > > 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)) {
> > > > >
> > > > > [2]
> > > > >
> > > > > > > > > + struct folio *folio = pmd_folio(*src_pmd);
> > > > >
> > > > > [3]
> > > > >
> > > > > > > > > +
> > > > > > > > > + 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.
> > > > > >
> > > > > > Hi Peter,
> > > > > > is_swap_pmd() check in __pmd_trans_huge_lock() allows for (!pmd_none()
> > > > > > && !pmd_present()) PMD to pass and that's when this crash is hit.
> > > > >
> > > > > First for all, thanks for looking into the issue with Lokesh; I am still
> > > > > catching up with emails after taking weeks off.
> > > > >
> > > > > I didn't yet read into the syzbot report, but I thought the bug was about
> > > > > referencing the folio on top of a swap entry after reading your current
> > > > > patch, which has:
> > > > >
> > > > > if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
> > > > > !pmd_none(dst_pmdval)) {
> > > > > struct folio *folio = pmd_folio(*src_pmd); <----
> > > > >
> > > > > Here looks like *src_pmd can be a migration entry. Is my understanding
> > > > > correct?
> > > >
> > > > Correct.
> > > >
> > > > >
> > > > > > If we drop the check at [1] then the path that takes us to
> > > > >
> > > > > If my above understanding is correct, IMHO it should be [2] above that
> > > > > makes sure the reference won't happen on a swap entry, not necessarily [1]?
> > > >
> > > > Yes, in case of migration entry this is what protects us.
> > > >
> > > > >
> > > > > > split_huge_pmd() will bail out inside split_huge_pmd_locked() with no
> > > > > > indication that split did not happen. Afterwards we will retry
> > > > >
> > > > > So we're talking about the case where it's a swap pmd entry, right?
> > > >
> > > > Hmm, my understanding is that it's being treated as a swap entry but
> > > > in reality is not. I thought THPs are always split before they get
> > > > swapped, no?
> > >
> > > Yes they should be split, afaiu.
> > >
> > > >
> > > > > Could you elaborate why the split would fail?
> > > >
> > > > Just looking at the code, split_huge_pmd_locked() checks for
> > > > (pmd_trans_huge(*pmd) || is_pmd_migration_entry(*pmd)).
> > > > pmd_trans_huge() is false if !pmd_present() and it's not a migration
> > > > entry, so __split_huge_pmd_locked() will be skipped.
> > >
> > > Here might be the major part of where confusion came from: I thought it
> > > must be a migration pmd entry to hit the issue, so it's not?
> > >
> > > I checked the code just now:
> > >
> > > __handle_mm_fault:
> > > if (unlikely(is_swap_pmd(vmf.orig_pmd))) {
> > > VM_BUG_ON(thp_migration_supported() &&
> > > !is_pmd_migration_entry(vmf.orig_pmd));
> > >
> > > So IIUC pmd migration entry is still the only possible way to have a swap
> > > entry. It doesn't look like we have "real" swap entries for PMD (which can
> > > further points to some swapfiles)?
> >
> > Correct. AFAIU here we stumble on a pmd entry which was allocated but
> > never populated.
>
> Do you mean a pmd_none()?
Yes.
>
> If so, that goes back to my original question, on why
> __pmd_trans_huge_lock() returns non-NULL if it's a pmd_none()? IMHO it
> really should have returned NULL for pmd_none().
That was exactly the answer I gave Lokesh when he theorized about the
cause of this crash but after reproducing it I saw that
pmd_trans_huge_lock() happily returns the PTL as long as PMD is not
pmd_none(). And that's because it passes as is_swap_pmd(). But even if
we change that we still need to implement the code to skip the entire
PMD.
>
> IOW, I still don't understand why below won't already work:
>
> ===8<===
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 52d7d5f144b8e..33e78f52ee9f5 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1880,13 +1880,15 @@ 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;
> + 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;
> + }
> }
>
> spin_unlock(ptl);
>
> ===8<===
>
> Likely I missed something important.. I'll be afk for a while soon, I'll
> also double check (maybe early next week) on the reproducer.
>
> Thanks,
>
> --
> Peter Xu
>
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