* [PATCH v3 1/1] userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs
@ 2025-08-06 15:40 Suren Baghdasaryan
2025-08-06 16:56 ` Peter Xu
0 siblings, 1 reply; 6+ messages in thread
From: Suren Baghdasaryan @ 2025-08-06 15:40 UTC (permalink / raw)
To: akpm
Cc: peterx, david, aarcange, lokeshgidra, surenb, linux-mm,
linux-kernel, syzbot+b446dbe27035ef6bd6c2, stable
When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it
encounters a non-present PMD (migration entry), it proceeds with folio
access even though the folio is not present. Add the missing check and
let split_huge_pmd() handle migration entries.
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Reported-by: syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: stable@vger.kernel.org
---
Changes since v2 [1]
- Updated the title and changelog, per David Hildenbrand
- Removed extra checks for non-present not-migration PMD entries,
per Peter Xu
[1] https://lore.kernel.org/all/20250731154442.319568-1-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 || (!is_huge_zero_folio(folio) &&
+ !PageAnonExclusive(&folio->page))) {
+ spin_unlock(ptl);
+ err = -EBUSY;
+ break;
+ }
}
spin_unlock(ptl);
base-commit: 8e7e0c6d09502e44aa7a8fce0821e042a6ec03d1
--
2.50.1.565.gc32cd1483b-goog
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH v3 1/1] userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs
2025-08-06 15:40 [PATCH v3 1/1] userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs Suren Baghdasaryan
@ 2025-08-06 16:56 ` Peter Xu
2025-08-06 17:09 ` Suren Baghdasaryan
0 siblings, 1 reply; 6+ messages in thread
From: Peter Xu @ 2025-08-06 16:56 UTC (permalink / raw)
To: Suren Baghdasaryan
Cc: akpm, david, aarcange, lokeshgidra, linux-mm, linux-kernel,
syzbot+b446dbe27035ef6bd6c2, stable
On Wed, Aug 06, 2025 at 08:40:15AM -0700, Suren Baghdasaryan wrote:
> When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it
The migration entry can appear with/without ALLOW_SRC_HOLES, right? Maybe
drop this line?
If we need another repost, the subject can further be tailored to mention
migration entry too rather than non-present. IMHO that's clearer on
explaining the issue this patch is fixing (e.g. a valid transhuge THP can
also have present bit cleared).
> encounters a non-present PMD (migration entry), it proceeds with folio
> access even though the folio is not present. Add the missing check and
IMHO "... even though folio is not present" is pretty vague. Maybe
"... even though it's a swap entry"? Fundamentally it's because of the
different layouts of normal THP v.s. a swap entry, hence pmd_folio() should
not be used on top of swap entries.
> let split_huge_pmd() handle migration entries.
>
> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Reported-by: syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Cc: stable@vger.kernel.org
> ---
> Changes since v2 [1]
> - Updated the title and changelog, per David Hildenbrand
> - Removed extra checks for non-present not-migration PMD entries,
> per Peter Xu
>
> [1] https://lore.kernel.org/all/20250731154442.319568-1-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 || (!is_huge_zero_folio(folio) &&
> + !PageAnonExclusive(&folio->page))) {
> + spin_unlock(ptl);
> + err = -EBUSY;
> + break;
> + }
> }
The change itself looks all correct, thanks. If you agree with above
commit message / subject updates, feel free to take this after some
amendment of the commit message:
Reviewed-by: Peter Xu <peterx@redhat.com>
>
> spin_unlock(ptl);
>
> base-commit: 8e7e0c6d09502e44aa7a8fce0821e042a6ec03d1
> --
> 2.50.1.565.gc32cd1483b-goog
>
--
Peter Xu
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v3 1/1] userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs
2025-08-06 16:56 ` Peter Xu
@ 2025-08-06 17:09 ` Suren Baghdasaryan
2025-08-06 18:09 ` Peter Xu
0 siblings, 1 reply; 6+ messages in thread
From: Suren Baghdasaryan @ 2025-08-06 17:09 UTC (permalink / raw)
To: Peter Xu
Cc: akpm, david, aarcange, lokeshgidra, linux-mm, linux-kernel,
syzbot+b446dbe27035ef6bd6c2, stable
On Wed, Aug 6, 2025 at 9:56 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Wed, Aug 06, 2025 at 08:40:15AM -0700, Suren Baghdasaryan wrote:
> > When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it
>
> The migration entry can appear with/without ALLOW_SRC_HOLES, right? Maybe
> drop this line?
Yes, you are right. I'll update.
>
> If we need another repost, the subject can further be tailored to mention
> migration entry too rather than non-present. IMHO that's clearer on
> explaining the issue this patch is fixing (e.g. a valid transhuge THP can
> also have present bit cleared).
>
> > encounters a non-present PMD (migration entry), it proceeds with folio
> > access even though the folio is not present. Add the missing check and
>
> IMHO "... even though folio is not present" is pretty vague. Maybe
> "... even though it's a swap entry"? Fundamentally it's because of the
> different layouts of normal THP v.s. a swap entry, hence pmd_folio() should
> not be used on top of swap entries.
Well, technically a migration entry is a non_swap_entry(), so calling
migration entries "swap entries" is confusing to me. Any better
wording we can use or do you think that's ok?
>
> > let split_huge_pmd() handle migration entries.
> >
> > Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> > Reported-by: syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com
> > Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > Cc: stable@vger.kernel.org
> > ---
> > Changes since v2 [1]
> > - Updated the title and changelog, per David Hildenbrand
> > - Removed extra checks for non-present not-migration PMD entries,
> > per Peter Xu
> >
> > [1] https://lore.kernel.org/all/20250731154442.319568-1-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 || (!is_huge_zero_folio(folio) &&
> > + !PageAnonExclusive(&folio->page))) {
> > + spin_unlock(ptl);
> > + err = -EBUSY;
> > + break;
> > + }
> > }
>
> The change itself looks all correct, thanks. If you agree with above
> commit message / subject updates, feel free to take this after some
> amendment of the commit message:
>
> Reviewed-by: Peter Xu <peterx@redhat.com>
>
> >
> > spin_unlock(ptl);
> >
> > base-commit: 8e7e0c6d09502e44aa7a8fce0821e042a6ec03d1
> > --
> > 2.50.1.565.gc32cd1483b-goog
> >
>
> --
> Peter Xu
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v3 1/1] userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs
2025-08-06 17:09 ` Suren Baghdasaryan
@ 2025-08-06 18:09 ` Peter Xu
2025-08-06 18:21 ` Suren Baghdasaryan
0 siblings, 1 reply; 6+ messages in thread
From: Peter Xu @ 2025-08-06 18:09 UTC (permalink / raw)
To: Suren Baghdasaryan
Cc: akpm, david, aarcange, lokeshgidra, linux-mm, linux-kernel,
syzbot+b446dbe27035ef6bd6c2, stable
On Wed, Aug 06, 2025 at 10:09:30AM -0700, Suren Baghdasaryan wrote:
> On Wed, Aug 6, 2025 at 9:56 AM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Wed, Aug 06, 2025 at 08:40:15AM -0700, Suren Baghdasaryan wrote:
> > > When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it
> >
> > The migration entry can appear with/without ALLOW_SRC_HOLES, right? Maybe
> > drop this line?
>
> Yes, you are right. I'll update.
>
> >
> > If we need another repost, the subject can further be tailored to mention
> > migration entry too rather than non-present. IMHO that's clearer on
> > explaining the issue this patch is fixing (e.g. a valid transhuge THP can
> > also have present bit cleared).
> >
> > > encounters a non-present PMD (migration entry), it proceeds with folio
> > > access even though the folio is not present. Add the missing check and
> >
> > IMHO "... even though folio is not present" is pretty vague. Maybe
> > "... even though it's a swap entry"? Fundamentally it's because of the
> > different layouts of normal THP v.s. a swap entry, hence pmd_folio() should
> > not be used on top of swap entries.
>
> Well, technically a migration entry is a non_swap_entry(), so calling
> migration entries "swap entries" is confusing to me. Any better
> wording we can use or do you think that's ok?
The more general definition of "swap entry" should follow what swp_entry_t
is defined, where, for example, is_migration_entry() itself takes
swp_entry_t as input. So it should be fine, but I agree it's indeed
confusing.
If we want to make it clearer, IMHO we could rename non_swap_entry()
instead to is_swapfile_entry() / is_real_swap_entry() / ... but that can be
discussed separately. Here, if we want to make it super accurate, we could
also use "swp_entry_t" instead of "swap entry", that'll be 100% accurate.
Thanks,
--
Peter Xu
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v3 1/1] userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs
2025-08-06 18:09 ` Peter Xu
@ 2025-08-06 18:21 ` Suren Baghdasaryan
2025-08-06 22:11 ` Suren Baghdasaryan
0 siblings, 1 reply; 6+ messages in thread
From: Suren Baghdasaryan @ 2025-08-06 18:21 UTC (permalink / raw)
To: Peter Xu
Cc: akpm, david, aarcange, lokeshgidra, linux-mm, linux-kernel,
syzbot+b446dbe27035ef6bd6c2, stable
On Wed, Aug 6, 2025 at 11:09 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Wed, Aug 06, 2025 at 10:09:30AM -0700, Suren Baghdasaryan wrote:
> > On Wed, Aug 6, 2025 at 9:56 AM Peter Xu <peterx@redhat.com> wrote:
> > >
> > > On Wed, Aug 06, 2025 at 08:40:15AM -0700, Suren Baghdasaryan wrote:
> > > > When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it
> > >
> > > The migration entry can appear with/without ALLOW_SRC_HOLES, right? Maybe
> > > drop this line?
> >
> > Yes, you are right. I'll update.
> >
> > >
> > > If we need another repost, the subject can further be tailored to mention
> > > migration entry too rather than non-present. IMHO that's clearer on
> > > explaining the issue this patch is fixing (e.g. a valid transhuge THP can
> > > also have present bit cleared).
> > >
> > > > encounters a non-present PMD (migration entry), it proceeds with folio
> > > > access even though the folio is not present. Add the missing check and
> > >
> > > IMHO "... even though folio is not present" is pretty vague. Maybe
> > > "... even though it's a swap entry"? Fundamentally it's because of the
> > > different layouts of normal THP v.s. a swap entry, hence pmd_folio() should
> > > not be used on top of swap entries.
> >
> > Well, technically a migration entry is a non_swap_entry(), so calling
> > migration entries "swap entries" is confusing to me. Any better
> > wording we can use or do you think that's ok?
>
> The more general definition of "swap entry" should follow what swp_entry_t
> is defined, where, for example, is_migration_entry() itself takes
> swp_entry_t as input. So it should be fine, but I agree it's indeed
> confusing.
>
> If we want to make it clearer, IMHO we could rename non_swap_entry()
> instead to is_swapfile_entry() / is_real_swap_entry() / ... but that can be
> discussed separately. Here, if we want to make it super accurate, we could
> also use "swp_entry_t" instead of "swap entry", that'll be 100% accurate.
Ok, that I think is our best option. I'll post an update shortly.
Thanks!
>
> Thanks,
>
> --
> Peter Xu
>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH v3 1/1] userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs
2025-08-06 18:21 ` Suren Baghdasaryan
@ 2025-08-06 22:11 ` Suren Baghdasaryan
0 siblings, 0 replies; 6+ messages in thread
From: Suren Baghdasaryan @ 2025-08-06 22:11 UTC (permalink / raw)
To: Peter Xu
Cc: akpm, david, aarcange, lokeshgidra, linux-mm, linux-kernel,
syzbot+b446dbe27035ef6bd6c2, stable
On Wed, Aug 6, 2025 at 11:21 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Wed, Aug 6, 2025 at 11:09 AM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Wed, Aug 06, 2025 at 10:09:30AM -0700, Suren Baghdasaryan wrote:
> > > On Wed, Aug 6, 2025 at 9:56 AM Peter Xu <peterx@redhat.com> wrote:
> > > >
> > > > On Wed, Aug 06, 2025 at 08:40:15AM -0700, Suren Baghdasaryan wrote:
> > > > > When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it
> > > >
> > > > The migration entry can appear with/without ALLOW_SRC_HOLES, right? Maybe
> > > > drop this line?
> > >
> > > Yes, you are right. I'll update.
> > >
> > > >
> > > > If we need another repost, the subject can further be tailored to mention
> > > > migration entry too rather than non-present. IMHO that's clearer on
> > > > explaining the issue this patch is fixing (e.g. a valid transhuge THP can
> > > > also have present bit cleared).
> > > >
> > > > > encounters a non-present PMD (migration entry), it proceeds with folio
> > > > > access even though the folio is not present. Add the missing check and
> > > >
> > > > IMHO "... even though folio is not present" is pretty vague. Maybe
> > > > "... even though it's a swap entry"? Fundamentally it's because of the
> > > > different layouts of normal THP v.s. a swap entry, hence pmd_folio() should
> > > > not be used on top of swap entries.
> > >
> > > Well, technically a migration entry is a non_swap_entry(), so calling
> > > migration entries "swap entries" is confusing to me. Any better
> > > wording we can use or do you think that's ok?
> >
> > The more general definition of "swap entry" should follow what swp_entry_t
> > is defined, where, for example, is_migration_entry() itself takes
> > swp_entry_t as input. So it should be fine, but I agree it's indeed
> > confusing.
> >
> > If we want to make it clearer, IMHO we could rename non_swap_entry()
> > instead to is_swapfile_entry() / is_real_swap_entry() / ... but that can be
> > discussed separately. Here, if we want to make it super accurate, we could
> > also use "swp_entry_t" instead of "swap entry", that'll be 100% accurate.
>
> Ok, that I think is our best option. I'll post an update shortly.
Posted at https://lore.kernel.org/all/20250806220022.926763-1-surenb@google.com/
Thanks!
> Thanks!
>
> >
> > Thanks,
> >
> > --
> > Peter Xu
> >
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2025-08-06 22:12 UTC | newest]
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2025-08-06 15:40 [PATCH v3 1/1] userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs Suren Baghdasaryan
2025-08-06 16:56 ` Peter Xu
2025-08-06 17:09 ` Suren Baghdasaryan
2025-08-06 18:09 ` Peter Xu
2025-08-06 18:21 ` Suren Baghdasaryan
2025-08-06 22:11 ` Suren Baghdasaryan
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