[syzbot] [io-uring?] KASAN: null-ptr-deref Read in io_sqe_buffer_register

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 4ac65880ebca Add linux-next specific files for 20250904
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1785fe62580000
kernel config: https://syzkaller.appspot.com/x/.config?x=fbc16d9faf3a88a4
dashboard link: https://syzkaller.appspot.com/bug?extid=1ab243d3eebb2aabf4a4
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=13f23e62580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12cb6312580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/36645a51612c/disk-4ac65880.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/bba80d634bef/vmlinux-4ac65880.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e58dd70dfd0f/bzImage-4ac65880.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+1ab243...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:68 [inline]
BUG: KASAN: null-ptr-deref in _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
BUG: KASAN: null-ptr-deref in PageCompound include/linux/page-flags.h:331 [inline]
BUG: KASAN: null-ptr-deref in io_buffer_account_pin io_uring/rsrc.c:668 [inline]
BUG: KASAN: null-ptr-deref in io_sqe_buffer_register+0x369/0x20a0 io_uring/rsrc.c:817
Read of size 8 at addr 0000000000000000 by task syz.0.17/6020

CPU: 0 UID: 0 PID: 6020 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
Call Trace:
<TASK>
dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
kasan_report+0x118/0x150 mm/kasan/report.c:595
check_region_inline mm/kasan/generic.c:-1 [inline]
kasan_check_range+0x2b0/0x2c0 mm/kasan/generic.c:200
instrument_atomic_read include/linux/instrumented.h:68 [inline]
_test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
PageCompound include/linux/page-flags.h:331 [inline]
io_buffer_account_pin io_uring/rsrc.c:668 [inline]
io_sqe_buffer_register+0x369/0x20a0 io_uring/rsrc.c:817
__io_sqe_buffers_update io_uring/rsrc.c:322 [inline]
__io_register_rsrc_update+0x55e/0x11b0 io_uring/rsrc.c:360
io_register_rsrc_update+0x196/0x1a0 io_uring/rsrc.c:391
__io_uring_register io_uring/register.c:736 [inline]
__do_sys_io_uring_register io_uring/register.c:926 [inline]
__se_sys_io_uring_register+0x795/0x11b0 io_uring/register.c:903
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f99b1f8ebe9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f99b2d88038 EFLAGS: 00000246 ORIG_RAX: 00000000000001ab
RAX: ffffffffffffffda RBX: 00007f99b21c5fa0 RCX: 00007f99b1f8ebe9
RDX: 00002000000003c0 RSI: 0000000000000010 RDI: 0000000000000003
RBP: 00007f99b2011e19 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000020 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f99b21c6038 R14: 00007f99b21c5fa0 R15: 00007ffeadfa5958
</TASK>
==================================================================


---
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#syz undup

[Jens Axboe]

This is from the mm-unstable changes in linux-next, adding David as I
ran a quick bisect and it said:

da6b34293ff8dbb78f8b9278c9a492925bbf1f87 is the first bad commit
commit da6b34293ff8dbb78f8b9278c9a492925bbf1f87
Author: David Hildenbrand <da...@redhat.com>
Date: Mon Sep 1 17:03:40 2025 +0200

mm/gup: remove record_subpages()

We can just cleanup the code by calculating the #refs earlier, so we can
just inline what remains of record_subpages().

Calculate the number of references/pages ahead of times, and record them
only once all our tests passed.

Link: https://lkml.kernel.org/r/20250901150359....@redhat.com
Signed-off-by: David Hildenbrand <da...@redhat.com>
Signed-off-by: Andrew Morton <ak...@linux-foundation.org>

I won't personally have time to look into this until after the weekend,
but as it's linux-next specific, not a huge deal right now.

Note that there's also a similar report, which is the same thing:

https://lore.kernel.org/all/68b9d130.a00a02...@google.com/

which I marked as dupe of this one.

--
Jens Axboe

[Andrew Morton]

On Thu, 4 Sep 2025 17:20:41 -0600 Jens Axboe <ax...@kernel.dk> wrote:

> > ==================================================================
>
> This is from the mm-unstable changes in linux-next, adding David as I
> ran a quick bisect and it said:
>
> da6b34293ff8dbb78f8b9278c9a492925bbf1f87 is the first bad commit
> commit da6b34293ff8dbb78f8b9278c9a492925bbf1f87
> Author: David Hildenbrand <da...@redhat.com>
> Date: Mon Sep 1 17:03:40 2025 +0200
>
> mm/gup: remove record_subpages()
>
> We can just cleanup the code by calculating the #refs earlier, so we can
> just inline what remains of record_subpages().
>
> Calculate the number of references/pages ahead of times, and record them
> only once all our tests passed.
>
> Link: https://lkml.kernel.org/r/20250901150359....@redhat.com
> Signed-off-by: David Hildenbrand <da...@redhat.com>
> Signed-off-by: Andrew Morton <ak...@linux-foundation.org>
>
> I won't personally have time to look into this until after the weekend,
> but as it's linux-next specific, not a huge deal right now.
>
> Note that there's also a similar report, which is the same thing:
>
> https://lore.kernel.org/all/68b9d130.a00a02...@google.com/
>
> which I marked as dupe of this one.

Thanks. Seems i can remove that patch without causing merge or build
damage, so I'll do that.

[David Hildenbrand]

Thanks, I shall go and figure out what I messed up there. I'll then
reply to the v2 of the patch with a fixup.

--
Cheers

David / dhildenb

[David Hildenbrand]

#syz test

From bfd07c995814354f6b66c5b6a72e96a7aa9fb73b Mon Sep 17 00:00:00 2001
From: David Hildenbrand <da...@redhat.com>
Date: Fri, 5 Sep 2025 08:38:43 +0200
Subject: [PATCH] fixup: mm/gup: remove record_subpages()

pages is not adjusted by the caller, but idnexed by existing *nr.

Signed-off-by: David Hildenbrand <da...@redhat.com>
---
mm/gup.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index 010fe56f6e132..22420f2069ee1 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2981,6 +2981,7 @@ static int gup_fast_pmd_leaf(pmd_t orig, pmd_t
*pmdp, unsigned long addr,
return 0;
}

+ pages += *nr;
*nr += refs;
for (; refs; refs--)
*(pages++) = page++;
@@ -3024,6 +3025,7 @@ static int gup_fast_pud_leaf(pud_t orig, pud_t
*pudp, unsigned long addr,
return 0;
}

+ pages += *nr;
*nr += refs;
for (; refs; refs--)
*(pages++) = page++;
--
2.50.1

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

failed to apply patch:
checking file mm/gup.c
patch: **** unexpected end of file in patch



Tested on:

commit: be5d4872 Add linux-next specific files for 20250905
git tree: linux-next

kernel config: https://syzkaller.appspot.com/x/.config?x=fbc16d9faf3a88a4
dashboard link: https://syzkaller.appspot.com/bug?extid=1ab243d3eebb2aabf4a4
compiler:

patch: https://syzkaller.appspot.com/x/patch.diff?x=1127e962580000

[David Hildenbrand]

#syz test https://github.com/davidhildenbrand/linux.git nth_page

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+1ab243...@syzkaller.appspotmail.com
Tested-by: syzbot+1ab243...@syzkaller.appspotmail.com

Tested on:

commit: bfd07c99 fixup: mm/gup: remove record_subpages()
git tree: https://github.com/davidhildenbrand/linux.git nth_page
console output: https://syzkaller.appspot.com/x/log.txt?x=15a5a134580000
kernel config: https://syzkaller.appspot.com/x/.config?x=366a4ffc91f4ab4
dashboard link: https://syzkaller.appspot.com/bug?extid=1ab243d3eebb2aabf4a4

compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

Note: no patches were applied.
Note: testing is done by a robot and is best-effort only.

[Andrew Morton]

On Fri, 5 Sep 2025 09:42:55 +0200 David Hildenbrand <da...@redhat.com> wrote:

> #syz test
>
> From bfd07c995814354f6b66c5b6a72e96a7aa9fb73b Mon Sep 17 00:00:00 2001
> From: David Hildenbrand <da...@redhat.com>
> Date: Fri, 5 Sep 2025 08:38:43 +0200
> Subject: [PATCH] fixup: mm/gup: remove record_subpages()
>
> pages is not adjusted by the caller, but idnexed by existing *nr.
>
> Signed-off-by: David Hildenbrand <da...@redhat.com>

Cool, I resurrected "mm/gup: remove record_subpages()" and added the -fix:

From: David Hildenbrand <da...@redhat.com>
Subject: fixup: mm/gup: remove record_subpages()
Date: Fri, 5 Sep 2025 08:38:43 +0200

pages is not adjusted by the caller, but indexed by existing *nr.

Link: https://lkml.kernel.org/r/cc7f03f8-da8b-407e...@redhat.com
Signed-off-by: David Hildenbrand <da...@redhat.com>
Reported-by: syzbot+1ab243...@syzkaller.appspotmail.com
Tested-by: syzbot+1ab243...@syzkaller.appspotmail.com
Reported-by: Jens Axboe <ax...@kernel.dk>
Cc: David Hildenbrand <da...@redhat.com>
Signed-off-by: Andrew Morton <ak...@linux-foundation.org>

---

mm/gup.c | 2 ++
1 file changed, 2 insertions(+)

--- a/mm/gup.c~mm-gup-remove-record_subpages-fix
+++ a/mm/gup.c
@@ -2966,6 +2966,7 @@ static int gup_fast_pmd_leaf(pmd_t orig,

return 0;
}

+ pages += *nr;
*nr += refs;
for (; refs; refs--)
*(pages++) = page++;

@@ -3009,6 +3010,7 @@ static int gup_fast_pud_leaf(pud_t orig,

return 0;
}

+ pages += *nr;
*nr += refs;
for (; refs; refs--)
*(pages++) = page++;

_