[v6.1] KASAN: null-ptr-deref Read in drop_buffers

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 228da13e907e Linux 6.1.175
git tree: linux-6.1.y
console output: https://syzkaller.appspot.com/x/log.txt?x=17920f2e580000
kernel config: https://syzkaller.appspot.com/x/.config?x=4dd3c1715f0a309b
dashboard link: https://syzkaller.appspot.com/bug?extid=838f9bf5573c02c8592a
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/a1730af56ab4/disk-228da13e.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/eafe9b37a422/vmlinux-228da13e.xz
kernel image: https://storage.googleapis.com/syzbot-assets/7353ac7f337a/bzImage-228da13e.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+838f9b...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:72 [inline]
BUG: KASAN: null-ptr-deref in atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]
BUG: KASAN: null-ptr-deref in buffer_busy fs/buffer.c:2784 [inline]
BUG: KASAN: null-ptr-deref in drop_buffers+0x65/0x6a0 fs/buffer.c:2796
Read of size 4 at addr 0000000000000060 by task kswapd0/83

CPU: 0 PID: 83 Comm: kswapd0 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
Call Trace:
<TASK>
dump_stack_lvl+0x188/0x24e lib/dump_stack.c:106
kasan_report+0x10b/0x140 mm/kasan/report.c:524
check_region_inline mm/kasan/generic.c:-1 [inline]
kasan_check_range+0x235/0x290 mm/kasan/generic.c:189
instrument_atomic_read include/linux/instrumented.h:72 [inline]
atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]
buffer_busy fs/buffer.c:2784 [inline]
drop_buffers+0x65/0x6a0 fs/buffer.c:2796
try_to_free_buffers+0x230/0x560 fs/buffer.c:2830
shrink_folio_list+0x2127/0x7aa0 mm/vmscan.c:2003
evict_folios+0xa80/0x22a0 mm/vmscan.c:5046
lru_gen_shrink_lruvec mm/vmscan.c:5229 [inline]
shrink_lruvec+0xc90/0x3d50 mm/vmscan.c:5924
shrink_node_memcgs mm/vmscan.c:6145 [inline]
shrink_node+0x680/0x28d0 mm/vmscan.c:6176
kswapd_shrink_node mm/vmscan.c:6968 [inline]
balance_pgdat mm/vmscan.c:7158 [inline]
kswapd+0x1c3e/0x3360 mm/vmscan.c:7418
kthread+0x29d/0x330 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
</TASK>
==================================================================
Kernel panic - not syncing: KASAN: panic_on_warn set ...
CPU: 0 PID: 83 Comm: kswapd0 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
Call Trace:
<TASK>
dump_stack_lvl+0x188/0x24e lib/dump_stack.c:106
panic+0x2e5/0x730 kernel/panic.c:339
check_panic_on_warn+0x80/0xa0 kernel/panic.c:235
end_report+0x66/0x110 mm/kasan/report.c:200
kasan_report+0x118/0x140 mm/kasan/report.c:526
check_region_inline mm/kasan/generic.c:-1 [inline]
kasan_check_range+0x235/0x290 mm/kasan/generic.c:189
instrument_atomic_read include/linux/instrumented.h:72 [inline]
atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]
buffer_busy fs/buffer.c:2784 [inline]
drop_buffers+0x65/0x6a0 fs/buffer.c:2796
try_to_free_buffers+0x230/0x560 fs/buffer.c:2830
shrink_folio_list+0x2127/0x7aa0 mm/vmscan.c:2003
evict_folios+0xa80/0x22a0 mm/vmscan.c:5046
lru_gen_shrink_lruvec mm/vmscan.c:5229 [inline]
shrink_lruvec+0xc90/0x3d50 mm/vmscan.c:5924
shrink_node_memcgs mm/vmscan.c:6145 [inline]
shrink_node+0x680/0x28d0 mm/vmscan.c:6176
kswapd_shrink_node mm/vmscan.c:6968 [inline]
balance_pgdat mm/vmscan.c:7158 [inline]
kswapd+0x1c3e/0x3360 mm/vmscan.c:7418
kthread+0x29d/0x330 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
</TASK>
Kernel Offset: disabled


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup