KASAN: null-ptr-deref Read in dlm_user_unlock

Status: moderation: reported on 2026/09/07 16:35
Subsystems: gfs2
Labels: prio:high
[Documentation on labels]
Reported-by: syzbot+da6dc573ce5e6624f505@syzkaller.appspotmail.com
First crash: 4d04h, last: 4d02h
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ID Workflow Result Correct Bug Created Started Finished Revision
7acfdd3e assessment-security DenialOfService: ✅ Exploitable: ❌ FilesystemTrigger: ❌ NetworkTrigger: ✅ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ❌ UserNamespace: ❌ VMGuestTrigger: ❌ VMHostTrigger: ❌ ❓ KASAN: null-ptr-deref Read in dlm_user_unlock 2026/09/05 00:01 2026/09/05 00:01 2026/09/05 01:24 7ce8fcf4

Sample crash report:
==================================================================
BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:82 [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 rsb_flag fs/dlm/dlm_internal.h:386 [inline]
BUG: KASAN: null-ptr-deref in hold_rsb fs/dlm/lock.c:334 [inline]
BUG: KASAN: null-ptr-deref in unlock_lock fs/dlm/lock.c:3333 [inline]
BUG: KASAN: null-ptr-deref in dlm_user_unlock+0x2ab/0x690 fs/dlm/lock.c:5956
Read of size 8 at addr 0000000000000050 by task syz.3.570/7893

CPU: 0 UID: 0 PID: 7893 Comm: syz.3.570 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/05/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 kasan_report+0x117/0x150 mm/kasan/report.c:595
 check_region_inline mm/kasan/generic.c:-1 [inline]
 kasan_check_range+0x264/0x2c0 mm/kasan/generic.c:200
 instrument_atomic_read include/linux/instrumented.h:82 [inline]
 _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
 rsb_flag fs/dlm/dlm_internal.h:386 [inline]
 hold_rsb fs/dlm/lock.c:334 [inline]
 unlock_lock fs/dlm/lock.c:3333 [inline]
 dlm_user_unlock+0x2ab/0x690 fs/dlm/lock.c:5956
 device_user_unlock+0x1ca/0x260 fs/dlm/user.c:321
 device_write+0x905/0xed0 fs/dlm/user.c:590
 vfs_write+0x296/0xba0 fs/read_write.c:685
 ksys_write+0x150/0x270 fs/read_write.c:739
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fe57459e159
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fe5753d1028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fe574826180 RCX: 00007fe57459e159
RDX: 0000000000000078 RSI: 0000200000000300 RDI: 000000000000000b
RBP: 00007fe574635024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fe574826218 R14: 00007fe574826180 R15: 00007ffc5e4d7fc8
 </TASK>
==================================================================

Crashes (2):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/09/03 17:34 upstream 940de590b839 6a988719 .config console log report info [disk image] [vmlinux] [kernel image] ci-upstream-kasan-gce-root KASAN: null-ptr-deref Read in dlm_user_unlock
2026/09/03 16:24 upstream 940de590b839 6a988719 .config console log report info [disk image] [vmlinux] [kernel image] ci-upstream-kasan-gce-selinux-root KASAN: null-ptr-deref Read in dlm_user_unlock
* Struck through repros no longer work on HEAD.