[syzbot] [bluetooth?] KASAN: null-ptr-deref Read in l2cap_chan_hold_unless_zero

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: e22254e9ddd8 Merge tag 'xfs-fixes-7.2-rc4' of git://git.ke..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13d46789580000
kernel config: https://syzkaller.appspot.com/x/.config?x=48ef5c5c0f192153
dashboard link: https://syzkaller.appspot.com/bug?extid=e6382a2f53f5fc7453ac
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=17b434b9580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17d46789580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-e22254e9.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c69a30d7d78f/vmlinux-e22254e9.xz
kernel image: https://storage.googleapis.com/syzbot-assets/f15a346cc7e1/bzImage-e22254e9.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e6382a...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:82 [inline]
BUG: KASAN: null-ptr-deref in atomic_read include/linux/atomic/atomic-instrumented.h:32 [inline]
BUG: KASAN: null-ptr-deref in refcount_read include/linux/refcount.h:170 [inline]
BUG: KASAN: null-ptr-deref in __refcount_add_not_zero include/linux/refcount.h:176 [inline]
BUG: KASAN: null-ptr-deref in __refcount_inc_not_zero include/linux/refcount.h:317 [inline]
BUG: KASAN: null-ptr-deref in refcount_inc_not_zero include/linux/refcount.h:335 [inline]
BUG: KASAN: null-ptr-deref in kref_get_unless_zero include/linux/kref.h:133 [inline]
BUG: KASAN: null-ptr-deref in l2cap_chan_hold_unless_zero+0x80/0x290 net/bluetooth/l2cap_core.c:511
Read of size 4 at addr 0000000000000008 by task syz.0.158/6397

CPU: 2 UID: 0 PID: 6397 Comm: syz.0.158 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
kasan_report+0xdf/0x1c0 mm/kasan/report.c:595
check_region_inline mm/kasan/generic.c:186 [inline]
kasan_check_range+0x10f/0x1e0 mm/kasan/generic.c:200
instrument_atomic_read include/linux/instrumented.h:82 [inline]
atomic_read include/linux/atomic/atomic-instrumented.h:32 [inline]
refcount_read include/linux/refcount.h:170 [inline]
__refcount_add_not_zero include/linux/refcount.h:176 [inline]
__refcount_inc_not_zero include/linux/refcount.h:317 [inline]
refcount_inc_not_zero include/linux/refcount.h:335 [inline]
kref_get_unless_zero include/linux/kref.h:133 [inline]
l2cap_chan_hold_unless_zero+0x80/0x290 net/bluetooth/l2cap_core.c:511
l2cap_sock_cleanup_listen+0x8a/0x4d0 net/bluetooth/l2cap_sock.c:1535
l2cap_sock_release+0x69/0x280 net/bluetooth/l2cap_sock.c:1480
__sock_release+0xb3/0x260 net/socket.c:710
sock_close+0x1c/0x30 net/socket.c:1501
__fput+0x3ff/0xb50 fs/file_table.c:512
task_work_run+0x150/0x240 kernel/task_work.c:233
resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
__exit_to_user_mode_loop kernel/entry/common.c:70 [inline]
exit_to_user_mode_loop+0x1d8/0x6f0 kernel/entry/common.c:101
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:230 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
do_syscall_64+0x666/0x870 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f3758d9de99
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:00007fff659f88f8 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: 0000000000000000 RBX: 00007f3759027da0 RCX: 00007f3758d9de99
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 00007f3759027da0 R08: 0000000000000006 R09: 0000000000000000
R10: 00007f3759027cb0 R11: 0000000000000246 R12: 0000000000046482
R13: 00007f375902609c R14: 00000000000461ac R15: 00007f3759026090
</TASK>
==================================================================


---
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[Hillf Danton]

> Date: Mon, 20 Jul 2026 16:56:27 -0700

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: e22254e9ddd8 Merge tag 'xfs-fixes-7.2-rc4' of git://git.ke..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=13d46789580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=48ef5c5c0f192153
> dashboard link: https://syzkaller.appspot.com/bug?extid=e6382a2f53f5fc7453ac
> compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=17b434b9580000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17d46789580000

#syz test

--- x/net/bluetooth/l2cap_sock.c
+++ y/net/bluetooth/l2cap_sock.c
@@ -1532,7 +1532,9 @@ static void l2cap_sock_cleanup_listen(st
struct l2cap_chan *chan;

lock_sock_nested(sk, L2CAP_NESTING_NORMAL);
- chan = l2cap_chan_hold_unless_zero(l2cap_pi(sk)->chan);
+ chan = l2cap_pi(sk)->chan;
+ if (chan)
+ chan = l2cap_chan_hold_unless_zero(l2cap_pi(sk)->chan);
release_sock(sk);
if (!chan) {
/* l2cap_conn_del() already tearing this child down */
--

[syzbot]

Hello,

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

Reported-by: syzbot+e6382a...@syzkaller.appspotmail.com
Tested-by: syzbot+e6382a...@syzkaller.appspotmail.com

Tested on:

commit: b95f03f0 Merge tag 'mm-hotfixes-stable-2026-07-20-11-3..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=123b8ab9580000

kernel config: https://syzkaller.appspot.com/x/.config?x=48ef5c5c0f192153
dashboard link: https://syzkaller.appspot.com/bug?extid=e6382a2f53f5fc7453ac
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44

patch: https://syzkaller.appspot.com/x/patch.diff?x=13b912b9580000

Note: testing is done by a robot and is best-effort only.