Re: [syzbot] [bluetooth?] KASAN: slab-use-after-free Read in l2cap_sock_new_connection_cb

[syzbot <syzbot+cdae834448ec8c3602fe@syzkaller.appspotmail.com>]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
possible deadlock in l2cap_sock_shutdown

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.2.19/6669 is trying to acquire lock:
ffff88803a345588 (&chan->lock/2){+.+.}-{4:4}, at: l2cap_chan_lock include/net/bluetooth/l2cap.h:827 [inline]
ffff88803a345588 (&chan->lock/2){+.+.}-{4:4}, at: l2cap_sock_shutdown+0x9f3/0x1130 net/bluetooth/l2cap_sock.c:1374

but task is already holding lock:
ffffffff8e9e3238 (ra_lock){+.+.}-{4:4}, at: class_mutex_constructor include/linux/mutex.h:228 [inline]
ffffffff8e9e3238 (ra_lock){+.+.}-{4:4}, at: l2cap_sock_release+0x5e/0x260 net/bluetooth/l2cap_sock.c:1427

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (ra_lock){+.+.}-{4:4}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       __mutex_lock_common kernel/locking/rtmutex_api.c:535 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:547
       class_mutex_constructor include/linux/mutex.h:228 [inline]
       l2cap_sock_new_connection_cb+0x4e/0x300 net/bluetooth/l2cap_sock.c:1477
       l2cap_connect_cfm+0x37a/0x1040 net/bluetooth/l2cap_core.c:7288
       hci_connect_cfm+0x95/0x140 include/net/bluetooth/hci_core.h:2107
       le_conn_complete_evt+0xfb8/0x1500 net/bluetooth/hci_event.c:5799
       hci_le_conn_complete_evt+0x187/0x450 net/bluetooth/hci_event.c:5825
       hci_event_func net/bluetooth/hci_event.c:7586 [inline]
       hci_event_packet+0x78f/0x1200 net/bluetooth/hci_event.c:7643
       hci_rx_work+0x46a/0xe80 net/bluetooth/hci_core.c:4099
       process_one_work kernel/workqueue.c:3263 [inline]
       process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3346
       worker_thread+0x8a0/0xda0 kernel/workqueue.c:3427
       kthread+0x711/0x8a0 kernel/kthread.c:463
       ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #0 (&chan->lock/2){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3165 [inline]
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3908
       __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
       __mutex_lock_common kernel/locking/rtmutex_api.c:535 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:547
       l2cap_chan_lock include/net/bluetooth/l2cap.h:827 [inline]
       l2cap_sock_shutdown+0x9f3/0x1130 net/bluetooth/l2cap_sock.c:1374
       l2cap_sock_release+0x82/0x260 net/bluetooth/l2cap_sock.c:1432
       __sock_release net/socket.c:662 [inline]
       sock_close+0xc3/0x240 net/socket.c:1455
       __fput+0x45b/0xa80 fs/file_table.c:468
       task_work_run+0x1d4/0x260 kernel/task_work.c:227
       resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
       exit_to_user_mode_loop+0xe9/0x130 kernel/entry/common.c:43
       exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
       syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
       syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
       do_syscall_64+0x2bd/0xfa0 arch/x86/entry/syscall_64.c:100
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(ra_lock);
                               lock(&chan->lock/2);
                               lock(ra_lock);
  lock(&chan->lock/2);

 *** DEADLOCK ***

2 locks held by syz.2.19/6669:
 #0: ffff8880480cd6f8 (&sb->s_type->i_mutex_key#10){+.+.}-{4:4}, at: inode_lock include/linux/fs.h:980 [inline]
 #0: ffff8880480cd6f8 (&sb->s_type->i_mutex_key#10){+.+.}-{4:4}, at: __sock_release net/socket.c:661 [inline]
 #0: ffff8880480cd6f8 (&sb->s_type->i_mutex_key#10){+.+.}-{4:4}, at: sock_close+0x9b/0x240 net/socket.c:1455
 #1: ffffffff8e9e3238 (ra_lock){+.+.}-{4:4}, at: class_mutex_constructor include/linux/mutex.h:228 [inline]
 #1: ffffffff8e9e3238 (ra_lock){+.+.}-{4:4}, at: l2cap_sock_release+0x5e/0x260 net/bluetooth/l2cap_sock.c:1427

stack backtrace:
CPU: 1 UID: 0 PID: 6669 Comm: syz.2.19 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_circular_bug+0x2ee/0x310 kernel/locking/lockdep.c:2043
 check_noncircular+0x134/0x160 kernel/locking/lockdep.c:2175
 check_prev_add kernel/locking/lockdep.c:3165 [inline]
 check_prevs_add kernel/locking/lockdep.c:3284 [inline]
 validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3908
 __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
 lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
 __mutex_lock_common kernel/locking/rtmutex_api.c:535 [inline]
 mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:547
 l2cap_chan_lock include/net/bluetooth/l2cap.h:827 [inline]
 l2cap_sock_shutdown+0x9f3/0x1130 net/bluetooth/l2cap_sock.c:1374
 l2cap_sock_release+0x82/0x260 net/bluetooth/l2cap_sock.c:1432
 __sock_release net/socket.c:662 [inline]
 sock_close+0xc3/0x240 net/socket.c:1455
 __fput+0x45b/0xa80 fs/file_table.c:468
 task_work_run+0x1d4/0x260 kernel/task_work.c:227
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 exit_to_user_mode_loop+0xe9/0x130 kernel/entry/common.c:43
 exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
 do_syscall_64+0x2bd/0xfa0 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc22a98f749
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:00007fff1d93f9c8 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: 0000000000000000 RBX: 00007fc22abe7da0 RCX: 00007fc22a98f749
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 00007fc22abe7da0 R08: 0000000000000000 R09: 000000141d93fcbf
R10: 000000000003fd20 R11: 0000000000000246 R12: 000000000002b06f
R13: 00007fff1d93fac0 R14: ffffffffffffffff R15: 00007fff1d93fae0
 </TASK>


Tested on:

commit:         ac3fd01e Linux 6.18-rc7
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=155ec57c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=38a0c4cddc846161
dashboard link: https://syzkaller.appspot.com/bug?extid=cdae834448ec8c3602fe
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=1280cf42580000