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 bt_accept_dequeue

============================================
WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
syz.3.20/6474 is trying to acquire lock:
ffff888028ff2358 (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1679 [inline]
ffff888028ff2358 (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}-{0:0}, at: bt_accept_dequeue+0x100/0x5b0 net/bluetooth/af_bluetooth.c:279

but task is already holding lock:
ffff88803b4e8358 (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1679 [inline]
ffff88803b4e8358 (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}-{0:0}, at: l2cap_sock_release+0x5f/0x230 net/bluetooth/l2cap_sock.c:1425

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP);
  lock(sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

2 locks held by syz.3.20/6474:
 #0: ffff88803d4c70f8 (&sb->s_type->i_mutex_key#10){+.+.}-{4:4}, at: inode_lock include/linux/fs.h:980 [inline]
 #0: ffff88803d4c70f8 (&sb->s_type->i_mutex_key#10){+.+.}-{4:4}, at: __sock_release net/socket.c:661 [inline]
 #0: ffff88803d4c70f8 (&sb->s_type->i_mutex_key#10){+.+.}-{4:4}, at: sock_close+0x9b/0x240 net/socket.c:1455
 #1: ffff88803b4e8358 (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1679 [inline]
 #1: ffff88803b4e8358 (sk_lock-AF_BLUETOOTH-BTPROTO_L2CAP){+.+.}-{0:0}, at: l2cap_sock_release+0x5f/0x230 net/bluetooth/l2cap_sock.c:1425

stack backtrace:
CPU: 0 UID: 0 PID: 6474 Comm: syz.3.20 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_deadlock_bug+0x28b/0x2a0 kernel/locking/lockdep.c:3041
 check_deadlock kernel/locking/lockdep.c:3093 [inline]
 validate_chain+0x1a3f/0x2140 kernel/locking/lockdep.c:3895
 __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
 lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
 lock_sock_nested+0x3e/0x130 net/core/sock.c:3720
 lock_sock include/net/sock.h:1679 [inline]
 bt_accept_dequeue+0x100/0x5b0 net/bluetooth/af_bluetooth.c:279
 l2cap_sock_cleanup_listen+0x2f/0x3e0 net/bluetooth/l2cap_sock.c:1454
 l2cap_sock_release+0x67/0x230 net/bluetooth/l2cap_sock.c:1426
 __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: 0033:0x7fe17a36f749
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:00007fff3eea6588 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: 0000000000000000 RBX: 00007fe17a5c7da0 RCX: 00007fe17a36f749
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 00007fe17a5c7da0 R08: 0000000000000000 R09: 000000143eea687f
R10: 000000000003fd20 R11: 0000000000000246 R12: 000000000002a91f
R13: 00007fff3eea6680 R14: ffffffffffffffff R15: 00007fff3eea66a0
 </TASK>


Tested on:

commit:         ac3fd01e Linux 6.18-rc7
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12c97612580000
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=100f4f42580000