BUG: unable to handle kernel NULL pointer dereference in l2cap_chan_put

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 961f830a Linux 4.19.138
git tree: linux-4.19.y
console output: https://syzkaller.appspot.com/x/log.txt?x=12278102900000
kernel config: https://syzkaller.appspot.com/x/.config?x=b89fed56745db52a
dashboard link: https://syzkaller.appspot.com/bug?extid=3a16a5803c261156be39
compiler: gcc (GCC) 10.1.0-syz 20200507
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12495026900000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+3a16a5...@syzkaller.appspotmail.com

BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
PGD 9a7e9067 P4D 9a7e9067 PUD 91aea067 PMD 0
Oops: 0002 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 6464 Comm: kworker/1:0 Not tainted 4.19.138-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events l2cap_chan_timeout
RIP: 0010:refcount_dec_and_test arch/x86/include/asm/refcount.h:76 [inline]
RIP: 0010:kref_put include/linux/kref.h:69 [inline]
RIP: 0010:l2cap_chan_put+0x10/0x1c0 net/bluetooth/l2cap_core.c:493
Code: ff e8 94 80 1e fb e9 fe fe ff ff 48 89 df e8 b7 7d 1e fb e9 ac fe ff ff 66 90 41 56 41 55 41 54 55 48 89 fd 53 e8 f0 5e e8 fa <f0> ff 4d 18 0f 88 11 37 b9 00 0f 94 c3 31 ff 89 de e8 0a 60 e8 fa
RSP: 0018:ffff888089c6fcb0 EFLAGS: 00010293
RAX: ffff8880923943c0 RBX: 0000000000000001 RCX: ffffffff8684aa52
RDX: 0000000000000000 RSI: ffffffff868150d0 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffff88808e438dc0
R13: ffff888096049478 R14: ffff88808e439228 R15: ffff8880ae730200
FS: 0000000000000000(0000) GS:ffff8880ae700000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000018 CR3: 000000008bc8f000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
l2cap_sock_kill.part.0+0x34/0x80 net/bluetooth/l2cap_sock.c:1053
l2cap_sock_kill net/bluetooth/l2cap_sock.c:1312 [inline]
l2cap_sock_close_cb+0xa7/0xd0 net/bluetooth/l2cap_sock.c:1311
l2cap_chan_timeout+0x1bb/0x210 net/bluetooth/l2cap_core.c:431
process_one_work+0x864/0x1570 kernel/workqueue.c:2155
worker_thread+0x64c/0x1130 kernel/workqueue.c:2298
kthread+0x30b/0x410 kernel/kthread.c:246
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:415
Modules linked in:
CR2: 0000000000000018
---[ end trace be7e3718029e52dc ]---
RIP: 0010:refcount_dec_and_test arch/x86/include/asm/refcount.h:76 [inline]
RIP: 0010:kref_put include/linux/kref.h:69 [inline]
RIP: 0010:l2cap_chan_put+0x10/0x1c0 net/bluetooth/l2cap_core.c:493
Code: ff e8 94 80 1e fb e9 fe fe ff ff 48 89 df e8 b7 7d 1e fb e9 ac fe ff ff 66 90 41 56 41 55 41 54 55 48 89 fd 53 e8 f0 5e e8 fa <f0> ff 4d 18 0f 88 11 37 b9 00 0f 94 c3 31 ff 89 de e8 0a 60 e8 fa
RSP: 0018:ffff888089c6fcb0 EFLAGS: 00010293
RAX: ffff8880923943c0 RBX: 0000000000000001 RCX: ffffffff8684aa52
RDX: 0000000000000000 RSI: ffffffff868150d0 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffff88808e438dc0
R13: ffff888096049478 R14: ffff88808e439228 R15: ffff8880ae730200
FS: 0000000000000000(0000) GS:ffff8880ae700000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000018 CR3: 000000008bc8f000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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