------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: lib/refcount.c:25 at 0x0, CPU#0: syz.3.1078/7204
Modules linked in:
CPU: 0 UID: 0 PID: 7204 Comm: syz.3.1078 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
RIP: 0010:refcount_warn_saturate+0xf9/0x120 lib/refcount.c:25
Code: 5d e9 7b 86 12 fd e8 76 86 12 fd 48 8d 3d 9f 9b e5 0b 67 48 0f b9 3a 5b 5d e9 63 86 12 fd e8 5e 86 12 fd 48 8d 3d 97 9b e5 0b <67> 48 0f b9 3a 5b 5d e9 4b 86 12 fd 48 89 ef e8 f3 5b 7c fd e9 35
RSP: 0018:ffffc9000ce4fd30 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000000002 RCX: ffffffff84ac5ccc
RDX: ffff8880214f24c0 RSI: ffffffff84ac5d62 RDI: ffffffff9091f900
RBP: ffff8880253a24c8 R08: 0000000000000005 R09: 0000000000000004
R10: 0000000000000002 R11: ffff8880214f2ff0 R12: ffff8880253a24c8
R13: ffff8880253a2400 R14: 0000000000000008 R15: ffff8880250f4000
FS:  00005555675b2500(0000) GS:ffff8880d68fa000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f8fcc02eb94 CR3: 000000002864e000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 __refcount_add include/linux/refcount.h:289 [inline]
 __refcount_inc include/linux/refcount.h:366 [inline]
 refcount_inc include/linux/refcount.h:383 [inline]
 ax25_dev_hold include/net/ax25.h:289 [inline]
 ax25_setsockopt+0x10b3/0x1220 net/ax25/af_ax25.c:705
 do_sock_setsockopt+0xf3/0x1d0 net/socket.c:2322
 __sys_setsockopt+0x1a0/0x230 net/socket.c:2347
 __do_sys_setsockopt net/socket.c:2353 [inline]
 __se_sys_setsockopt net/socket.c:2350 [inline]
 __x64_sys_setsockopt+0xbd/0x160 net/socket.c:2350
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f8fcbf8f7c9
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:00007fff7e4d0698 EFLAGS: 00000246 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 00007f8fcc1e5fa0 RCX: 00007f8fcbf8f7c9
RDX: 0000000000000019 RSI: 0000000000000101 RDI: 0000000000000004
RBP: 00007f8fcc013f91 R08: 0000000000000010 R09: 0000000000000000
R10: 0000200000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f8fcc1e5fa0 R14: 00007f8fcc1e5fa0 R15: 0000000000000005
 </TASK>
----------------
Code disassembly (best guess):
   0:	5d                   	pop    %rbp
   1:	e9 7b 86 12 fd       	jmp    0xfd128681
   6:	e8 76 86 12 fd       	call   0xfd128681
   b:	48 8d 3d 9f 9b e5 0b 	lea    0xbe59b9f(%rip),%rdi        # 0xbe59bb1
  12:	67 48 0f b9 3a       	ud1    (%edx),%rdi
  17:	5b                   	pop    %rbx
  18:	5d                   	pop    %rbp
  19:	e9 63 86 12 fd       	jmp    0xfd128681
  1e:	e8 5e 86 12 fd       	call   0xfd128681
  23:	48 8d 3d 97 9b e5 0b 	lea    0xbe59b97(%rip),%rdi        # 0xbe59bc1
* 2a:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2f:	5b                   	pop    %rbx
  30:	5d                   	pop    %rbp
  31:	e9 4b 86 12 fd       	jmp    0xfd128681
  36:	48 89 ef             	mov    %rbp,%rdi
  39:	e8 f3 5b 7c fd       	call   0xfd7c5c31
  3e:	e9                   	.byte 0xe9
  3f:	35                   	.byte 0x35