syzbot

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refcount_t: addition on 0; use-after-free.
WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x111/0x130 lib/refcount.c:25, CPU#2: syz.0.446/6575
Modules linked in:
CPU: 2 UID: 0 PID: 6575 Comm: syz.0.446 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
RIP: 0010:refcount_warn_saturate+0x111/0x130 lib/refcount.c:25
Code: 06 e8 33 cf 1b fd 48 8d 3d 9c 65 f7 0b 67 48 0f b9 3a e8 22 cf 1b fd 5b 5d c3 cc cc cc cc e8 16 cf 1b fd 48 8d 3d 8f 65 f7 0b <67> 48 0f b9 3a e8 05 cf 1b fd 5b 5d c3 cc cc cc cc 48 89 df e8 56
RSP: 0018:ffffc90007257d38 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffff88802b513cc8 RCX: ffffffff84eb3e7b
RDX: ffff88803236c900 RSI: ffffffff84eb3f2a RDI: ffffffff90e2a4c0
RBP: 0000000000000002 R08: 0000000000000005 R09: 0000000000000004
R10: 0000000000000002 R11: 0000000000000000 R12: 0000000000000000
R13: ffff88802b513cc8 R14: ffff88802b513c00 R15: ffff88802b74a000
FS:  0000555566a7a500(0000) GS:ffff8880d659d000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f88453e42f8 CR3: 0000000049682000 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:285 [inline]
 ax25_setsockopt+0x10d7/0x1230 net/ax25/af_ax25.c:705
 do_sock_setsockopt+0xf3/0x1d0 net/socket.c:2322
 __sys_setsockopt+0x195/0x220 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+0x106/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f8a7099bf79
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:00007ffdcbe48d78 EFLAGS: 00000246 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 00007f8a70c15fa0 RCX: 00007f8a7099bf79
RDX: 0000000000000019 RSI: 0000000000000101 RDI: 0000000000000009
RBP: 00007f8a70a327e0 R08: 0000000000000010 R09: 0000000000000000
R10: 00002000000001c0 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f8a70c15fac R14: 00007f8a70c15fa0 R15: 00007f8a70c15fa0
 </TASK>
----------------
Code disassembly (best guess), 1 bytes skipped:
   0:	e8 33 cf 1b fd       	call   0xfd1bcf38
   5:	48 8d 3d 9c 65 f7 0b 	lea    0xbf7659c(%rip),%rdi        # 0xbf765a8
   c:	67 48 0f b9 3a       	ud1    (%edx),%rdi
  11:	e8 22 cf 1b fd       	call   0xfd1bcf38
  16:	5b                   	pop    %rbx
  17:	5d                   	pop    %rbp
  18:	c3                   	ret
  19:	cc                   	int3
  1a:	cc                   	int3
  1b:	cc                   	int3
  1c:	cc                   	int3
  1d:	e8 16 cf 1b fd       	call   0xfd1bcf38
  22:	48 8d 3d 8f 65 f7 0b 	lea    0xbf7658f(%rip),%rdi        # 0xbf765b8
* 29:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2e:	e8 05 cf 1b fd       	call   0xfd1bcf38
  33:	5b                   	pop    %rbx
  34:	5d                   	pop    %rbp
  35:	c3                   	ret
  36:	cc                   	int3
  37:	cc                   	int3
  38:	cc                   	int3
  39:	cc                   	int3
  3a:	48 89 df             	mov    %rbx,%rdi
  3d:	e8                   	.byte 0xe8
  3e:	56                   	push   %rsi