Re: WARNING: refcount bug in j1939_netdev_start

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    9805a683 Merge branch 'x86-urgent-for-linus' of git://git...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=133de01ce00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=896c87b73c6fcda6
dashboard link: https://syzkaller.appspot.com/bug?extid=afd421337a736d6c1ee6
compiler:       clang version 9.0.0 (/home/glider/llvm/clang  
80fee25776c2fb61e74c1ecb1a523375c2500b69)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10d713c6e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+afd421337a736d6c1ee6@xxxxxxxxxxxxxxxxxxxxxxxxx

------------[ cut here ]------------
refcount_t: increment on 0; use-after-free.
WARNING: CPU: 1 PID: 8514 at lib/refcount.c:156  
refcount_inc_checked+0x4b/0x50 lib/refcount.c:156
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 8514 Comm: syz-executor.3 Not tainted 5.4.0-rc6+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1fb/0x318 lib/dump_stack.c:118
 panic+0x264/0x7a9 kernel/panic.c:221
 __warn+0x20e/0x210 kernel/panic.c:582
 report_bug+0x1b6/0x2f0 lib/bug.c:195
 fixup_bug arch/x86/kernel/traps.c:179 [inline]
 do_error_trap+0xd7/0x440 arch/x86/kernel/traps.c:272
 do_invalid_op+0x36/0x40 arch/x86/kernel/traps.c:291
 invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1028
RIP: 0010:refcount_inc_checked+0x4b/0x50 lib/refcount.c:156
Code: 3d f7 ab 75 05 01 75 08 e8 12 e2 2d fe 5b 5d c3 e8 0a e2 2d fe c6 05  
e1 ab 75 05 01 48 c7 c7 b4 c5 40 88 31 c0 e8 a5 78 00 fe <0f> 0b eb df 90  
55 48 89 e5 41 57 41 56 41 55 41 54 53 48 83 ec 10
RSP: 0018:ffff8880816dfd10 EFLAGS: 00010246
RAX: 8358cd98639ffe00 RBX: ffff8880975b50f0 RCX: ffff8880a4010540
RDX: 0000000000000000 RSI: 0000000080000000 RDI: 0000000000000000
RBP: ffff8880816dfd18 R08: ffffffff815ca054 R09: ffffed1015d640d2
R10: ffffed1015d640d2 R11: 0000000000000000 R12: ffff88808e820588
R13: dffffc0000000000 R14: ffff88808e820000 R15: 1ffff11011d04047
 j1939_netdev_start+0x47c/0x730 net/can/j1939/main.c:267
 j1939_sk_bind+0x2c0/0xac0 net/can/j1939/socket.c:438
 __sys_bind+0x2c2/0x3a0 net/socket.c:1647
 __do_sys_bind net/socket.c:1658 [inline]
 __se_sys_bind net/socket.c:1656 [inline]
 __x64_sys_bind+0x7a/0x90 net/socket.c:1656
 do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45a219
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 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 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007efc734cbc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000031
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000045a219
RDX: 0000000000000018 RSI: 0000000020000240 RDI: 0000000000000005
RBP: 000000000075c070 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007efc734cc6d4
R13: 00000000004c057e R14: 00000000004d2c50 R15: 00000000ffffffff
Kernel Offset: disabled
Rebooting in 86400 seconds..