Re: WARNING: refcount bug in j1939_netdev_start (2)

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    f8788d86 Linux 5.6-rc3
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12a93c2de00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5d2e033af114153f
dashboard link: https://syzkaller.appspot.com/bug?extid=85d9878b19c94f9019ad
compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=137fe929e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+85d9878b19c94f9019ad@xxxxxxxxxxxxxxxxxxxxxxxxx

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refcount_t: addition on 0; use-after-free.
WARNING: CPU: 0 PID: 8790 at lib/refcount.c:25 refcount_warn_saturate+0x147/0x1b0 lib/refcount.c:25
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 8790 Comm: syz-executor.4 Not tainted 5.6.0-rc3-syzkaller #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+0x209/0x210 kernel/panic.c:582
 report_bug+0x1b6/0x2f0 lib/bug.c:195
 fixup_bug arch/x86/kernel/traps.c:174 [inline]
 do_error_trap+0xcf/0x1c0 arch/x86/kernel/traps.c:267
 do_invalid_op+0x36/0x40 arch/x86/kernel/traps.c:286
 invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1027
RIP: 0010:refcount_warn_saturate+0x147/0x1b0 lib/refcount.c:25
Code: c7 ed e2 f0 88 31 c0 e8 d7 2d a8 fd 0f 0b eb a1 e8 0e 68 d6 fd c6 05 c7 31 c5 05 01 48 c7 c7 24 e3 f0 88 31 c0 e8 b9 2d a8 fd <0f> 0b eb 83 e8 f0 67 d6 fd c6 05 aa 31 c5 05 01 48 c7 c7 50 e3 f0
RSP: 0018:ffffc900024e7d00 EFLAGS: 00010246
RAX: 11ed52ed4a02e700 RBX: 0000000000000002 RCX: ffff888097334280
RDX: 0000000000000000 RSI: 0000000080000000 RDI: 0000000000000000
RBP: ffffc900024e7d10 R08: ffffffff81600324 R09: ffffed1015d44592
R10: ffffed1015d44592 R11: 0000000000000000 R12: 1ffff11011de2046
R13: ffff8880a6558000 R14: 0000000000000002 R15: 0000000000000005
 j1939_netdev_start+0x83a/0x920 include/linux/refcount.h:191
 j1939_sk_bind+0x2ae/0xa90 net/can/j1939/socket.c:469
 __sys_bind+0x2bd/0x3a0 net/socket.c:1662
 __do_sys_bind net/socket.c:1673 [inline]
 __se_sys_bind net/socket.c:1671 [inline]
 __x64_sys_bind+0x7a/0x90 net/socket.c:1671
 do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45c449
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:00007f439cc91c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000031
RAX: ffffffffffffffda RBX: 00007f439cc926d4 RCX: 000000000045c449
RDX: 0000000000000018 RSI: 0000000020000240 RDI: 0000000000000005
RBP: 000000000076bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000000030 R14: 00000000004c28fe R15: 000000000076bf2c
Kernel Offset: disabled
Rebooting in 86400 seconds..