WARNING in kcm_write_msgs

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 4520f06b Linux 4.14.175
git tree: linux-4.14.y
console output: https://syzkaller.appspot.com/x/log.txt?x=17c8d807e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=93cf891381c0c347
dashboard link: https://syzkaller.appspot.com/bug?extid=886469e548a81296444b
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1573cd9fe00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15aaabb3e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+886469...@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: CPU: 0 PID: 8406 at net/kcm/kcmsock.c:629 kcm_write_msgs+0xf72/0x1740 net/kcm/kcmsock.c:629
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 8406 Comm: syz-executor414 Not tainted 4.14.175-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x13e/0x194 lib/dump_stack.c:58
panic+0x1f9/0x42d kernel/panic.c:183
__warn.cold+0x2f/0x30 kernel/panic.c:547
report_bug+0x20a/0x248 lib/bug.c:186
fixup_bug arch/x86/kernel/traps.c:177 [inline]
fixup_bug arch/x86/kernel/traps.c:172 [inline]
do_error_trap+0x195/0x2d0 arch/x86/kernel/traps.c:295
invalid_op+0x1b/0x40 arch/x86/entry/entry_64.S:963
RIP: 0010:kcm_write_msgs+0xf72/0x1740 net/kcm/kcmsock.c:629
RSP: 0018:ffff8880973679f8 EFLAGS: 00010297
RAX: ffff8880a4efe340 RBX: 00000000000000c0 RCX: 0000000000000007
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8880a5a41b42
RBP: ffff8880a5a41b40 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000008000
R13: ffff88808a69d380 R14: ffff8880a5a41a80 R15: 0000000000000000
kcm_sendmsg+0x1841/0x1df0 net/kcm/kcmsock.c:1035
sock_sendmsg_nosec net/socket.c:646 [inline]
sock_sendmsg+0xc5/0x100 net/socket.c:656
sock_write_iter+0x22c/0x370 net/socket.c:925
call_write_iter include/linux/fs.h:1778 [inline]
new_sync_write fs/read_write.c:469 [inline]
__vfs_write+0x44e/0x630 fs/read_write.c:482
vfs_write+0x192/0x4e0 fs/read_write.c:544
SYSC_write fs/read_write.c:590 [inline]
SyS_write+0xf2/0x210 fs/read_write.c:582
do_syscall_64+0x1d5/0x640 arch/x86/entry/common.c:292
entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x446b79
RSP: 002b:00007f2b8e2d3d98 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00000000006dbc48 RCX: 0000000000446b79
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000005
RBP: 00000000006dbc40 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dbc4c
R13: 00000000200003c0 R14: 00000000004aea28 R15: 000000000000d4f5
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches