Re: WARNING in kcm_write_msgs

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    29f3490b net: use indirect call wrappers for skb_copy_data..
git tree:       net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=12876b91e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8d5bbd990b8ab319
dashboard link: https://syzkaller.appspot.com/bug?extid=52624bdfbf2746d37d70
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1394bd39e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10da50ade00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+52624bdfbf2746d37d70@xxxxxxxxxxxxxxxxxxxxxxxxx

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WARNING: CPU: 1 PID: 11822 at net/kcm/kcmsock.c:628 kcm_write_msgs+0xfb6/0x1760 net/kcm/kcmsock.c:628
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 11822 Comm: syz-executor408 Not tainted 5.6.0-rc5-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+0x188/0x20d lib/dump_stack.c:118
 panic+0x2e3/0x75c kernel/panic.c:221
 __warn.cold+0x2f/0x35 kernel/panic.c:582
 report_bug+0x27b/0x2f0 lib/bug.c:195
 fixup_bug arch/x86/kernel/traps.c:174 [inline]
 fixup_bug arch/x86/kernel/traps.c:169 [inline]
 do_error_trap+0x12b/0x220 arch/x86/kernel/traps.c:267
 do_invalid_op+0x32/0x40 arch/x86/kernel/traps.c:286
 invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1027
RIP: 0010:kcm_write_msgs+0xfb6/0x1760 net/kcm/kcmsock.c:628
Code: fa 48 c1 ea 03 80 3c 02 00 0f 85 5e 05 00 00 48 8b 44 24 50 45 31 e4 48 8b 74 24 08 48 89 70 10 e9 95 f4 ff ff e8 7a 8a 53 fa <0f> 0b 41 bc ea ff ff ff e9 83 f4 ff ff e8 68 8a 53 fa 0f 0b e9 1c
RSP: 0018:ffffc900015e7978 EFLAGS: 00010293
RAX: ffff888085688240 RBX: ffff88808571e800 RCX: ffffffff871e8c3c
RDX: 0000000000000000 RSI: ffffffff871e9496 RDI: 0000000000000001
RBP: 0000000000000000 R08: ffff888085688240 R09: fffffbfff185158c
R10: fffffbfff185158b R11: ffffffff8c28ac5f R12: 0000000000008000
R13: ffff8880a65b96c0 R14: 00000000000000c0 R15: ffff8880a6fe6d40
 kcm_sendmsg+0x1b67/0x2129 net/kcm/kcmsock.c:1034
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:672
 sock_write_iter+0x283/0x3c0 net/socket.c:1004
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write+0x49c/0x700 fs/read_write.c:483
 __vfs_write+0xc9/0x100 fs/read_write.c:496
 vfs_write+0x262/0x5c0 fs/read_write.c:558
 ksys_write+0x1e8/0x250 fs/read_write.c:611
 do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x446b79
Code: e8 0c e8 ff ff 48 83 c4 18 c3 0f 1f 80 00 00 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 0f 83 5b 07 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f2e8f3e5d98 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..