general protection fault in skb_unlink

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    0044cdeb7313 Merge branch 'for-linus' of git://git.kernel...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=157e3d2f800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=968b0b23c7854c0b
dashboard link: https://syzkaller.appspot.com/bug?extid=278279efdd2730dd14bf
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=16939b2f800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1046c54f800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+278279efdd2730dd14bf@xxxxxxxxxxxxxxxxxxxxxxxxx

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 8484 Comm: syz-executor919 Not tainted 4.17.0-rc7+ #74
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:__skb_unlink include/linux/skbuff.h:1844 [inline]
RIP: 0010:skb_unlink+0xc1/0x160 net/core/skbuff.c:2921
RSP: 0018:ffff8801d012f6f0 EFLAGS: 00010002
RAX: 0000000000000286 RBX: ffff8801d6e073c0 RCX: 0000000000000001
RDX: dffffc0000000000 RSI: 0000000000000004 RDI: 0000000000000008
RBP: ffff8801d012f718 R08: ffffed0038bb3b6d R09: ffffed0038bb3b6c
R10: ffffed0038bb3b6c R11: ffff8801c5d9db63 R12: 0000000000000000
R13: 0000000000000000 R14: ffff8801c5d9db60 R15: ffff8801d012fce0
FS:  0000000000ab7880(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020e5b000 CR3: 00000001c31fb000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 kcm_recvmsg+0x48d/0x590 net/kcm/kcmsock.c:1160
 sock_recvmsg_nosec+0x8c/0xb0 net/socket.c:802
 ___sys_recvmsg+0x2b6/0x680 net/socket.c:2279
 __sys_recvmmsg+0x2f9/0xb80 net/socket.c:2391
 do_sys_recvmmsg+0xe4/0x190 net/socket.c:2472
 __do_sys_recvmmsg net/socket.c:2485 [inline]
 __se_sys_recvmmsg net/socket.c:2481 [inline]
 __x64_sys_recvmmsg+0xbe/0x150 net/socket.c:2481
 do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4417a9
RSP: 002b:00007ffe27282838 EFLAGS: 00000206 ORIG_RAX: 000000000000012b
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004417a9
RDX: 00000000040000f7 RSI: 00000000200002c0 RDI: 0000000000000006
RBP: 0000000000000000 R08: 0000000020000200 R09: 00007ffe272829f8
R10: 0000000000000060 R11: 0000000000000206 R12: 00000000000001f3
R13: 000000000001f871 R14: 0000000000000000 R15: 0000000000000000
Code: 00 00 00 49 8d 7d 08 4c 8b 63 08 48 ba 00 00 00 00 00 fc ff df 48 c7  
43 08 00 00 00 00 48 89 f9 48 c7 03 00 00 00 00 48 c1 e9 03 <80> 3c 11 00  
75 5b 4c 89 e1 4d 89 65 08 48 ba 00 00 00 00 00 fc
RIP: __skb_unlink include/linux/skbuff.h:1844 [inline] RSP: ffff8801d012f6f0
RIP: skb_unlink+0xc1/0x160 net/core/skbuff.c:2921 RSP: ffff8801d012f6f0
---[ end trace 7338a6b7e627a89e ]---


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