general protection fault in rds_sendmsg

Hello,

syzbot hit the following crash on net-next commit
0ea5f58ebc83edfd13b0b254fbbc4ac191351c18 (Wed Feb 21 22:51:04 2018 +0000)
Merge branch 'ipvlan-deps'

So far this crash happened 2 times on net-next.
C reproducer is attached.
syzkaller reproducer is attached.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.

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Reported-by: syzbot+f893ae7bb2f6456dfbc3@syzkaller.appspotmail.com
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audit: type=1400 audit(1519323871.266:6): avc:  denied  { map } for   
pid=4129 comm="bash" path="/bin/bash" dev="sda1" ino=1457  
scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023  
tcontext=system_u:object_r:file_t:s0 tclass=file permissive=1
audit: type=1400 audit(1519323877.529:7): avc:  denied  { map } for   
pid=4143 comm="syzkaller165096" path="/root/syzkaller165096144" dev="sda1"  
ino=16481 scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023  
tcontext=unconfined_u:object_r:user_home_t:s0 tclass=file permissive=1
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: 4143 Comm: syzkaller165096 Not tainted 4.16.0-rc2+ #237
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:rds_cmsg_zcopy net/rds/send.c:947 [inline]
RIP: 0010:rds_cmsg_send net/rds/send.c:994 [inline]
RIP: 0010:rds_sendmsg+0x10fd/0x2550 net/rds/send.c:1167
RSP: 0018:ffff8801b5f7f690 EFLAGS: 00010202
RAX: 0000000000000004 RBX: ffff8801b5f7f978 RCX: ffffffff8530dbed
RDX: 000000003f1c97ab RSI: dffffc0000000000 RDI: 0000000000000020
RBP: ffff8801b5f7f9a0 R08: 0000000000000000 R09: 1ffff10036befe3a
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801b5f7fb10
R13: ffff8801b5f7fd30 R14: ffff8801b1ca0000 R15: 0000000000000000
FS:  0000000001769880(0000) GS:ffff8801db400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055c7207450f0 CR3: 00000001b599a005 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  sock_sendmsg_nosec net/socket.c:629 [inline]
  sock_sendmsg+0xca/0x110 net/socket.c:639
  ___sys_sendmsg+0x767/0x8b0 net/socket.c:2047
  __sys_sendmsg+0xe5/0x210 net/socket.c:2081
  SYSC_sendmsg net/socket.c:2092 [inline]
  SyS_sendmsg+0x2d/0x50 net/socket.c:2088
  do_syscall_64+0x280/0x940 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x43fe49
RSP: 002b:00007ffd031511f8 EFLAGS: 00000217 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 000000000043fe49
RDX: 0000000000000000 RSI: 0000000020000300 RDI: 0000000000000003
RBP: 00000000006ca018 R08: 00000000004002c8 R09: 00000000004002c8
R10: 00000000004002c8 R11: 0000000000000217 R12: 0000000000401770
R13: 0000000000401800 R14: 0000000000000000 R15: 0000000000000000
Code: e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 41 14 00 00 49 8d 7f 20 48 be  
00 00 00 00 00 fc ff df 41 8b 54 24 10 48 89 f8 48 c1 e8 03 <0f> b6 04 30  
84 c0 74 08 3c 03 0f 8e 6b 11 00 00 41 89 57 20 e8
RIP: rds_cmsg_zcopy net/rds/send.c:947 [inline] RSP: ffff8801b5f7f690
RIP: rds_cmsg_send net/rds/send.c:994 [inline] RSP: ffff8801b5f7f690
RIP: rds_sendmsg+0x10fd/0x2550 net/rds/send.c:1167 RSP: ffff8801b5f7f690
---[ end trace a6cffb297815c791 ]---
Kernel panic - not syncing: Fatal exception
Dumping ftrace buffer:
    (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


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