Hello,

syzbot hit the following crash on upstream commit
8f5fd927c3a7576d57248a2d7a0861c3f2795973 (Fri Mar 16 20:37:42 2018 +0000)
Merge tag 'for-4.16-rc5-tag' of  
git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux

So far this crash happened 2 times on upstream.
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+6304bf97ef436580fede@syzkaller.appspotmail.com
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audit: type=1400 audit(1521377060.016:6): avc:  denied  { map } for   
pid=4210 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(1521377077.866:7): avc:  denied  { map } for   
pid=4228 comm="syzkaller050160" path="/root/syzkaller050160487" 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
------------[ cut here ]------------
kernel BUG at drivers/vhost/vhost.c:1655!
invalid opcode: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
    (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 4228 Comm: syzkaller050160 Not tainted 4.16.0-rc5+ #357
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:set_bit_to_user drivers/vhost/vhost.c:1655 [inline]
RIP: 0010:log_write+0x3ca/0x490 drivers/vhost/vhost.c:1679
RSP: 0018:ffff8801b0fa77b0 EFLAGS: 00010293
RAX: ffff8801af534240 RBX: dffffc0000000000 RCX: ffffffff8443f50a
RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8801af535618
RBP: ffff8801b0fa78f0 R08: 0000000000000040 R09: 0000000000000001
R10: ffff8801b0fa76d0 R11: 0000000000000002 R12: 0001ffffffffffff
R13: ffffed00361f4f09 R14: ffff8801b0fa78c8 R15: ffff8801b0fa7848
FS:  00000000007df880(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020d7c000 CR3: 00000001d3e5b005 CR4: 00000000001606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  vhost_update_used_flags+0x379/0x480 drivers/vhost/vhost.c:1726
  vhost_vq_init_access+0xca/0x540 drivers/vhost/vhost.c:1766
  vhost_net_set_backend drivers/vhost/net.c:1166 [inline]
  vhost_net_ioctl+0xee0/0x1920 drivers/vhost/net.c:1320
  vfs_ioctl fs/ioctl.c:46 [inline]
  do_vfs_ioctl+0x1b1/0x1520 fs/ioctl.c:686
  SYSC_ioctl fs/ioctl.c:701 [inline]
  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:692
  do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x43ff09
RSP: 002b:00007ffe94d57fc8 EFLAGS: 00000207 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 000000000043ff09
RDX: 0000000020d7c000 RSI: 000000004008af30 RDI: 0000000000000003
RBP: 00000000006ca018 R08: 00000000004002c8 R09: 00000000004002c8
R10: 00000000004002c8 R11: 0000000000000207 R12: 0000000000401830
R13: 00000000004018c0 R14: 0000000000000000 R15: 0000000000000000
Code: 5e 41 5f 5d c3 31 c0 eb a6 e8 b3 22 2d fd 4c 89 ef e8 5b bb 4d fd 4c  
89 f8 48 c1 e8 03 c6 04 18 f8 e9 3a ff ff ff e8 96 22 2d fd <0f> 0b e8 8f  
22 2d fd 4d 8d 6c 24 ff e9 89 fe ff ff e8 80 22 2d
RIP: set_bit_to_user drivers/vhost/vhost.c:1655 [inline] RSP:  
ffff8801b0fa77b0
RIP: log_write+0x3ca/0x490 drivers/vhost/vhost.c:1679 RSP: ffff8801b0fa77b0
---[ end trace 867ce9e35847b153 ]---
Kernel panic - not syncing: Fatal exception
Dumping ftrace buffer:
    (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
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