general protection fault in tun_do_read

Hello,

syzbot hit the following crash on net-next commit
43df215d99e6049d4680309c54232689e16ddd6b (Wed Jan 24 01:24:32 2018 +0000)
Merge branch 'bpf-and-netdevsim-test-updates'

So far this crash happened 2 times on net-next.
Unfortunately, I don't have any reproducer for this crash yet.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+885a488f0e7e392e9022@syzkaller.appspotmail.com
It will help syzbot understand when the bug is fixed. See footer for  
details.
If you forward the report, please keep this part and the footer.

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: 19625 Comm: syz-executor7 Not tainted 4.15.0-rc9+ #206
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:__ptr_ring_peek include/linux/ptr_ring.h:190 [inline]
RIP: 0010:__ptr_ring_consume include/linux/ptr_ring.h:286 [inline]
RIP: 0010:ptr_ring_consume include/linux/ptr_ring.h:322 [inline]
RIP: 0010:tun_ring_recv drivers/net/tun.c:2127 [inline]
RIP: 0010:tun_do_read+0xf05/0x2600 drivers/net/tun.c:2178
RSP: 0018:ffff8801ae21f920 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000010 RCX: 0000000000000000
RDX: 0000000000000010 RSI: 0000000000000002 RDI: ffff8801affc1c4c
RBP: ffff8801ae21fc88 R08: 1ffff10035c43e63 R09: 0000000000000004
R10: ffff8801ae21f7d8 R11: 0000000000000004 R12: ffff8801ae21fc60
R13: ffff8801affc1580 R14: 0000000000000800 R15: 0000000000000000
netlink: 'syz-executor6': attribute type 1 has an invalid length.
netlink: 'syz-executor6': attribute type 1 has an invalid length.
FS:  0000000002472940(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000047e5d0 CR3: 00000001ae1d8002 CR4: 00000000001606f0
netlink: 'syz-executor6': attribute type 1 has an invalid length.
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  tun_chr_read_iter+0xe5/0x1e0 drivers/net/tun.c:2210
  call_read_iter include/linux/fs.h:1766 [inline]
  new_sync_read fs/read_write.c:401 [inline]
  __vfs_read+0x6a7/0xa00 fs/read_write.c:413
  vfs_read+0x11e/0x350 fs/read_write.c:447
  SYSC_read fs/read_write.c:573 [inline]
  SyS_read+0xef/0x220 fs/read_write.c:566
  entry_SYSCALL_64_fastpath+0x29/0xa0
RIP: 0033:0x40d130
RSP: 002b:0000000000a2fa18 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 0000000002472914 RCX: 000000000040d130
RDX: 00000000000003e8 RSI: 0000000000a2fb50 RDI: 0000000000000012
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000002472940
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000002eba
R13: 0000000000000005 R14: 0000000000071d6e R15: 0000000000000001
Code: ea 03 80 3c 02 00 0f 85 0a 12 00 00 49 8b 95 08 07 00 00 49 63 cf 48  
b8 00 00 00 00 00 fc ff df 48 8d 1c ca 48 89 de 48 c1 ee 03 <80> 3c 06 00  
0f 85 b8 11 00 00 48 8b 1b 48 89 8d b8 fc ff ff 48
RIP: __ptr_ring_peek include/linux/ptr_ring.h:190 [inline] RSP:  
ffff8801ae21f920
RIP: __ptr_ring_consume include/linux/ptr_ring.h:286 [inline] RSP:  
ffff8801ae21f920
RIP: ptr_ring_consume include/linux/ptr_ring.h:322 [inline] RSP:  
ffff8801ae21f920
RIP: tun_ring_recv drivers/net/tun.c:2127 [inline] RSP: ffff8801ae21f920
RIP: tun_do_read+0xf05/0x2600 drivers/net/tun.c:2178 RSP: ffff8801ae21f920
---[ end trace 51dd5d313aab4725 ]---
Kernel panic - not syncing: Fatal exception
Dumping ftrace buffer:
    (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
This bug is generated by a dumb bot. It may contain errors.
See https://goo.gl/tpsmEJ for details.
Direct all questions to syzkaller@googlegroups.com.

syzbot will keep track of this bug report.
If you forgot to add the Reported-by tag, once the fix for this bug is  
merged
into any tree, please reply to this email with:
#syz fix: exact-commit-title
To mark this as a duplicate of another syzbot report, please reply with:
#syz dup: exact-subject-of-another-report
If it's a one-off invalid bug report, please reply with:
#syz invalid
Note: if the crash happens again, it will cause creation of a new bug  
report.
Note: all commands must start from beginning of the line in the email body.