WARNING in kmem_cache_create_usercopy

[syzbot <syzbot+0c1d61e4db7db94102ca@syzkaller.appspotmail.com>]

Hello,

syzbot found the following crash on:

HEAD commit:    4794a36bf08d Add linux-next specific files for 20180928
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=124f814e400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b0ba9bb377f8ae91
dashboard link: https://syzkaller.appspot.com/bug?extid=0c1d61e4db7db94102ca
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1511532a400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1701f831400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+0c1d61e4db7db94102ca@syzkaller.appspotmail.com

WARNING: CPU: 0 PID: 6065 at mm/slab_common.c:473  
kmem_cache_create_usercopy+0xad/0x240 mm/slab_common.c:473
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 6065 Comm: syz-executor140 Not tainted  
4.19.0-rc5-next-20180928+ #84
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+0x1d3/0x2c4 lib/dump_stack.c:113
  panic+0x238/0x4e7 kernel/panic.c:184
  __warn.cold.8+0x163/0x1ba kernel/panic.c:536
  report_bug+0x254/0x2d0 lib/bug.c:186
  fixup_bug arch/x86/kernel/traps.c:178 [inline]
  do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:271
  do_invalid_op+0x36/0x40 arch/x86/kernel/traps.c:290
  invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:969
RIP: 0010:kmem_cache_create_usercopy+0xad/0x240 mm/slab_common.c:473
Code: 44 89 f0 25 00 60 de 04 45 85 ed 89 45 cc 75 0b 8b 45 d0 85 c0 0f 85  
8e 01 00 00 44 39 eb 72 0a 89 d8 44 29 e8 3b 45 d0 73 7e <0f> 0b c7 45 d0  
00 00 00 00 4c 8b 45 10 44 89 fa 89 de 4c 89 e7 8b
RSP: 0018:ffff8801bc23f5d0 EFLAGS: 00010213
RAX: 0000000000000000 RBX: 0000000000000008 RCX: 0000000000000006
RDX: 0000000000000000 RSI: 0000000000000020 RDI: ffffffff88b04b20
RBP: ffff8801bc23f608 R08: fffffbfff1283a2d R09: fffffbfff1283a2c
R10: ffff8801bc23f5c0 R11: ffffffff8941d167 R12: ffffffff88b04b20
R13: 00000000fffffffd R14: 0000000000000000 R15: 0000000000000000
  p9_client_create+0xa58/0x1769 net/9p/client.c:1054
  v9fs_session_init+0x217/0x1bb0 fs/9p/v9fs.c:421
  v9fs_mount+0x7c/0x8f0 fs/9p/vfs_super.c:135
  legacy_get_tree+0x131/0x460 fs/fs_context.c:718
  vfs_get_tree+0x1cb/0x5c0 fs/super.c:1795
  do_new_mount fs/namespace.c:2648 [inline]
  do_mount+0x70c/0x1d90 fs/namespace.c:2974
  ksys_mount+0x12d/0x140 fs/namespace.c:3190
  __do_sys_mount fs/namespace.c:3204 [inline]
  __se_sys_mount fs/namespace.c:3201 [inline]
  __x64_sys_mount+0xbe/0x150 fs/namespace.c:3201
  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
  entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440189
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 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 fb 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffdd30ec3c8 EFLAGS: 00000202 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0030656c69662f2e RCX: 0000000000440189
RDX: 00000000200008c0 RSI: 0000000020000000 RDI: 0000000000000000
RBP: 00000000006ca018 R08: 0000000020000a80 R09: 00000000004002c8
R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000401a10
R13: 0000000000401aa0 R14: 0000000000000000 R15: 0000000000000000
Kernel Offset: disabled
Rebooting in 86400 seconds..


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