WARNING: refcount bug in p9_req_put

[syzbot <syzbot+edec7868af5997928fe9@syzkaller.appspotmail.com>]

Hello,

syzbot found the following crash on:

HEAD commit:    ccda4af0f4b9 Linux 4.20-rc2
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=166b66a3400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4a0a89f12ca9b0f5
dashboard link: https://syzkaller.appspot.com/bug?extid=edec7868af5997928fe9
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+edec7868af5997928fe9@syzkaller.appspotmail.com

audit: type=1400 audit(1542156774.207:23570): avc:  denied  { map } for   
pid=3935 comm="blkid" path="/sbin/blkid" dev="sda1" ino=16128  
scontext=system_u:system_r:kernel_t:s0 tcontext=system_u:object_r:file_t:s0  
tclass=file permissive=1
XFS (loop1): device supports 512 byte sectors (not 0)
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 3966 at lib/refcount.c:187  
refcount_sub_and_test_checked+0x2c9/0x310 lib/refcount.c:187
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 3966 Comm: syz-executor0 Not tainted 4.20.0-rc2+ #112
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+0x244/0x39d lib/dump_stack.c:113
  panic+0x2ad/0x55c kernel/panic.c:188
  __warn.cold.8+0x20/0x45 kernel/panic.c:540
  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:refcount_sub_and_test_checked+0x2c9/0x310 lib/refcount.c:187
Code: 89 de e8 ea 1a ed fd 84 db 74 07 31 db e9 4d ff ff ff e8 0a 1a ed fd  
48 c7 c7 20 ae 60 88 c6 05 7b fd 7e 06 01 e8 67 7d b6 fd <0f> 0b 31 db e9  
2c ff ff ff 48 89 cf e8 a6 67 30 fe e9 41 fe ff ff
RSP: 0018:ffff88817e87f330 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffc90005e51000
RDX: 00000000000222c2 RSI: ffffffff8165e7e5 RDI: 0000000000000005
RBP: ffff88817e87f418 R08: ffff8881866ba640 R09: ffffed103b5c5020
R10: ffffed103b5c5020 R11: ffff8881dae28107 R12: ffff88817c7a7008
R13: 00000000ffffffff R14: ffff88817e87f3f0 R15: ffff8881c1dc9d68
  refcount_dec_and_test_checked+0x1a/0x20 lib/refcount.c:212
  kref_put include/linux/kref.h:69 [inline]
  p9_req_put+0x20/0x60 net/9p/client.c:395
  p9_conn_destroy net/9p/trans_fd.c:880 [inline]
  p9_fd_close+0x39f/0x6b0 net/9p/trans_fd.c:913
  p9_client_create+0xbd0/0x1674 net/9p/client.c:1062
  v9fs_session_init+0x217/0x1bb0 fs/9p/v9fs.c:421
  v9fs_mount+0x7c/0x8f0 fs/9p/vfs_super.c:135
  mount_fs+0xae/0x31d fs/super.c:1261
  vfs_kern_mount.part.35+0xdc/0x4f0 fs/namespace.c:961
  vfs_kern_mount fs/namespace.c:951 [inline]
  do_new_mount fs/namespace.c:2469 [inline]
  do_mount+0x581/0x31f0 fs/namespace.c:2801
  ksys_mount+0x12d/0x140 fs/namespace.c:3017
  __do_sys_mount fs/namespace.c:3031 [inline]
  __se_sys_mount fs/namespace.c:3028 [inline]
  __x64_sys_mount+0xbe/0x150 fs/namespace.c:3028
  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
  entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457569
Code: fd b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 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 cb b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f1d0faf5c78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 0000000000457569
RDX: 0000000020000600 RSI: 00000000200005c0 RDI: 0000000000000000
RBP: 000000000072bf00 R08: 0000000020000240 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f1d0faf66d4
R13: 00000000004c2b12 R14: 00000000004d4278 R15: 00000000ffffffff
Kernel Offset: disabled
Rebooting in 86400 seconds..


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