NULL pointer dereference in SMB2_close_free() with v4.19

[Oleksandr Natalenko]

Hi.

After upgrading to v4.19 I've noticed the following in my dmesg:

===
[10287.637871] BUG: unable to handle kernel NULL pointer dereference at 
0000000000000000
[10287.637886] PGD 0 P4D 0
[10287.637900] Oops: 0000 [#1] PREEMPT SMP PTI
[10287.637909] CPU: 3 PID: 9321 Comm: file.so Not tainted 4.19.0-pf1 #1
[10287.637914] Hardware name: Dell Inc.          Vostro 3360/0F5DWF, 
BIOS A18 09/25/2013
[10287.637980] RIP: 0010:SMB2_close_free+0x8/0x10 [cifs]
[10287.637987] Code: 65 48 33 1c 25 28 00 00 00 75 09 48 83 c4 18 5b 5d 
41 5c c3 e8 d9 d3 8d f9 66 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 8b 
07 <48> 8b 38 e9 70 42 fe ff 0f 1f 44 00 00 55 48 89 e5 41 57 41 56 49
[10287.637993] RSP: 0018:ffffa5d589213bb8 EFLAGS: 00010246
[10287.638000] RAX: 0000000000000000 RBX: ffff990ed85d2800 RCX: 
0000000000000000
[10287.638006] RDX: 00000000ffffff90 RSI: 0000000000000206 RDI: 
ffffa5d589213d68
[10287.638010] RBP: ffffa5d589213df0 R08: 0000000000021070 R09: 
0000000000000001
[10287.638015] R10: 0000000000001553 R11: 0000000000000005 R12: 
ffffa5d589213c50
[10287.638020] R13: ffff990ea821e800 R14: ffff990ed2e93800 R15: 
0000000000000000
[10287.638027] FS:  00007fb0cef22640(0000) GS:ffff990eef0c0000(0000) 
knlGS:0000000000000000
[10287.638036] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[10287.638043] CR2: 0000000000000000 CR3: 000000028fd5a001 CR4: 
00000000001606e0
[10287.638050] Call Trace:
[10287.638112]  smb2_queryfs+0x15b/0x360 [cifs]
[10287.638130]  ? lookup_fast+0xc8/0x2f0
[10287.638138]  ? walk_component+0x48/0x2d0
[10287.638145]  ? legitimize_path.isra.9+0x2d/0x60
[10287.638151]  ? unlazy_walk+0x50/0xb0
[10287.638194]  ? cifs_statfs+0xb1/0x300 [cifs]
[10287.638243]  cifs_statfs+0xb1/0x300 [cifs]
[10287.638259]  statfs_by_dentry+0x4c/0x70
[10287.638268]  vfs_statfs+0x16/0xc0
[10287.638276]  user_statfs+0x54/0xa0
[10287.638285]  __se_sys_statfs+0x25/0x60
[10287.638300]  do_syscall_64+0x5b/0x170
[10287.638310]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
===

This is the SMB client having 2 mountpoints like this:

===
//1.2.3.4/share on /mnt/share type cifs 
(rw,nosuid,nodev,relatime,vers=3.0,cache=strict,username=guest,uid=1000,forceuid,gid=1000,forcegid,addr=1.2.3.4,file_mode=0755,dir_mode=0755,soft,nounix,serverino,mapposix,rsize=1048576,wsize=1048576,echo_interval=60,actimeo=1)
===

The printout repeats several times, and the call trace is consistent, 
although ATM I don't know what triggers it. Any ideas on this would be 
appreciated.

I'm going to try reproduce it locally on some VM reliably, but if it is 
something that is already known, or if there's some patch you'd like me 
to test, please let me know.

Thanks.

P.S. Please CC me if answering, I'm not subscribed to the CIFS list.

--
  Oleksandr Natalenko (post-factum)