If server replied SMB2_NEGOTIATE with a zero SecurityBufferOffset,
smb2_get_data_area() sets @len to non-zero but return NULL, so
decode_negTokeninit() ends up being called with a NULL @security_blob:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: 0000 [#1] PREEMPT SMP NOPTI
  CPU: 2 PID: 871 Comm: mount.cifs Not tainted 6.7.0-rc4 #2
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.2-3-gd478f380-rebuilt.opensuse.org 04/01/2014
  RIP: 0010:asn1_ber_decoder+0x173/0xc80
  Code: 01 4c 39 2c 24 75 09 45 84 c9 0f 85 2f 03 00 00 48 8b 14 24 4c 29 ea 48 83 fa 01 0f 86 1e 07 00 00 48 8b 74 24 28 4d 8d 5d 01 <42> 0f b6 3c 2e 89 fa 40 88 7c 24 5c f7 d2 83 e2 1f 0f 84 3d 07 00
  RSP: 0018:ffffc9000063f950 EFLAGS: 00010202
  RAX: 0000000000000002 RBX: 0000000000000000 RCX: 000000000000004a
  RDX: 000000000000004a RSI: 0000000000000000 RDI: 0000000000000000
  RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
  R10: 0000000000000002 R11: 0000000000000001 R12: 0000000000000000
  R13: 0000000000000000 R14: 000000000000004d R15: 0000000000000000
  FS:  00007fce52b0fbc0(0000) GS:ffff88806ba00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000000 CR3: 000000001ae64000 CR4: 0000000000750ef0
  PKRU: 55555554
  Call Trace:
   <TASK>
   ? __die+0x23/0x70
   ? page_fault_oops+0x181/0x480
   ? __stack_depot_save+0x1e6/0x480
   ? exc_page_fault+0x6f/0x1c0
   ? asm_exc_page_fault+0x26/0x30
   ? asn1_ber_decoder+0x173/0xc80
   ? check_object+0x40/0x340
   decode_negTokenInit+0x1e/0x30 [cifs]
   SMB2_negotiate+0xc99/0x17c0 [cifs]
   ? smb2_negotiate+0x46/0x60 [cifs]
   ? srso_alias_return_thunk+0x5/0xfbef5
   smb2_negotiate+0x46/0x60 [cifs]
   cifs_negotiate_protocol+0xae/0x130 [cifs]
   cifs_get_smb_ses+0x517/0x1040 [cifs]
   ? srso_alias_return_thunk+0x5/0xfbef5
   ? srso_alias_return_thunk+0x5/0xfbef5
   ? queue_delayed_work_on+0x5d/0x90
   cifs_mount_get_session+0x78/0x200 [cifs]
   dfs_mount_share+0x13a/0x9f0 [cifs]
   ? srso_alias_return_thunk+0x5/0xfbef5
   ? lock_acquire+0xbf/0x2b0
   ? find_nls+0x16/0x80
   ? srso_alias_return_thunk+0x5/0xfbef5
   cifs_mount+0x7e/0x350 [cifs]
   cifs_smb3_do_mount+0x128/0x780 [cifs]
   smb3_get_tree+0xd9/0x290 [cifs]
   vfs_get_tree+0x2c/0x100
   ? capable+0x37/0x70
   path_mount+0x2d7/0xb80
   ? srso_alias_return_thunk+0x5/0xfbef5
   ? _raw_spin_unlock_irqrestore+0x44/0x60
   __x64_sys_mount+0x11a/0x150
   do_syscall_64+0x47/0xf0
   entry_SYSCALL_64_after_hwframe+0x6f/0x77
  RIP: 0033:0x7fce52c2ab1e

Fix this by setting @len to zero when @off == 0 so callers won't
attempt to dereference non-existing data areas.
Reported-by: Robert Morris <rtm@csail.mit.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>