The UAF bug is due to smb2_reconnect_server() accessing a session that
is already being teared down by another thread that is executing
__cifs_put_smb_ses().  This can happen when (a) the client has
connection to the server but no session or (b) another thread ends up
setting @ses->ses_status again to something different than
SES_EXITING.

To fix this, we need to make sure to unconditionally set
@ses->ses_status to SES_EXITING and prevent any other threads from
setting a new status while we're still tearing it down.

The following can be reproduced by adding some delay to right after
the ipc is freed in __cifs_put_smb_ses() - which will give
smb2_reconnect_server() worker a chance to run and then accessing
@ses->ipc:

kinit ...
mount.cifs //srv/share /mnt/1 -o sec=krb5,nohandlecache,echo_interval=10
[disconnect srv]
ls /mnt/1 &>/dev/null
sleep 30
kdestroy
[reconnect srv]
sleep 10
umount /mnt/1
...
CIFS: VFS: Verify user has a krb5 ticket and keyutils is installed
CIFS: VFS: \\srv Send error in SessSetup = -126
CIFS: VFS: Verify user has a krb5 ticket and keyutils is installed
CIFS: VFS: \\srv Send error in SessSetup = -126
general protection fault, probably for non-canonical address
0x6b6b6b6b6b6b6b6b: 0000 [#1] PREEMPT SMP NOPTI
CPU: 3 PID: 50 Comm: kworker/3:1 Not tainted 6.9.0-rc2 #1
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-1.fc39
04/01/2014
Workqueue: cifsiod smb2_reconnect_server [cifs]
RIP: 0010:__list_del_entry_valid_or_report+0x33/0xf0
Code: 4f 08 48 85 d2 74 42 48 85 c9 74 59 48 b8 00 01 00 00 00 00 ad
de 48 39 c2 74 61 48 b8 22 01 00 00 00 00 74 69 <48> 8b 01 48 39 f8 75
7b 48 8b 72 08 48 39 c6 0f 85 88 00 00 00 b8
RSP: 0018:ffffc900001bfd70 EFLAGS: 00010a83
RAX: dead000000000122 RBX: ffff88810da53838 RCX: 6b6b6b6b6b6b6b6b
RDX: 6b6b6b6b6b6b6b6b RSI: ffffffffc02f6878 RDI: ffff88810da53800
RBP: ffff88810da53800 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000001 R12: ffff88810c064000
R13: 0000000000000001 R14: ffff88810c064000 R15: ffff8881039cc000
FS: 0000000000000000(0000) GS:ffff888157c00000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fe3728b1000 CR3: 000000010caa4000 CR4: 0000000000750ef0
PKRU: 55555554
Call Trace:
 <TASK>
 ? die_addr+0x36/0x90
 ? exc_general_protection+0x1c1/0x3f0
 ? asm_exc_general_protection+0x26/0x30
 ? __list_del_entry_valid_or_report+0x33/0xf0
 __cifs_put_smb_ses+0x1ae/0x500 [cifs]
 smb2_reconnect_server+0x4ed/0x710 [cifs]
 process_one_work+0x205/0x6b0
 worker_thread+0x191/0x360
 ? __pfx_worker_thread+0x10/0x10
 kthread+0xe2/0x110
 ? __pfx_kthread+0x10/0x10
 ret_from_fork+0x34/0x50
 ? __pfx_kthread+0x10/0x10
 ret_from_fork_asm+0x1a/0x30
 </TASK>

Cc: stable@vger.kernel.org
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>