ksmbd: fix use-after-free bug in smb2_tree_disconect

smb2_tree_disconnect() freed the struct ksmbd_tree_connect,
but it left the dangling pointer. It can be accessed
again under compound requests.

This bug can lead an oops looking something link:

[ 1685.468014 ] BUG: KASAN: use-after-free in ksmbd_tree_conn_disconnect+0x131/0x160 [ksmbd]
[ 1685.468068 ] Read of size 4 at addr ffff888102172180 by task kworker/1:2/4807
...
[ 1685.468130 ] Call Trace:
[ 1685.468132 ]  <TASK>
[ 1685.468135 ]  dump_stack_lvl+0x49/0x5f
[ 1685.468141 ]  print_report.cold+0x5e/0x5cf
[ 1685.468145 ]  ? ksmbd_tree_conn_disconnect+0x131/0x160 [ksmbd]
[ 1685.468157 ]  kasan_report+0xaa/0x120
[ 1685.468194 ]  ? ksmbd_tree_conn_disconnect+0x131/0x160 [ksmbd]
[ 1685.468206 ]  __asan_report_load4_noabort+0x14/0x20
[ 1685.468210 ]  ksmbd_tree_conn_disconnect+0x131/0x160 [ksmbd]
[ 1685.468222 ]  smb2_tree_disconnect+0x175/0x250 [ksmbd]
[ 1685.468235 ]  handle_ksmbd_work+0x30e/0x1020 [ksmbd]
[ 1685.468247 ]  process_one_work+0x778/0x11c0
[ 1685.468251 ]  ? _raw_spin_lock_irq+0x8e/0xe0
[ 1685.468289 ]  worker_thread+0x544/0x1180
[ 1685.468293 ]  ? __cpuidle_text_end+0x4/0x4
[ 1685.468297 ]  kthread+0x282/0x320
[ 1685.468301 ]  ? process_one_work+0x11c0/0x11c0
[ 1685.468305 ]  ? kthread_complete_and_exit+0x30/0x30
[ 1685.468309 ]  ret_from_fork+0x1f/0x30

Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-17816
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

Author: namjaejeon

smb2pdu.c

@@ -2078,6 +2078,7 @@ int smb2_tree_disconnect(struct ksmbd_work *work)
 
 	ksmbd_close_tree_conn_fds(work);
 	ksmbd_tree_conn_disconnect(sess, tcon);
+	work->tcon = NULL;
 	return 0;
 }