scsi: iscsi: target: Set conn->sess to NULL when iscsi_login_set_conn_values fails

Fixes a use-after-free reported by KASAN when later
iscsi_target_login_sess_out gets called and it tries to access
conn->sess->se_sess:

Disabling lock debugging due to kernel taint
iSCSI Login timeout on Network Portal [::]:3260
iSCSI Login negotiation failed.
==================================================================
BUG: KASAN: use-after-free in
iscsi_target_login_sess_out.cold.12+0x58/0xff [iscsi_target_mod]
Read of size 8 at addr ffff880109d070c8 by task iscsi_np/980

CPU: 1 PID: 980 Comm: iscsi_np Tainted: G           O
4.17.8kasan.sess.connops+ #4
Hardware name: To be filled by O.E.M. To be filled by O.E.M./Aptio CRB,
BIOS 5.6.5 05/19/2014
Call Trace:
 dump_stack+0x71/0xac
 print_address_description+0x65/0x22e
 ? iscsi_target_login_sess_out.cold.12+0x58/0xff [iscsi_target_mod]
 kasan_report.cold.6+0x241/0x2fd
 iscsi_target_login_sess_out.cold.12+0x58/0xff [iscsi_target_mod]
 iscsi_target_login_thread+0x1086/0x1710 [iscsi_target_mod]
 ? __sched_text_start+0x8/0x8
 ? iscsi_target_login_sess_out+0x250/0x250 [iscsi_target_mod]
 ? __kthread_parkme+0xcc/0x100
 ? parse_args.cold.14+0xd3/0xd3
 ? iscsi_target_login_sess_out+0x250/0x250 [iscsi_target_mod]
 kthread+0x1a0/0x1c0
 ? kthread_bind+0x30/0x30
 ret_from_fork+0x35/0x40

Allocated by task 980:
 kasan_kmalloc+0xbf/0xe0
 kmem_cache_alloc_trace+0x112/0x210
 iscsi_target_login_thread+0x816/0x1710 [iscsi_target_mod]
 kthread+0x1a0/0x1c0
 ret_from_fork+0x35/0x40

Freed by task 980:
 __kasan_slab_free+0x125/0x170
 kfree+0x90/0x1d0
 iscsi_target_login_thread+0x1577/0x1710 [iscsi_target_mod]
 kthread+0x1a0/0x1c0
 ret_from_fork+0x35/0x40

The buggy address belongs to the object at ffff880109d06f00
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 456 bytes inside of
 512-byte region [ffff880109d06f00, ffff880109d07100)
The buggy address belongs to the page:
page:ffffea0004274180 count:1 mapcount:0 mapping:0000000000000000
index:0x0 compound_mapcount: 0
flags: 0x17fffc000008100(slab|head)
raw: 017fffc000008100 0000000000000000 0000000000000000 00000001000c000c
raw: dead000000000100 dead000000000200 ffff88011b002e00 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff880109d06f80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff880109d07000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff880109d07080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                              ^
 ffff880109d07100: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff880109d07180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
[rebased against idr/ida changes and to handle ret review comments from Matthew]
Signed-off-by: Mike Christie <mchristi@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/iscsi/iscsi_target_login.c

@@ -310,11 +310,9 @@ static int iscsi_login_zero_tsih_s1(
 		return -ENOMEM;
 	}
 
-	ret = iscsi_login_set_conn_values(sess, conn, pdu->cid);
-	if (unlikely(ret)) {
-		kfree(sess);
-		return ret;
-	}
+	if (iscsi_login_set_conn_values(sess, conn, pdu->cid))
+		goto free_sess;
+
 	sess->init_task_tag	= pdu->itt;
 	memcpy(&sess->isid, pdu->isid, 6);
 	sess->exp_cmd_sn	= be32_to_cpu(pdu->cmdsn);