scsi: target: iscsi: Prevent login threads from racing between each other

The tpg->np_login_sem is a semaphore that is used to serialize the login
process when multiple login threads run concurrently against the same
target portal group.

The iscsi_target_locate_portal() function finds the tpg, calls
iscsit_access_np() against the np_login_sem semaphore and saves the tpg
pointer in conn->tpg;

If iscsi_target_locate_portal() fails, the caller will check for the
conn->tpg pointer and, if it's not NULL, then it will assume that
iscsi_target_locate_portal() called iscsit_access_np() on the semaphore.

Make sure that conn->tpg gets initialized only if iscsit_access_np() was
successful, otherwise iscsit_deaccess_np() may end up being called against
a semaphore we never took, allowing more than one thread to access the same
tpg.

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Link: https://lore.kernel.org/r/20230508162219.1731964-4-mlombard@redhat.com
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Author: maurizio-lombardi

drivers/target/iscsi/iscsi_target_nego.c

@@ -1122,6 +1122,7 @@ int iscsi_target_locate_portal(
 	iscsi_target_set_sock_callbacks(conn);
 
 	login->np = np;
+	conn->tpg = NULL;
 
 	login_req = (struct iscsi_login_req *) login->req;
 	payload_length = ntoh24(login_req->dlength);
@@ -1189,7 +1190,6 @@ int iscsi_target_locate_portal(
 	 */
 	sessiontype = strncmp(s_buf, DISCOVERY, 9);
 	if (!sessiontype) {
-		conn->tpg = iscsit_global->discovery_tpg;
 		if (!login->leading_connection)
 			goto get_target;
 
@@ -1206,9 +1206,11 @@ int iscsi_target_locate_portal(
 		 * Serialize access across the discovery struct iscsi_portal_group to
 		 * process login attempt.
 		 */
+		conn->tpg = iscsit_global->discovery_tpg;
 		if (iscsit_access_np(np, conn->tpg) < 0) {
 			iscsit_tx_login_rsp(conn, ISCSI_STATUS_CLS_TARGET_ERR,
 				ISCSI_LOGIN_STATUS_SVC_UNAVAILABLE);
+			conn->tpg = NULL;
 			ret = -1;
 			goto out;
 		}