net/sched: taprio: always validate TCA_TAPRIO_ATTR_PRIOMAP

If one TCA_TAPRIO_ATTR_PRIOMAP attribute has been provided,
taprio_parse_mqprio_opt() must validate it, or userspace
can inject arbitrary data to the kernel, the second time
taprio_change() is called.

First call (with valid attributes) sets dev->num_tc
to a non zero value.

Second call (with arbitrary mqprio attributes)
returns early from taprio_parse_mqprio_opt()
and bad things can happen.

Fixes: a3d43c0d ("taprio: Add support adding an admin schedule")
Reported-by: Noam Rathaus <noamr@ssd-disclosure.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20240604181511.769870-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/sch_taprio.c

@@ -1176,16 +1176,13 @@ static int taprio_parse_mqprio_opt(struct net_device *dev,
 {
 	bool allow_overlapping_txqs = TXTIME_ASSIST_IS_ENABLED(taprio_flags);
 
-	if (!qopt && !dev->num_tc) {
-		NL_SET_ERR_MSG(extack, "'mqprio' configuration is necessary");
-		return -EINVAL;
-	}
-
-	/* If num_tc is already set, it means that the user already
-	 * configured the mqprio part
-	 */
-	if (dev->num_tc)
+	if (!qopt) {
+		if (!dev->num_tc) {
+			NL_SET_ERR_MSG(extack, "'mqprio' configuration is necessary");
+			return -EINVAL;
+		}
 		return 0;
+	}
 
 	/* taprio imposes that traffic classes map 1:n to tx queues */
 	if (qopt->num_tc > dev->num_tx_queues) {