net/sched: stop qdisc_tree_reduce_backlog on TC_H_ROOT

In qdisc_tree_reduce_backlog, Qdiscs with major handle ffff: are assumed
to be either root or ingress. This assumption is bogus since it's valid
to create egress qdiscs with major handle ffff:
Budimir Markovic found that for qdiscs like DRR that maintain an active
class list, it will cause a UAF with a dangling class pointer.

In 066a3b5, the concern was to avoid iterating over the ingress
qdisc since its parent is itself. The proper fix is to stop when parent
TC_H_ROOT is reached because the only way to retrieve ingress is when a
hierarchy which does not contain a ffff: major handle call into
qdisc_lookup with TC_H_MAJ(TC_H_ROOT).

In the scenario where major ffff: is an egress qdisc in any of the tree
levels, the updates will also propagate to TC_H_ROOT, which then the
iteration must stop.

Fixes: 066a3b5 ("[NET_SCHED] sch_api: fix qdisc_tree_decrease_qlen() loop")
Reported-by: Budimir Markovic <[email protected]>
Suggested-by: Jamal Hadi Salim <[email protected]>
Tested-by: Victor Nogueira <[email protected]>
Signed-off-by: Pedro Tammela <[email protected]>
Signed-off-by: Jamal Hadi Salim <[email protected]>

 net/sched/sch_api.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
Reviewed-by: Simon Horman <[email protected]>

Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: tammela

net/sched/sch_api.c

@@ -791,7 +791,7 @@ void qdisc_tree_reduce_backlog(struct Qdisc *sch, int n, int len)
 	drops = max_t(int, n, 0);
 	rcu_read_lock();
 	while ((parentid = sch->parent)) {
-		if (TC_H_MAJ(parentid) == TC_H_MAJ(TC_H_INGRESS))
+		if (parentid == TC_H_ROOT)
 			break;
 
 		if (sch->flags & TCQ_F_NOPARENT)