CVE-2024-53057: net/sched: stop qdisc_tree_reduce_backlog on TC_H_ROOT

In the Linux kernel, the following vulnerability has been resolved:

net/sched: stop qdisctreereducebacklog on TCHROOT

In qdisctreereducebacklog, 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 066a3b5b2346, the concern was to avoid iterating over the ingress qdisc since its parent is itself. The proper fix is to stop when parent TCHROOT is reached because the only way to retrieve ingress is when a hierarchy which does not contain a ffff: major handle call into qdisclookup with TCHMAJ(TCHROOT).

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

net/sched/schapi.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)