Commit 28b09a06 authored Apr 17, 2025 by Cong Wang Committed by Greg Kroah-Hartman May 02, 2025

net_sched: hfsc: Fix a UAF vulnerability in class handling



[ Upstream commit 3df275ef0a6ae181e8428a6589ef5d5231e58b5c ]

This patch fixes a Use-After-Free vulnerability in the HFSC qdisc class
handling. The issue occurs due to a time-of-check/time-of-use condition
in hfsc_change_class() when working with certain child qdiscs like netem
or codel.

The vulnerability works as follows:
1. hfsc_change_class() checks if a class has packets (q.qlen != 0)
2. It then calls qdisc_peek_len(), which for certain qdiscs (e.g.,
   codel, netem) might drop packets and empty the queue
3. The code continues assuming the queue is still non-empty, adding
   the class to vttree
4. This breaks HFSC scheduler assumptions that only non-empty classes
   are in vttree
5. Later, when the class is destroyed, this can lead to a Use-After-Free

The fix adds a second queue length check after qdisc_peek_len() to verify
the queue wasn't emptied.

Fixes: 21f4d5cc ("net_sched/hfsc: fix curve activation in hfsc_change_class()")
Reported-by: Gerrard Tai <gerrard.tai@starlabs.sg>
Reviewed-by: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20250417184732.943057-2-xiyou.wangcong@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent a3df5601

net/sched/sch_hfsc.c

+7 −2

Original line number	Diff line number	Diff line
		@@ -959,6 +959,7 @@ hfsc_change_class(struct Qdisc *sch, u32 classid, u32 parentid,

		if (cl != NULL) {
		int old_flags;
		int len = 0;

		if (parentid) {
		if (cl->cl_parent &&
		@@ -989,9 +990,13 @@ hfsc_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
		if (usc != NULL)
		hfsc_change_usc(cl, usc, cur_time);

		if (cl->qdisc->q.qlen != 0)
		len = qdisc_peek_len(cl->qdisc);
		/* Check queue length again since some qdisc implementations
		* (e.g., netem/codel) might empty the queue during the peek
		* operation.
		*/
		if (cl->qdisc->q.qlen != 0) {
		int len = qdisc_peek_len(cl->qdisc);

		if (cl->cl_flags & HFSC_RSC) {
		if (old_flags & HFSC_RSC)
		update_ed(cl, len);