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net_sched: hfsc: Fix a UAF vulnerability in class handling

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[ Upstream commit 3df275ef0a ]

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: 21f4d5cc25 ("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>
This commit is contained in:
Cong Wang
2025-04-17 11:47:30 -07:00
committed by Greg Kroah-Hartman
parent 4f435c1f4c
commit 3aa852e360
1 changed files with 7 additions and 2 deletions
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net/sched/sch_hfsc.c
+7 -2
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@@ -958,6 +958,7 @@ 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 &&
@@ -988,9 +989,13 @@ 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);