Commit 3df275ef0a6a for kernel

commit 3df275ef0a6ae181e8428a6589ef5d5231e58b5c
Author: Cong Wang <xiyou.wangcong@gmail.com>
Date:   Thu Apr 17 11:47:30 2025 -0700

    net_sched: hfsc: Fix a UAF vulnerability in class handling

    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: 21f4d5cc25ec ("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>

diff --git a/net/sched/sch_hfsc.c b/net/sched/sch_hfsc.c
index ce5045eea065..b368ac0595d5 100644
--- a/net/sched/sch_hfsc.c
+++ b/net/sched/sch_hfsc.c
@@ -961,6 +961,7 @@ 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 &&
@@ -991,9 +992,13 @@ 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);