net/sched: sch_qfq: Fix race condition on qfq_aggregate

jira VULN-89287
cve CVE-2025-38477
commit-author Xiang Mei <xmei5@asu.edu>
commit 5e28d5a

A race condition can occur when 'agg' is modified in qfq_change_agg
(called during qfq_enqueue) while other threads access it
concurrently. For example, qfq_dump_class may trigger a NULL
dereference, and qfq_delete_class may cause a use-after-free.

This patch addresses the issue by:

1. Moved qfq_destroy_class into the critical section.

2. Added sch_tree_lock protection to qfq_dump_class and
qfq_dump_class_stats.

Fixes: 462dbc9 ("pkt_sched: QFQ Plus: fair-queueing service at DRR cost")
	Signed-off-by: Xiang Mei <xmei5@asu.edu>
	Reviewed-by: Cong Wang <xiyou.wangcong@gmail.com>
	Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 5e28d5a)
	Signed-off-by: Brett Mastbergen <bmastbergen@ciq.com>

Author: bmastbergen

net/sched/sch_qfq.c

@@ -414,7 +414,7 @@ static int qfq_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
 	bool existing = false;
 	struct nlattr *tb[TCA_QFQ_MAX + 1];
 	struct qfq_aggregate *new_agg = NULL;
-	u32 weight, lmax, inv_w;
+	u32 weight, lmax, inv_w, old_weight, old_lmax;
 	int err;
 	int delta_w;
 
@@ -449,12 +449,16 @@ static int qfq_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
 	inv_w = ONE_FP / weight;
 	weight = ONE_FP / inv_w;
 
-	if (cl != NULL &&
-	    lmax == cl->agg->lmax &&
-	    weight == cl->agg->class_weight)
-		return 0; /* nothing to change */
+	if (cl != NULL) {
+		sch_tree_lock(sch);
+		old_weight = cl->agg->class_weight;
+		old_lmax   = cl->agg->lmax;
+		sch_tree_unlock(sch);
+		if (lmax == old_lmax && weight == old_weight)
+			return 0; /* nothing to change */
+	}
 
-	delta_w = weight - (cl ? cl->agg->class_weight : 0);
+	delta_w = weight - (cl ? old_weight : 0);
 
 	if (q->wsum + delta_w > QFQ_MAX_WSUM) {
 		pr_notice("qfq: total weight out of range (%d + %u)\n",
@@ -557,10 +561,10 @@ static int qfq_delete_class(struct Qdisc *sch, unsigned long arg)
 
 	qfq_purge_queue(cl);
 	qdisc_class_hash_remove(&q->clhash, &cl->common);
+	qfq_destroy_class(sch, cl);
 
 	sch_tree_unlock(sch);
 
-	qfq_destroy_class(sch, cl);
 	return 0;
 }
 
@@ -625,6 +629,7 @@ static int qfq_dump_class(struct Qdisc *sch, unsigned long arg,
 {
 	struct qfq_class *cl = (struct qfq_class *)arg;
 	struct nlattr *nest;
+	u32 class_weight, lmax;
 
 	tcm->tcm_parent	= TC_H_ROOT;
 	tcm->tcm_handle	= cl->common.classid;
@@ -633,8 +638,13 @@ static int qfq_dump_class(struct Qdisc *sch, unsigned long arg,
 	nest = nla_nest_start(skb, TCA_OPTIONS);
 	if (nest == NULL)
 		goto nla_put_failure;
-	if (nla_put_u32(skb, TCA_QFQ_WEIGHT, cl->agg->class_weight) ||
-	    nla_put_u32(skb, TCA_QFQ_LMAX, cl->agg->lmax))
+
+	sch_tree_lock(sch);
+	class_weight	= cl->agg->class_weight;
+	lmax		= cl->agg->lmax;
+	sch_tree_unlock(sch);
+	if (nla_put_u32(skb, TCA_QFQ_WEIGHT, class_weight) ||
+	    nla_put_u32(skb, TCA_QFQ_LMAX, lmax))
 		goto nla_put_failure;
 	return nla_nest_end(skb, nest);
 
@@ -651,8 +661,10 @@ static int qfq_dump_class_stats(struct Qdisc *sch, unsigned long arg,
 
 	memset(&xstats, 0, sizeof(xstats));
 
+	sch_tree_lock(sch);
 	xstats.weight = cl->agg->class_weight;
 	xstats.lmax = cl->agg->lmax;
+	sch_tree_unlock(sch);
 
 	if (gnet_stats_copy_basic(qdisc_root_sleeping_running(sch),
 				  d, NULL, &cl->bstats) < 0 ||