[PATCH] net/sched: sch_qfq: prevent aggregate self-replacement

[David Lee <david.lee@trailofbits.com>]

qfq_change_class() snapshots the current aggregate settings while
holding the qdisc tree lock, but drops the lock before selecting the
destination aggregate. During that gap, qfq_enqueue() can move the
class to an aggregate matching the requested settings.

When qfq_change_class() resumes, qfq_find_agg() then returns cl->agg.
If it is a singleton, qfq_deact_rm_from_agg() frees the aggregate
before qfq_add_to_agg() immediately accesses the same pointer, causing
a use-after-free.

While holding the tree lock, skip the replacement when the destination
is already the current aggregate. Any estimator replacement has already
completed, so the class change can finish normally.

Fixes: 462dbc9101ac ("pkt_sched: QFQ Plus: fair-queueing service at DRR cost")
Bug found and triaged by OpenAI Security Research and 
validated by Trail of Bits.

Assisted-by: Codex:gpt-5.6-sol gpt-5.5-cyber
Signed-off-by: Kyle Zeng <kylebot@openai.com>
---
Trail of Bits has a reproducer for this bug that triggers a
KASAN use-after-free and can share if needed.

 net/sched/sch_qfq.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/sched/sch_qfq.c b/net/sched/sch_qfq.c
index 6f3b7273c..e900890e9 100644
--- a/net/sched/sch_qfq.c
+++ b/net/sched/sch_qfq.c
@@ -517,11 +517,14 @@ static int qfq_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
 		sch_tree_lock(sch);
 		qfq_init_agg(q, new_agg, lmax, weight);
 	}
+	if (existing && new_agg == cl->agg)
+		goto unlock;
 	if (existing)
 		qfq_deact_rm_from_agg(q, cl);
 	else
 		qdisc_class_hash_insert(&q->clhash, &cl->common);
 	qfq_add_to_agg(q, new_agg, cl);
+unlock:
 	sch_tree_unlock(sch);
 	qdisc_class_hash_grow(sch, &q->clhash);
 
-- 
2.53.0