net/sched: sch_codel: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen

[ Upstream commit 52f1da3 ]

Whenever codel drops packets during peek, it calls
qdisc_tree_reduce_backlog. An issue arises because it calls
qdisc_tree_reduce_backlog before it reincrements the qlen. If qlen drops
to zero, but peek returns an skb, the parent's qlen_notify callback will
be executed even though codel still has 1 packet on the queue and, thus,
will mistakenly deactivate the parent's class causing issues like a wild
memory access when qfq has codel as a child:

[   36.339843][  T370] Oops: general protection fault, probably for non-canonical address 0xfbd59c0000000024: 0000 [#1] SMP KASAN NOPTI
[   36.340408][  T370] KASAN: maybe wild-memory-access in range [0xdead000000000120-0xdead000000000127]
[   36.340737][  T370] CPU: 2 UID: 0 PID: 370 Comm: tc Not tainted 7.1.0-rc5-00287-g66e13b626592 #87 PREEMPT(full)
[   36.341113][  T370] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   36.341357][  T370] RIP: 0010:qfq_deactivate_agg (include/linux/list.h:1029 (discriminator 2) include/linux/list.h:1043 (discriminator 2) net/sched/sch_qfq.c:1369 (discriminator 2) net/sched/sch_qfq.c:1395 (discriminator 2)) sch_qfq
[   36.342221][  T370] RSP: 0018:ffff8881100ef370 EFLAGS: 00010216
[   36.342422][  T370] RAX: 0000000000000000 RBX: ffff8881058a9568 RCX: dffffc0000000000
[   36.342664][  T370] RDX: 1ffff11021064dc3 RSI: ffff888108326e00 RDI: dffffc0000000000
[   36.342905][  T370] RBP: ffff8881058a8280 R08: dead000000000122 R09: 1bd5a00000000024
[   36.343140][  T370] R10: fffffbfff2940329 R11: fffffbfff2940329 R12: 0000000000000000
[   36.343383][  T370] R13: dead000000000100 R14: ffff8881058a9580 R15: ffff8881058a9578
[   36.343631][  T370] FS:  00007fc04b0ca780(0000) GS:ffff888184fef000(0000) knlGS:0000000000000000
[   36.343911][  T370] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   36.344116][  T370] CR2: 0000557c02c02000 CR3: 000000010e0ba000 CR4: 0000000000750ef0
[   36.344359][  T370] PKRU: 55555554
[   36.344481][  T370] Call Trace:
...
[   36.345054][  T370] qfq_reset_qdisc (net/sched/sch_qfq.c:357 net/sched/sch_qfq.c:1487) sch_qfq
[   36.345222][  T370]  qdisc_reset (net/sched/sch_generic.c:1057)
[   36.345503][  T370]  __qdisc_destroy (net/sched/sch_generic.c:1096)
[   36.345677][  T370]  qdisc_graft (net/sched/sch_api.c:1062 net/sched/sch_api.c:1053 net/sched/sch_api.c:1159)
[   36.346335][  T370]  tc_get_qdisc (net/sched/sch_api.c:1528 net/sched/sch_api.c:1556)

Fix this by only calling qdisc_tree_reduce_backlog in peek after the
qlen is restored.

Fixes: 342debc ("codel: remove sch->q.qlen check before qdisc_tree_reduce_backlog()")
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20260610192855.3121513-3-victor@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: vbnogueira

net/sched/sch_codel.c

@@ -86,7 +86,7 @@ static void drop_func(struct sk_buff *skb, void *ctx)
 	qdisc_qstats_drop(sch);
 }
 
-static struct sk_buff *codel_qdisc_dequeue(struct Qdisc *sch)
+static struct sk_buff *__codel_qdisc_dequeue(struct Qdisc *sch)
 {
 	struct codel_sched_data *q = qdisc_priv(sch);
 	struct sk_buff *skb;
@@ -95,13 +95,51 @@ static struct sk_buff *codel_qdisc_dequeue(struct Qdisc *sch)
 			    &q->stats, qdisc_pkt_len, codel_get_enqueue_time,
 			    drop_func, dequeue_func);
 
+	if (skb)
+		qdisc_bstats_update(sch, skb);
+	return skb;
+}
+
+static void codel_dequeue_drop(struct Qdisc *sch)
+{
+	struct codel_sched_data *q = qdisc_priv(sch);
+
 	if (q->stats.drop_count) {
-		qdisc_tree_reduce_backlog(sch, q->stats.drop_count, q->stats.drop_len);
+		qdisc_tree_reduce_backlog(sch, q->stats.drop_count,
+					  q->stats.drop_len);
 		q->stats.drop_count = 0;
 		q->stats.drop_len = 0;
 	}
-	if (skb)
-		qdisc_bstats_update(sch, skb);
+}
+
+static struct sk_buff *codel_qdisc_dequeue(struct Qdisc *sch)
+{
+	struct sk_buff *skb;
+
+	skb = __codel_qdisc_dequeue(sch);
+
+	codel_dequeue_drop(sch);
+
+	return skb;
+}
+
+static struct sk_buff *codel_peek(struct Qdisc *sch)
+{
+	struct sk_buff *skb = skb_peek(&sch->gso_skb);
+
+	if (!skb) {
+		skb = __codel_qdisc_dequeue(sch);
+
+		if (skb) {
+			__skb_queue_head(&sch->gso_skb, skb);
+			/* it's still part of the queue */
+			qdisc_qstats_backlog_inc(sch, skb);
+			sch->q.qlen++;
+		}
+
+		codel_dequeue_drop(sch);
+	}
+
 	return skb;
 }
 
@@ -276,7 +314,7 @@ static struct Qdisc_ops codel_qdisc_ops __read_mostly = {
 
 	.enqueue	=	codel_qdisc_enqueue,
 	.dequeue	=	codel_qdisc_dequeue,
-	.peek		=	qdisc_peek_dequeued,
+	.peek		=	codel_peek,
 	.init		=	codel_init,
 	.reset		=	codel_reset,
 	.change 	=	codel_change,