netdev - [RFC PATCH net 1/4] net_sched: drr: Fix double list add in class with netem as child qdisc

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Message-ID: <20250415000316.3122018-2-victor@mojatatu.com>
Date: Mon, 14 Apr 2025 21:03:13 -0300
From: Victor Nogueira <victor@...atatu.com>
To: netdev@...r.kernel.org
Cc: jhs@...atatu.com,
	xiyou.wangcong@...il.com,
	jiri@...nulli.us,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	toke@...hat.com,
	gerrard.tai@...rlabs.sg,
	pctammela@...atatu.com
Subject: [RFC PATCH net 1/4] net_sched: drr: Fix double list add in class with netem as child qdisc

As described in Gerrard's report [1], there are use cases where a netem
child qdisc will make the parent qdisc's enqueue callback reentrant.
In the case of drr, there won't be a UAF, but the code will add the same
classifier to the list twice, which will cause memory corruption.

This patch checks, in parallel with the qlen being zero, whether the
class was already added to the active_list (cl_is_initialised) before
adding to the list.

Fixes: 37d9cf1a3ce3 ("sched: Fix detection of empty queues in child qdiscs")

[1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/

Signed-off-by: Victor Nogueira <victor@...atatu.com>
---
 net/sched/sch_drr.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/sched/sch_drr.c b/net/sched/sch_drr.c
index e0a81d313aa7..942073214e80 100644
--- a/net/sched/sch_drr.c
+++ b/net/sched/sch_drr.c
@@ -35,6 +35,11 @@ struct drr_sched {
 	struct Qdisc_class_hash		clhash;
 };
 
+static bool cl_is_initialised(struct drr_class *cl)
+{
+	return !list_empty(&cl->alist);
+}
+
 static struct drr_class *drr_find_class(struct Qdisc *sch, u32 classid)
 {
 	struct drr_sched *q = qdisc_priv(sch);
@@ -336,8 +341,8 @@ static int drr_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	unsigned int len = qdisc_pkt_len(skb);
 	struct drr_sched *q = qdisc_priv(sch);
 	struct drr_class *cl;
+	bool is_empty;
 	int err = 0;
-	bool first;
 
 	cl = drr_classify(skb, sch, &err);
 	if (cl == NULL) {
@@ -347,7 +352,7 @@ static int drr_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		return err;
 	}
 
-	first = !cl->qdisc->q.qlen;
+	is_empty = !cl->qdisc->q.qlen;
 	err = qdisc_enqueue(skb, cl->qdisc, to_free);
 	if (unlikely(err != NET_XMIT_SUCCESS)) {
 		if (net_xmit_drop_count(err)) {
@@ -357,7 +362,7 @@ static int drr_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		return err;
 	}
 
-	if (first) {
+	if (is_empty && !cl_is_initialised(cl)) {
 		list_add_tail(&cl->alist, &q->active);
 		cl->deficit = cl->quantum;
 	}
-- 
2.34.1