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.

In addition to checking for qlen being zero, this patch checks whether the
class was already added to the active_list (cl_is_active) before adding
to the list to cover for the reentrant case.

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

Fixes: 37d9cf1a3ce3 ("sched: Fix detection of empty queues in child qdiscs")
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20250425220710.3964791-2-victor@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/sched/sch_drr.c b/net/sched/sch_drr.c
index e0a81d313aa767d09a28d84dba03ed3270b185ff..9b6d79bd87371208cc68ca44aa563ac6bbbb4aae 100644 (file)
--- 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_active(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);
@@ -337,7 +342,6 @@ static int drr_enqueue(struct sk_buff *skb, struct Qdisc *sch,
        struct drr_sched *q = qdisc_priv(sch);
        struct drr_class *cl;
        int err = 0;
-       bool first;
 
        cl = drr_classify(skb, sch, &err);
        if (cl == NULL) {
@@ -347,7 +351,6 @@ static int drr_enqueue(struct sk_buff *skb, struct Qdisc *sch,
                return err;
        }
 
-       first = !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 +360,7 @@ static int drr_enqueue(struct sk_buff *skb, struct Qdisc *sch,
                return err;
        }
 
-       if (first) {
+       if (!cl_is_active(cl)) {
                list_add_tail(&cl->alist, &q->active);
                cl->deficit = cl->quantum;
        }