[PATCH 2/2] net/sched: sch_multiq: Replace direct dequeue call with peek and qdisc_dequeue_peeked

[Bryam Vargas via B4 Relay]

From: Bryam Vargas <hexlabsecurity@xxxxxxxxx>

multiq_dequeue() takes a packet from a band's child with a direct
->dequeue() call after multiq_peek() peeked it. When the child is
non-work-conserving the peek stashes the skb in the child's gso_skb, so
the direct dequeue returns a different skb and orphans the stash,
desyncing the child's qlen/backlog. With a qfq child reached through a
peeking parent (e.g. tbf) this re-enters the child on an emptied list and
dereferences NULL, panicking the kernel from softirq on ordinary egress.

Take the packet through qdisc_dequeue_peeked(), as sch_prio already does
and as sch_red and sch_sfb were just fixed to do. The helper is a no-op
when the child has no stash, so a work-conserving child is unaffected.

Fixes: 77be155cba4e ("pkt_sched: Add peek emulation for non-work-conserving qdiscs.")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Bryam Vargas <hexlabsecurity@xxxxxxxxx>
---
 net/sched/sch_multiq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/sch_multiq.c b/net/sched/sch_multiq.c
index 4e465d11e3d7..a467dd122369 100644
--- a/net/sched/sch_multiq.c
+++ b/net/sched/sch_multiq.c
@@ -103,7 +103,7 @@ static struct sk_buff *multiq_dequeue(struct Qdisc *sch)
 		if (!netif_xmit_stopped(
 		    netdev_get_tx_queue(qdisc_dev(sch), q->curband))) {
 			qdisc = q->queues[q->curband];
-			skb = qdisc->dequeue(qdisc);
+			skb = qdisc_dequeue_peeked(qdisc);
 			if (skb) {
 				qdisc_bstats_update(sch, skb);
 				qdisc_qlen_dec(sch);

-- 
2.43.0