[PATCH 5.15.y] net/sched: sch_taprio: Replace direct dequeue call with peek and qdisc_dequeue_peeked

[Sasha Levin <sashal@kernel.org>]

From: Bryam Vargas <hexlabsecurity@proton.me>

[ Upstream commit e056e1dfcddca877dd46d704e8ec9860cfc9ec44 ]

When taprio's software path peeks a non-work-conserving child qdisc, the
child stashes the peeked skb in its gso_skb; taprio_dequeue_from_txq()
then takes the packet with a direct child ->dequeue() call, which ignores
that stash, orphans the peeked skb and desyncs the child's qlen/backlog.
With a qfq child 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_red and sch_sfb
now do. The helper returns the child's stashed skb first and is a no-op
when there is none, so a work-conserving child is unaffected and the
gated path now consumes the skb whose length was charged to the budget.

Fixes: 5a781ccbd19e ("tc: Add support for configuring the taprio scheduler")
Cc: stable@vger.kernel.org
Cc: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260625-b4-disp-31bcb279-v1-1-85c40b83c529@proton.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ applied the peek/dequeue conversion to both `child->ops->dequeue(child)` call sites, since 6.1 lacks the upstream cleanup that merged them into one ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/sched/sch_taprio.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 8e7c0a3034cc41..9e6a1e0f075e94 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -558,7 +558,7 @@ static struct sk_buff *taprio_dequeue_from_txq(struct Qdisc *sch, int txq,
 		return NULL;
 
 	if (TXTIME_ASSIST_IS_ENABLED(q->flags)) {
-		skb = child->ops->dequeue(child);
+		skb = qdisc_dequeue_peeked(child);
 		if (!skb)
 			return NULL;
 		goto skb_found;
@@ -589,7 +589,7 @@ static struct sk_buff *taprio_dequeue_from_txq(struct Qdisc *sch, int txq,
 	    atomic_sub_return(len, &entry->budget) < 0)
 		return NULL;
 
-	skb = child->ops->dequeue(child);
+	skb = qdisc_dequeue_peeked(child);
 	if (unlikely(!skb))
 		return NULL;
 
-- 
2.53.0