[SRU][F/J/N][PATCH 1/1] pfifo_tail_enqueue: Drop new packet when sch->limit == 0

[Tim Whisonant]

From: Quang Le <quanglex97 at gmail.com>

Expected behaviour:
In case we reach scheduler's limit, pfifo_tail_enqueue() will drop a
packet in scheduler's queue and decrease scheduler's qlen by one.
Then, pfifo_tail_enqueue() enqueue new packet and increase
scheduler's qlen by one. Finally, pfifo_tail_enqueue() return
`NET_XMIT_CN` status code.

Weird behaviour:
In case we set `sch->limit == 0` and trigger pfifo_tail_enqueue() on a
scheduler that has no packet, the 'drop a packet' step will do nothing.
This means the scheduler's qlen still has value equal 0.
Then, we continue to enqueue new packet and increase scheduler's qlen by
one. In summary, we can leverage pfifo_tail_enqueue() to increase qlen by
one and return `NET_XMIT_CN` status code.

The problem is:
Let's say we have two qdiscs: Qdisc_A and Qdisc_B.
 - Qdisc_A's type must have '->graft()' function to create parent/child relationship.
   Let's say Qdisc_A's type is `hfsc`. Enqueue packet to this qdisc will trigger `hfsc_enqueue`.
 - Qdisc_B's type is pfifo_head_drop. Enqueue packet to this qdisc will trigger `pfifo_tail_enqueue`.
 - Qdisc_B is configured to have `sch->limit == 0`.
 - Qdisc_A is configured to route the enqueued's packet to Qdisc_B.

Enqueue packet through Qdisc_A will lead to:
 - hfsc_enqueue(Qdisc_A) -> pfifo_tail_enqueue(Qdisc_B)
 - Qdisc_B->q.qlen += 1
 - pfifo_tail_enqueue() return `NET_XMIT_CN`
 - hfsc_enqueue() check for `NET_XMIT_SUCCESS` and see `NET_XMIT_CN` => hfsc_enqueue() don't increase qlen of Qdisc_A.

The whole process lead to a situation where Qdisc_A->q.qlen == 0 and Qdisc_B->q.qlen == 1.
Replace 'hfsc' with other type (for example: 'drr') still lead to the same problem.
This violate the design where parent's qlen should equal to the sum of its childrens'qlen.

Bug impact: This issue can be used for user->kernel privilege escalation when it is reachable.

Fixes: 57dbb2d83d10 ("sched: add head drop fifo queue")
Reported-by: Quang Le <quanglex97 at gmail.com>
Signed-off-by: Quang Le <quanglex97 at gmail.com>
Signed-off-by: Cong Wang <cong.wang at bytedance.com>
Link: https://patch.msgid.link/20250204005841.223511-2-xiyou.wangcong@gmail.com
Signed-off-by: Jakub Kicinski <kuba at kernel.org>
(backported from commit 647cef20e649c576dff271e018d5d15d998b629d)
[tswhison: adjusted context due to missing commits
01daf66b791e ("net_sched: sch_fifo: implement lockless __fifo_dump()")]
CVE-2025-21702
Signed-off-by: Tim Whisonant <tim.whisonant at canonical.com>
---
 net/sched/sch_fifo.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/sched/sch_fifo.c b/net/sched/sch_fifo.c
index 8b7110cbcce4..518b01254048 100644
--- a/net/sched/sch_fifo.c
+++ b/net/sched/sch_fifo.c
@@ -42,6 +42,9 @@ static int pfifo_tail_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 {
 	unsigned int prev_backlog;
 
+	if (unlikely(READ_ONCE(sch->limit) == 0))
+		return qdisc_drop(skb, sch, to_free);
+
 	if (likely(sch->q.qlen < sch->limit))
 		return qdisc_enqueue_tail(skb, sch);
 
-- 
2.43.0