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Date:   Fri, 21 Oct 2022 16:40:58 +0800
From:   Zhengchao Shao <shaozhengchao@...wei.com>
To:     <netdev@...r.kernel.org>, <jhs@...atatu.com>,
        <xiyou.wangcong@...il.com>, <jiri@...nulli.us>,
        <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
        <pabeni@...hat.com>
CC:     <weiyongjun1@...wei.com>, <yuehaibing@...wei.com>,
        <shaozhengchao@...wei.com>
Subject: [PATCH net] net: sched: fq_codel: fix null-ptr-deref issue in fq_codel_enqueue()

As [0] see, it will cause null-ptr-deref issue.
The following is the process of triggering the problem:
fq_codel_enqueue()
	...
	idx = fq_codel_classify()        --->if idx != 0
	flow = &q->flows[idx];
	flow_queue_add(flow, skb);       --->add skb to flow[idex]
	q->backlogs[idx] += qdisc_pkt_len(skb); --->backlogs = 0
	...
	fq_codel_drop()          --->set sch->limit = 0, always
				     drop packets
		...
		idx = i          --->because backlogs in every
				     flows is 0, so idx = 0
		...
		flow = &q->flows[idx];   --->get idx=0 flow
		...
		dequeue_head()
			skb = flow->head; --->flow->head = NULL
			flow->head = skb->next; --->cause null-ptr-deref

So, only need to discard the packets whose len is 0 on dropping path of
enqueue. Then, the correct flow id can be obtained by fq_codel_drop() on
next enqueuing.

[0]: https://syzkaller.appspot.com/bug?id=0b84da80c2917757915afa89f7738a9d16ec96c5

Fixes: 4b549a2ef4be ("fq_codel: Fair Queue Codel AQM")
Signed-off-by: Zhengchao Shao <shaozhengchao@...wei.com>
---
 net/sched/sch_fq_codel.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
index 99d318b60568..3bbe7f69dfb5 100644
--- a/net/sched/sch_fq_codel.c
+++ b/net/sched/sch_fq_codel.c
@@ -187,6 +187,7 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	struct fq_codel_sched_data *q = qdisc_priv(sch);
 	unsigned int idx, prev_backlog, prev_qlen;
 	struct fq_codel_flow *flow;
+	struct sk_buff *drop_skb;
 	int ret;
 	unsigned int pkt_len;
 	bool memory_limited;
@@ -222,6 +223,13 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 
 	/* save this packet length as it might be dropped by fq_codel_drop() */
 	pkt_len = qdisc_pkt_len(skb);
+
+	/* drop skb if len = 0, so fq_codel_drop could get the right flow idx*/
+	if (unlikely(!pkt_len)) {
+		drop_skb = dequeue_head(flow);
+		__qdisc_drop(drop_skb, to_free);
+		return NET_XMIT_SUCCESS;
+	}
 	/* fq_codel_drop() is quite expensive, as it performs a linear search
 	 * in q->backlogs[] to find a fat flow.
 	 * So instead of dropping a single packet, drop half of its backlog
-- 
2.17.1

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