netdev - [PATCH v2 net] net: sched: prevent a use after free

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Message-ID: <20200205115330.7x2qgaks7racy5wj@kili.mountain>
Date:   Wed, 5 Feb 2020 14:53:30 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Mohit Bhasi <mohitbhasi1998@...il.com>,
        "V. Saicharan" <vsaicharan1998@...il.com>,
        Leslie Monis <lesliemonis@...il.com>,
        "Sachin D. Patil" <sdp.sachin@...il.com>,
        Gautam Ramakrishnan <gautamramk@...il.com>,
        netdev@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: [PATCH v2 net] net: sched: prevent a use after free

The bug is that we call kfree_skb(skb) and then pass "skb" to
qdisc_pkt_len(skb) on the next line, which is a use after free.
Also Cong Wang points out that it's better to delay the actual
frees until we drop the rtnl lock so we should use rtnl_kfree_skbs()
instead of kfree_skb().

Cc: Cong Wang <xiyou.wangcong@...il.com>
Fixes: ec97ecf1ebe4 ("net: sched: add Flow Queue PIE packet scheduler")
Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
---
v2: Use rtnl_kfree_skbs() instead of kfree_skb().  From static analysis.
    Not tested, but I have audited the code pretty close and I think
    switing to rtnl_kfree_skbs() is harmless.

 net/sched/sch_fq_pie.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/sch_fq_pie.c b/net/sched/sch_fq_pie.c
index bbd0dea6b6b9..214657eb3dfd 100644
--- a/net/sched/sch_fq_pie.c
+++ b/net/sched/sch_fq_pie.c
@@ -349,9 +349,9 @@ static int fq_pie_change(struct Qdisc *sch, struct nlattr *opt,
 	while (sch->q.qlen > sch->limit) {
 		struct sk_buff *skb = fq_pie_qdisc_dequeue(sch);
 
-		kfree_skb(skb);
 		len_dropped += qdisc_pkt_len(skb);
 		num_dropped += 1;
+		rtnl_kfree_skbs(skb, skb);
 	}
 	qdisc_tree_reduce_backlog(sch, num_dropped, len_dropped);
 
-- 
2.11.0