netdev - [Patch net] sch_sfb: fix a crash in sfb

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Date:   Wed, 11 Sep 2019 11:34:45 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     netdev@...r.kernel.org
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        syzbot+d5870a903591faaca4ae@...kaller.appspotmail.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: [Patch net] sch_sfb: fix a crash in sfb_destroy()

When tcf_block_get() fails in sfb_init(), q->qdisc is still a NULL
pointer which leads to a crash in sfb_destroy().

Linus suggested three solutions for this problem, the simplest fix
is just moving the noop_qdisc assignment before tcf_block_get()
so that qdisc_put() would become a nop.

Fixes: 6529eaba33f0 ("net: sched: introduce tcf block infractructure")
Reported-by: syzbot+d5870a903591faaca4ae@...kaller.appspotmail.com
Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Jamal Hadi Salim <jhs@...atatu.com>
Cc: Jiri Pirko <jiri@...nulli.us>
Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
---
 net/sched/sch_sfb.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/sched/sch_sfb.c b/net/sched/sch_sfb.c
index 1dff8506a715..db1c8eb521a2 100644
--- a/net/sched/sch_sfb.c
+++ b/net/sched/sch_sfb.c
@@ -552,11 +552,11 @@ static int sfb_init(struct Qdisc *sch, struct nlattr *opt,
 	struct sfb_sched_data *q = qdisc_priv(sch);
 	int err;
 
+	q->qdisc = &noop_qdisc;
+
 	err = tcf_block_get(&q->block, &q->filter_list, sch, extack);
 	if (err)
 		return err;
-
-	q->qdisc = &noop_qdisc;
 	return sfb_change(sch, opt, extack);
 }
 
-- 
2.21.0