Re: [PATCH v2 net] net: sched: fix uses after free

[John Fastabend <john.fastabend@gmail.com>]

On 03/14/2018 08:10 PM, John Fastabend wrote:
> On 03/14/2018 06:53 PM, Eric Dumazet wrote:
>> syzbot reported one use-after-free in pfifo_fast_enqueue() [1]
>>
>> Issue here is that we can not reuse skb after a successful skb_array_produce()
>> since another cpu might have consumed it already.
>>
>> I believe a similar problem exists in try_bulk_dequeue_skb_slow()
>> in case we put an skb into qdisc_enqueue_skb_bad_txq() for lockless qdisc.
>>
> 
> [...]
> 
>> Fixes: c5ad119fb6c0 ("net: sched: pfifo_fast use skb_array")
>> Signed-off-by: Eric Dumazet <edumazet@google.com>
>> Reported-by: syzbot+ed43b6903ab968b16f54@syzkaller.appspotmail.com
>> Cc: John Fastabend <john.fastabend@gmail.com>
>> Cc: Jamal Hadi Salim <jhs@mojatatu.com>
>> Cc:	Cong Wang <xiyou.wangcong@gmail.com>
>> Cc:	Jiri Pirko <jiri@resnulli.us>
>> ---
>>  net/sched/sch_generic.c | 22 +++++++++++++---------
>>  1 file changed, 13 insertions(+), 9 deletions(-)
>>
> 
> Thanks!
> 
> Acked-by: John Fastabend <john.fastabend@gmail.com>
> 
>> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
>> index 190570f21b208d5a17943360a3a6f85e1c2a2187..7e3fbe9cc936be376b66a5b12bf8957c3b601f2c 100644
>> --- a/net/sched/sch_generic.c
>> +++ b/net/sched/sch_generic.c
>> @@ -106,6 +106,14 @@ static inline void qdisc_enqueue_skb_bad_txq(struct Qdisc *q,
>>  
>>  	__skb_queue_tail(&q->skb_bad_txq, skb);
>>  
>> +	if (qdisc_is_percpu_stats(q)) {
>> +		qdisc_qstats_cpu_backlog_inc(q, skb);
> 
> So I guess the skb access above needs to be removed as
> well per your comment in the commit description. But that
> can be another patch.
> 

Actually this is fine I read this too quickly on first
read.

Sorry for the noise.  Looks good to me. 

>> +		qdisc_qstats_cpu_qlen_inc(q);
>> +	} else {
>> +		qdisc_qstats_backlog_inc(q, skb);
>> +		q->q.qlen++;
>> +	}
>> +
>>  	if (lock)
>>  		spin_unlock(lock);
>>  }
> 
>