From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net] net/sched: act_pedit: limit negative offset
Date: Mon, 28 Nov 2016 00:49:36 -0500 (EST)
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From: Cong Wang <xiyou.wangcong@gmail.com>
Date: Sun, 27 Nov 2016 21:39:33 -0800

> On Sun, Nov 27, 2016 at 7:58 AM, Amir Vadai <amir@vadai.me> wrote:
>> Should not allow setting a negative offset that goes below the skb head.
> ...
>> diff --git a/net/sched/act_pedit.c b/net/sched/act_pedit.c
>> index b54d56d4959b..e79e8a88f2d2 100644
>> --- a/net/sched/act_pedit.c
>> +++ b/net/sched/act_pedit.c
>> @@ -154,8 +154,11 @@ static int tcf_pedit(struct sk_buff *skb, const struct tc_action *a,
>>                         }
>>
>>                         ptr = skb_header_pointer(skb, off + offset, 4, &_data);
>> -                       if (!ptr)
>> +                       if ((unsigned char *)ptr < skb->head) {
> 
> 
> ptr returned could be &_data, which is on stack, so why this comparison
> makes sense for this case?

Indeed, this will definitely do the wrong thing when the on-stack area
passed back to ptr.