netdev - Re: [PATCH net] net/sched: act_pedit: limit negative offset

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Message-ID: <CAM_iQpX1at1w=3OfRFchn3STTDJGXj640tGetxXToR=EyddpEw@mail.gmail.com>
Date:   Sun, 27 Nov 2016 21:39:33 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Amir Vadai <amir@...ai.me>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Hadar Har-Zion <hadarh@...lanox.com>,
        Jiri Pirko <jiri@...lanox.com>
Subject: Re: [PATCH net] net/sched: act_pedit: limit negative offset

On Sun, Nov 27, 2016 at 7:58 AM, Amir Vadai <amir@...ai.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?


> +                               pr_info("tc filter pedit offset out of bounds\n");
>                                 goto bad;
> +                       }
> +
>                         /* just do it, baby */
>                         *ptr = ((*ptr & tkey->mask) ^ tkey->val);
>                         if (ptr == &_data)
> --
> 2.10.2
>