netdev - Re: [PATCH] net: check the length of the data before dereferencing it

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Date:	Mon, 2 Apr 2012 14:27:27 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, kaber@...sh.net,
	pablo@...filter.org, netfilter-devel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH] net: check the length of the data before dereferencing it

On Mon, Apr 2, 2012 at 12:54 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Mon, 2012-04-02 at 12:47 +0800, Changli Gao wrote:
>
>> Got it. Thanks. FYI, the tcp options are copied to the stack before
>> being parsed.
>>
>
> What do you mean ?
>
> code looks like :
>
> const struct tcphdr *th = tcp_hdr(skb);
> int length = (th->doff * 4) - sizeof(struct tcphdr);
>
> ptr = (const unsigned char *)(th + 1);
>
>
>
> Therefore ptr points somewhere in skb->head ...
>
>
>

Oh, sorry. I forgot to add the condition when I wrote it down. I mean
the code in netfilter.

        unsigned char buff[(15 * 4) - sizeof(struct tcphdr)];
        const unsigned char *ptr;
        int length = (tcph->doff*4) - sizeof(struct tcphdr);

        if (!length)
                return;

        ptr = skb_header_pointer(skb, dataoff + sizeof(struct tcphdr),
                                 length, buff);
        BUG_ON(ptr == NULL);

-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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