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

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Message-ID: <1333348981.2325.6960.camel@edumazet-glaptop>
Date:	Mon, 02 Apr 2012 08:43:01 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...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, 2012-04-02 at 14:27 +0800, Changli Gao wrote:
> 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);
> 

Doesnt really save us, skb_header_pointer() copies only if block not
directly and fully accessible in skb->head

So if skb->head contains exactly the tcp options, we still can read one
uninit byte.

So potential problem in netfilter too.



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