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

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Date:	Sun, 1 Apr 2012 18:43:25 +0200
From:	Pablo Neira Ayuso <pablo@...filter.org>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	Patrick McHardy <kaber@...sh.net>,
	"David S. Miller" <davem@...emloft.net>,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] netfilter: check the length of the data before
 dereferencing it

On Sun, Apr 01, 2012 at 10:22:50PM +0800, Changli Gao wrote:
> We should check the length of the data before dereferencing it when parsing
> the TCP options.
> 
> Signed-off-by: Changli Gao <xiaosuo@...il.com>
> ---
>  net/netfilter/nf_conntrack_proto_tcp.c |    4 ++++
>  1 file changed, 4 insertions(+)
> diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
> index 361eade..9e446c5 100644
> --- a/net/netfilter/nf_conntrack_proto_tcp.c
> +++ b/net/netfilter/nf_conntrack_proto_tcp.c
> @@ -404,6 +404,8 @@ static void tcp_options(const struct sk_buff *skb,
>  			length--;
>  			continue;
>  		default:
> +			if (length < 2)
> +				return;
>  			opsize=*ptr++;
>  			if (opsize < 2) /* "silly options" */
>  				return;

length is always multiple of 4:

int length = (tcph->doff*4) - sizeof(struct tcphdr);
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