Re: [PATCH] filter: add SKF_AD_NLATTR_NEST to look for nested attributes

[Patrick McHardy <kaber@trash.net>]

David Miller wrote:
> From: Pablo Neira Ayuso <pablo@netfilter.org>
>> SKF_AD_NLATTR allows us to find the first matching attribute in a
>> stream of netlink attributes from one offset to the end of the
>> netlink message. This is not suitable to look for a specific
>> matching inside a set of nested attributes.
>>
>> For example, in ctnetlink messages, if we look for the CTA_V6_SRC
>> attribute in a message that talks about an IPv4 connection,
>> SKF_AD_NLATTR returns the offset of CTA_STATUS which has the same
>> value of CTA_V6_SRC but outside the nest. To differenciate
>> CTA_STATUS and CTA_V6_SRC, we would have to make assumptions on the
>> size of the attribute and the usual offset, resulting in horrible
>> BSF code.
>>
>> This patch adds SKF_AD_NLATTR_NEST, which is a variant of
>> SKF_AD_NLATTR, that looks for an attribute inside the limits of
>> a nested attributes, but not further.
>>
>> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
> 
> This looks fine to me, Patrick is it ok with you too?
> 
> If Patrick has no objections I'll apply it to net-next-2.6

I think this needs more validation to avoid overruning the skb data:

> +		case SKF_AD_NLATTR_NEST: {
> +			struct nlattr *nla;
> +
> +			if (skb_is_nonlinear(skb))
> +				return 0;
> +			if (A > skb->len - sizeof(struct nlattr))
> +				return 0;
> +
> +			nla = nla_find_nested((struct nlattr *)&skb->data[A],X);

This interprets skb->data as nlattr and takes the nla_len member
as limit for nla_find(), so it needs to be validated not to exceed
the skb length.

Besides that, the patch looks fine.