Subject: Re: [PATCH v3] ipv6:introduce function to find route for redirect

From: Duan Jiong <[email-masked]>
Date: Mon, 02 Sep 2013 10:38:20 +0800

于 2013年09月02日 05:08, Hannes Frederic Sowa 写道:
> On Sat, Aug 31, 2013 at 01:09:33PM +0800, Duan Jiong wrote:
>> RFC 4861 says that the IP source address of the Redirect is the
>> same as the current first-hop router for the specified ICMP
>> Destination Address, so the gateway should be taken into
>> consideration when we find the route for redirect.
>>
>> Signed-off-by: Duan Jiong <[email-masked]>
> 
> This patch looks good!
> 
> Maybe you could also point to commit
> a6279458c534d01ccc39498aba61c93083ee0372 ("NDISC: Search over all possible
> rules on receipt of redirect.") and where the check went away?
> 
> The bug is only "exploitable" on layer-2 because the source address of the
> redirect is checked to be a valid link-local address but it makes spoofing a
> lot easier in the same L2 domain nonetheless.
> 
> Nice work!
> 
> Some smaller comments inline:
> 
Thanks very much for you comments， and i will correct those problems.
>> ---
>>  Changes for v3:
>>  1.Fix the comments style problems
>>
>>  net/ipv6/ah6.c     |  2 +-
>>  net/ipv6/esp6.c    |  2 +-
>>  net/ipv6/icmp.c    |  2 +-
>>  net/ipv6/ipcomp6.c |  2 +-
>>  net/ipv6/ndisc.c   |  3 ++-
>>  net/ipv6/route.c   | 78 +++++++++++++++++++++++++++++++++++++++++++++++++-----
>>  6 files changed, 78 insertions(+), 11 deletions(-)
>>
>> diff --git a/net/ipv6/ah6.c b/net/ipv6/ah6.c
>> index bb02e17..73784c3 100644
>> --- a/net/ipv6/ah6.c
>> +++ b/net/ipv6/ah6.c
>> @@ -628,7 +628,7 @@ static void ah6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
>>  		return;
>>  
>>  	if (type == NDISC_REDIRECT)
>> -		ip6_redirect(skb, net, 0, 0);
>> +		ip6_redirect(skb, net, skb->dev->ifindex, 0);
>>  	else
>>  		ip6_update_pmtu(skb, net, info, 0, 0);
>>  	xfrm_state_put(x);
>> diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
>> index aeac0dc..d3618a7 100644
>> --- a/net/ipv6/esp6.c
>> +++ b/net/ipv6/esp6.c
>> @@ -447,7 +447,7 @@ static void esp6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
>>  		return;
>>  
>>  	if (type == NDISC_REDIRECT)
>> -		ip6_redirect(skb, net, 0, 0);
>> +		ip6_redirect(skb, net, skb->dev->ifindex, 0);
>>  	else
>>  		ip6_update_pmtu(skb, net, info, 0, 0);
>>  	xfrm_state_put(x);
>> diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
>> index 7cfc8d2..73681c2 100644
>> --- a/net/ipv6/icmp.c
>> +++ b/net/ipv6/icmp.c
>> @@ -92,7 +92,7 @@ static void icmpv6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
>>  	if (type == ICMPV6_PKT_TOOBIG)
>>  		ip6_update_pmtu(skb, net, info, 0, 0);
>>  	else if (type == NDISC_REDIRECT)
>> -		ip6_redirect(skb, net, 0, 0);
>> +		ip6_redirect(skb, net, skb->dev->ifindex, 0);
>>  
>>  	if (!(type & ICMPV6_INFOMSG_MASK))
>>  		if (icmp6->icmp6_type == ICMPV6_ECHO_REQUEST)
>> diff --git a/net/ipv6/ipcomp6.c b/net/ipv6/ipcomp6.c
>> index 7af5aee..5636a91 100644
>> --- a/net/ipv6/ipcomp6.c
>> +++ b/net/ipv6/ipcomp6.c
>> @@ -76,7 +76,7 @@ static void ipcomp6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
>>  		return;
>>  
>>  	if (type == NDISC_REDIRECT)
>> -		ip6_redirect(skb, net, 0, 0);
>> +		ip6_redirect(skb, net, skb->dev->ifindex, 0);
>>  	else
>>  		ip6_update_pmtu(skb, net, info, 0, 0);
>>  	xfrm_state_put(x);
>> diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
>> index 04d31c2..90f474b 100644
>> --- a/net/ipv6/ndisc.c
>> +++ b/net/ipv6/ndisc.c
>> @@ -1370,7 +1370,8 @@ static void ndisc_redirect_rcv(struct sk_buff *skb)
>>  		return;
>>  
>>  	if (!ndopts.nd_opts_rh) {
>> -		ip6_redirect_no_header(skb, dev_net(skb->dev), 0, 0);
>> +		ip6_redirect_no_header(skb, dev_net(skb->dev),
>> +							skb->dev->ifindex, 0);
> 
> This is not indented correctly. skb->dev->ifindex should be placed right below
> the upper skb variable.
> 
>>  		return;
>>  	}
>>  
>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>> index 8d9a93e..2e1d378 100644
>> --- a/net/ipv6/route.c
>> +++ b/net/ipv6/route.c
>> @@ -1157,6 +1157,74 @@ void ip6_sk_update_pmtu(struct sk_buff *skb, struct sock *sk, __be32 mtu)
>>  }
>>  EXPORT_SYMBOL_GPL(ip6_sk_update_pmtu);
>>  
>> +/* Handle redirects */
>> +struct ip6rd_flowi {
>> +	struct flowi6 fl6;
>> +	struct in6_addr gateway;
>> +};
>> +
>> +static struct rt6_info *__ip6_route_redirect(struct net *net,
>> +					     struct fib6_table *table,
>> +					     struct flowi6 *fl6,
>> +					     int flags)
>> +{
>> +	struct ip6rd_flowi *rdfl = (struct ip6rd_flowi *)fl6;
>> +	struct rt6_info *rt;
>> +	struct fib6_node *fn;
>> +
>> +	/* Get the "current" route for this destination and
>> +	 * check if the redirect has come from approriate router.
>> +	 *
>> +	 * RFC 4861 specifies that redirects should only be
>> +	 * accepted if they come from the nexthop to the target.
>> +	 * Due to the way the routes are chosen, this notion
>> +	 * is a bit fuzzy and one might need to check all possible
>> +	 * routes.
>> +	 */
>> +
>> +	read_lock_bh(&table->tb6_lock);
>> +	fn = fib6_lookup(&table->tb6_root, &fl6->daddr, &fl6->saddr);
>> +restart:
>> +	for (rt = fn->leaf; rt; rt = rt->dst.rt6_next) {
>> +		if (rt6_check_expired(rt))
>> +			continue;
> 
> [1] (backreference for a comment below)
> 
>> +		if (!(rt->rt6i_flags & RTF_GATEWAY))
>> +			continue;
>> +		if (fl6->flowi6_oif != rt->dst.dev->ifindex)
>> +			continue;
>> +		if (!ipv6_addr_equal(&rdfl->gateway, &rt->rt6i_gateway))
>> +			continue;
>> +		break;
>> +	}
>> +
>> +	if (!rt)
>> +		rt = net->ipv6.ip6_null_entry;
>> +	BACKTRACK(net, &fl6->saddr);
>> +out:
>> +	dst_hold(&rt->dst);
>> +
>> +	read_unlock_bh(&table->tb6_lock);
>> +
>> +	return rt;
>> +};
>> +
>> +static struct dst_entry *ip6_route_redirect(const struct flowi6 *fl6,
>> +					   const struct in6_addr *gateway,
>> +					   struct net *net)
> 
> The ordering of the arguments could be in the same style as the other
> ip6_route_* functions (net, flow, gateway). But that is not that important.
> 
>> +{
>> +	int flags = RT6_LOOKUP_F_HAS_SADDR;
>> +	struct ip6rd_flowi rdfl;
>> +
>> +	rdfl.fl6 = *fl6;
>> +	rdfl.gateway = *gateway;
>> +
>> +	if (rt6_need_strict(&fl6->daddr))
>> +		flags |= RT6_LOOKUP_F_IFACE;
> 
> This is not needed because you do the matching on interfaces yourself
> in __ip6_route_redirect. The flag should currently not be checked on
> this code path down in fib6_rule_lookup.
> 
>> +
>> +	return fib6_rule_lookup(net, &rdfl.fl6,
>> +						   flags, __ip6_route_redirect);
> 
> This line jumped a bit too far to the right, too. ;)
> 
>> +}
>> +
>>  void ip6_redirect(struct sk_buff *skb, struct net *net, int oif, u32 mark)
> 
> Maybe we should rename oif to iif now?
Yes, that looks no ambiguity.
> 
>>  {
>>  	const struct ipv6hdr *iph = (struct ipv6hdr *) skb->data;
>> @@ -1171,9 +1239,8 @@ void ip6_redirect(struct sk_buff *skb, struct net *net, int oif, u32 mark)
>>  	fl6.saddr = iph->saddr;
>>  	fl6.flowlabel = ip6_flowinfo(iph);
>>  
>> -	dst = ip6_route_output(net, NULL, &fl6);
>> -	if (!dst->error)
>> -		rt6_do_redirect(dst, NULL, skb);
>> +	dst = ip6_route_redirect(&fl6, &ipv6_hdr(skb)->saddr, net);
>> +	rt6_do_redirect(dst, NULL, skb);
> 
> What is the reason you left out the dst.error check? E.g. if a system had a
> prohbit rule it is possible to circumvent this with a redirect packet now.
> 
That's my fault. i just understand the meaning of dst.error incorrectly, and
i will modify the patch according to your comment below.
> I would think about placing this check at [1] and fail the lookup
> early. rt6_do_redirect does check for null-entry, so you could omit the
> check here, then.
> 

Thanks,
  Duan