Subject: Re: [PATCH] ipv4: Restore accept_local behaviour in fib_validate_source()

From: Hannes Frederic Sowa <[email-masked]>
Date: Thu, 14 Aug 2014 15:36:32 +0200

On Do, 2014-08-14 at 14:45 +0200, Sébastien Barré wrote:
> Commit 7a9bc9b introduced a short-circuit to avoid calling
> fib_validate_source when not needed. That change took rp_filter
> into account, but not accept_local. This resulted in a change
> of behaviour: with rp_filter and accept_local off, incoming
> packets with a local address in the source field should be
> dropped.
> 
> Here is how to reproduce the change pre/post 7a9bc9b commit:
> -configure the same IPv4 address on hosts A and B.
> -try to send an arp request from B to A.
> -The arp request will be dropped before commit 7a9bc9b,
> but accepted and answered after that commit.
> 
> This adds a check for ACCEPT_LOCAL, to maintain full
> fib validation in case it is 0.
> 
> Signed-off-by: Sébastien Barré <[email-masked]>
> 
> ---
>  net/ipv4/fib_frontend.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
> index 255aa99..fa1fb2d 100644
> --- a/net/ipv4/fib_frontend.c
> +++ b/net/ipv4/fib_frontend.c
> @@ -321,6 +321,7 @@ int fib_validate_source(struct sk_buff *skb, __be32 src, __be32 dst,
>  	int r = secpath_exists(skb) ? 0 : IN_DEV_RPFILTER(idev);
>  
>  	if (!r && !fib_num_tclassid_users(dev_net(dev)) &&
> +	    IN_DEV_ACCEPT_LOCAL(idev) &&
>  	    (dev->ifindex != oif || !IN_DEV_TX_REDIRECTS(idev))) {
>  		*itag = 0;
>  		return 0;

But fib_validate_check is too strict for this situation. In case
rp_filter and accept_local are disabled, you should also leave
__fib_validate_source directly after the RTN_UNICAST checks.

Bye,
Hannes