netdev - Re: [PATCH v3] icmp: support rfc5837

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Date:   Fri, 19 Mar 2021 17:54:54 -0600
From:   David Ahern <dsahern@...il.com>
To:     Ishaan Gandhi <ishaangandhi@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        willemb@...gle.com,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH v3] icmp: support rfc5837

On 3/19/21 10:11 AM, Ishaan Gandhi wrote:
> Thank you. Would it be better to do instead:
> 
> +	if_index = skb->skb_iif;
> 
> or
> 
> +	if_index = ip_version == 4 ? inet_iif(skb) : skb->skb_iif;
> 

If the packet comes in via an interface assigned to a VRF, skb_iif is
most likely the VRF index which is not what you want.

The general problem of relying on skb_iif was discussed on v1 and v2 of
your patch. Returning an iif that is a VRF, as an example, leaks
information about the networking configuration of the device which from
a quick reading of the RFC is not the intention.

Further, the Security Considerations section recommends controls on what
information can be returned where you have added a single sysctl that
determines if all information or none is returned. Further, it is not a
a per-device control but a global one that applies to all net devices -
though multiple entries per netdevice has a noticeable cost in memory at
scale.

In the end it seems to me the cost benefit is not there for a feature
like this.