netdev - Re: [PATCH net-next] ipv6: Don't use dst gateway directly in ip6_confirm

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Date:   Thu, 12 Sep 2019 00:52:45 +0200 (CEST)
From:   David Miller <davem@...emloft.net>
To:     sbrivio@...hat.com
Cc:     gnault@...hat.com, ja@....bg, nicolas.dichtel@...nd.com,
        dsahern@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] ipv6: Don't use dst gateway directly in
 ip6_confirm_neigh()

From: Stefano Brivio <sbrivio@...hat.com>
Date: Mon,  9 Sep 2019 22:44:06 +0200

> This is the equivalent of commit 2c6b55f45d53 ("ipv6: fix neighbour
> resolution with raw socket") for ip6_confirm_neigh(): we can send a
> packet with MSG_CONFIRM on a raw socket for a connected route, so the
> gateway would be :: here, and we should pick the next hop using
> rt6_nexthop() instead.
> 
> This was found by code review and, to the best of my knowledge, doesn't
> actually fix a practical issue: the destination address from the packet
> is not considered while confirming a neighbour, as ip6_confirm_neigh()
> calls choose_neigh_daddr() without passing the packet, so there are no
> similar issues as the one fixed by said commit.
> 
> A possible source of issues with the existing implementation might come
> from the fact that, if we have a cached dst, we won't consider it,
> while rt6_nexthop() takes care of that. I might just not be creative
> enough to find a practical problem here: the only way to affect this
> with cached routes is to have one coming from an ICMPv6 redirect, but
> if the next hop is a directly connected host, there should be no
> topology for which a redirect applies here, and tests with redirected
> routes show no differences for MSG_CONFIRM (and MSG_PROBE) packets on
> raw sockets destined to a directly connected host.
> 
> However, directly using the dst gateway here is not consistent anymore
> with neighbour resolution, and, in general, as we want the next hop,
> using rt6_nexthop() looks like the only sane way to fetch it.
> 
> Reported-by: Guillaume Nault <gnault@...hat.com>
> Signed-off-by: Stefano Brivio <sbrivio@...hat.com>

Applied.