netdev - Re: [PATCH net] ipv6: drop incoming packets having a v4mapped source address

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Date:   Wed, 2 Oct 2019 11:47:04 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Florian Westphal <fw@...len.de>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ipv6: drop incoming packets having a v4mapped source address

On Wed, Oct 2, 2019 at 11:38 AM Florian Westphal <fw@...len.de> wrote:
>
> Eric Dumazet <edumazet@...gle.com> wrote:
> > The dual stack API automatically forces the traffic to be IPv4
> > if v4mapped addresses are used at bind() or connect(), so it makes
> > no sense to allow IPv6 traffic to use the same v4mapped class.
> >
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Cc: Florian Westphal <fw@...len.de>
> > Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
> > Reported-by: syzbot <syzkaller@...glegroups.com>
> > ---
> >  net/ipv6/ip6_input.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/net/ipv6/ip6_input.c b/net/ipv6/ip6_input.c
> > index d432d0011c160f41aec09640e95179dd7b364cfc..2bb0b66181a741c7fb73cacbdf34c5160f52d186 100644
> > --- a/net/ipv6/ip6_input.c
> > +++ b/net/ipv6/ip6_input.c
> > @@ -223,6 +223,16 @@ static struct sk_buff *ip6_rcv_core(struct sk_buff *skb, struct net_device *dev,
> >       if (ipv6_addr_is_multicast(&hdr->saddr))
> >               goto err;
> >
> > +     /* While RFC4291 is not explicit about v4mapped addresses
> > +      * in IPv6 headers, it seems clear linux dual-stack
> > +      * model can not deal properly with these.
> > +      * Security models could be fooled by ::ffff:127.0.0.1 for example.
> > +      *
> > +      * https://tools.ietf.org/html/draft-itojun-v6ops-v4mapped-harmful-02
> > +      */
> > +     if (ipv6_addr_v4mapped(&hdr->saddr))
> > +             goto err;
> > +
>
> Any reason to only consider ->saddr instead of checking daddr as well?

I do not see reasons the packet should be accepted for sane configurations ?

I would rather have a separate patch for daddr if someone needs it.

(This also comes at a cpu cost for all received packets :/ )