netdev - Re: [PATCH nf-next v4 5/5] af_packet: Introduce egress hook

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Date:   Sat, 30 Jan 2021 11:58:06 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        netfilter-devel <netfilter-devel@...r.kernel.org>,
        coreteam@...filter.org,
        Network Development <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Thomas Graf <tgraf@...g.ch>,
        Laura Garcia Liebana <nevola@...il.com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH nf-next v4 5/5] af_packet: Introduce egress hook

On Sat, Jan 30, 2021 at 11:26 AM Lukas Wunner <lukas@...ner.de> wrote:
>
> On Sun, Jan 24, 2021 at 11:18:00AM -0500, Willem de Bruijn wrote:
> > On Sun, Jan 24, 2021 at 6:14 AM Lukas Wunner <lukas@...ner.de> wrote:
> > > On Fri, Jan 22, 2021 at 11:13:19AM -0500, Willem de Bruijn wrote:
> > > > On Fri, Jan 22, 2021 at 4:44 AM Lukas Wunner <lukas@...ner.de> wrote:
> > > > > Add egress hook for AF_PACKET sockets that have the PACKET_QDISC_BYPASS
> > > > > socket option set to on, which allows packets to escape without being
> > > > > filtered in the egress path.
> > > > >
> > > > > This patch only updates the AF_PACKET path, it does not update
> > > > > dev_direct_xmit() so the XDP infrastructure has a chance to bypass
> > > > > Netfilter.
> > > >
> > > > Isn't the point of PACKET_QDISC_BYPASS to skip steps like this?
> > >
> > > I suppose PACKET_QDISC_BYPASS "was introduced to bypass qdisc,
> > > not to bypass everything."
> > >
> > > (The quote is taken from this message by Eric Dumazet:
> > > https://lore.kernel.org/netfilter-devel/a9006cf7-f4ba-81b1-fca1-fd2e97939fdc@gmail.com/
> > > )
> >
> > I see. I don't understand the value of a short-cut fast path if we
> > start chipping away at its characteristic feature.
>
> The point is to filter traffic coming in through af_packet.
> Exempting PACKET_QDISC_BYPASS from filtering would open up a
> trivial security hole.

Sure. But that argument is no different for TC_EGRESS.

That's why packet sockets require CAP_NET_RAW. It is perhaps
unfortunately that it is ns_capable instead of capable. But there is
nothing netfilter specific about this.