linux-kernel - Re: [RFC PATCH 1/3] ipv4: Run a reverse sk

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Message-ID: <Zu2pev10zUAEnbYm@GHGHG14>
Date: Fri, 20 Sep 2024 17:57:30 +0100
From: Tiago Lam <tiagolam@...udflare.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>,
	David Ahern <dsahern@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
	Jakub Sitnicki <jakub@...udflare.com>, kernel-team@...udflare.com
Subject: Re: [RFC PATCH 1/3] ipv4: Run a reverse sk_lookup on sendmsg.

On Wed, Sep 18, 2024 at 08:45:23AM -0400, Willem de Bruijn wrote:
> Tiago Lam wrote:
> > In order to check if egress traffic should be allowed through, we run a
> > reverse socket lookup (i.e. normal socket lookup with the src/dst
> > addresses and ports reversed) to check if the corresponding ingress
> > traffic is allowed in.
> 
> The subject and this description makes it sound that the change always
> runs a reverse sk_lookup on sendmsg.
> 
> It also focuses on the mechanism, rather than the purpose.
> 
> The feature here adds IP_ORIGDSTADDR as a way to respond from a
> user configured address. With the sk_lookup limited to this new
> special case, as a safety to allow it.
> 
> If I read this correctly, I suggest rewording the cover letter and
> commit to make this intent and behavior more explicit.
> 

I think that makes sense, given this is really about two things:
1. Allowing users to use IP_ORIGDSTADDR to set src address and/or port
on sendmsg();
2. When they do, allow for that return traffic to exit without any extra
configuration, thus limiting how users can take advantage of this new
functionality.

I've made a few changes which hopefully makes that clearer in v2, which
I'm sending shortly. Thanks for these suggestions!

Tiago.