netdev - Re: [PATCH nf-next v5 0/6] Netfilter egress hook

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Message-ID: <e4f1700c-c299-7091-1c23-60ec329a5b8d@iogearbox.net>
Date:   Thu, 30 Sep 2021 08:08:53 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Pablo Neira Ayuso <pablo@...filter.org>,
        netfilter-devel@...r.kernel.org
Cc:     davem@...emloft.net, netdev@...r.kernel.org, kuba@...nel.org,
        lukas@...ner.de, kadlec@...filter.org, fw@...len.de,
        ast@...nel.org, edumazet@...gle.com, tgraf@...g.ch,
        nevola@...il.com, john.fastabend@...il.com, willemb@...gle.com
Subject: Re: [PATCH nf-next v5 0/6] Netfilter egress hook

On 9/28/21 11:55 AM, Pablo Neira Ayuso wrote:
> Hi,
> 
> This patchset v5 that re-adds the Netfilter egress:
> 
> 1) Rename linux/netfilter_ingress.h to linux/netfilter_netdev.h
>     from Lukas Wunner.
> 
> 2) Generalize ingress hook file to accomodate egress support,
>     from Lukas Wunner.
> 
> 3) Modularize Netfilter ingress hook into nf_tables_netdev: Daniel
>     Borkmann is requesting for a mechanism to allow to blacklist
>     Netfilter, this allows users to blacklist this new module that
>     includes ingress chain and the new egress chain for the netdev
>     family. There is no other in-tree user of the ingress and egress
>     hooks than this which might interfer with his matter.
> 
> 4) Place the egress hook again before the tc egress hook as requested
>     by Daniel Borkmann. Patch to add egress hook from Lukas Wunner.
>     The Netfilter egress hook remains behind the static key, if unused
>     performance degradation is negligible.
> 
> 5) Add netfilter egress handling to af_packet.
> 
> Arguably, distributors might decide to compile nf_tables_netdev
> built-in. Traditionally, distributors have compiled their kernels using
> the default configuration that Netfilter Kconfig provides (ie. use
> modules whenever possible). In any case, I consider that distributor
> policy is out of scope in this discussion, providing a mechanism to
> allow Daniel to prevent Netfilter ingress and egress chains to be loaded
> should be sufficient IMHO.

Hm, so in the case of SRv6 users were running into a similar issue and commit
7a3f5b0de364 ("netfilter: add netfilter hooks to SRv6 data plane") [0] added
a new hook along with a sysctl which defaults the new hook to off.

The rationale for it was given as "the hooks are enabled via nf_hooks_lwtunnel
sysctl to make sure existing netfilter rulesets do not break." [0,1]

If the suggestion to flag the skb [2] one way or another from the tc forwarding
path (e.g. skb bit or per-cpu marker) is not technically feasible, then why not
do a sysctl toggle like in the SRv6 case?

   [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7a3f5b0de3647c854e34269c3332d7a1e902901a
   [1] https://lore.kernel.org/netdev/20210830093852.21654-1-pablo@netfilter.org/
   [2] https://lore.kernel.org/netdev/11584665-e3b5-3afe-d72c-ef92ad0b9313@iogearbox.net/