netdev - Re: [PATCH] net: introduce ip_local_unbindable

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Message-ID: <CANP3RGePJ+z1t8oq-QS1tcwEYWanPHPargKpHkZZGiT4jMa6xw@mail.gmail.com>
Date:   Wed, 27 Nov 2019 12:50:39 -0800
From:   Maciej Żenczykowski <zenczykowski@...il.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Linux NetDev <netdev@...r.kernel.org>,
        Sean Tranchetti <stranche@...eaurora.org>,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Linux SCTP <linux-sctp@...r.kernel.org>
Subject: Re: [PATCH] net: introduce ip_local_unbindable_ports sysctl

On Wed, Nov 27, 2019 at 5:14 AM Marcelo Ricardo Leitner
<marcelo.leitner@...il.com> wrote:
>
> On Tue, Nov 26, 2019 at 04:13:13PM -0800, Maciej Żenczykowski wrote:
> > From: Maciej Żenczykowski <maze@...gle.com>
> >
> > and associated inet_is_local_unbindable_port() helper function:
> > use it to make explicitly binding to an unbindable port return
> > -EPERM 'Operation not permitted'.
> >
> > Autobind doesn't honour this new sysctl since:
> >   (a) you can simply set both if that's the behaviour you desire
> >   (b) there could be a use for preventing explicit while allowing auto
> >   (c) it's faster in the relatively critical path of doing port selection
> >       during connect() to only check one bitmap instead of both
> ...
> > If we *know* that certain ports are simply unusable, then it's better
> > nothing even gets the opportunity to try to use them.  This way we at
> > least get a quick failure, instead of some sort of timeout (or possibly
> > even corruption of the data stream of the non-kernel based use case).
>
> This is doable with SELinux today, no?

Perhaps, but SELinux isn't used by many distros, including the servers
where I have nics that steal some ports.  It's also much much
more difficult, requiring a policy, compilers, etc... and it gets even
more complex if you need to dynamically modify the set of ports,
which requires extra tools and runtime permissions.