netdev - Re: [PATCH] net: introduce ip_local_unbindable

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Message-Id: <20191127.143333.1387731429786481895.davem@davemloft.net>
Date:   Wed, 27 Nov 2019 14:33:33 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     zenczykowski@...il.com
Cc:     marcelo.leitner@...il.com, netdev@...r.kernel.org,
        stranche@...eaurora.org, subashab@...eaurora.org,
        edumazet@...gle.com, linux-sctp@...r.kernel.org
Subject: Re: [PATCH] net: introduce ip_local_unbindable_ports sysctl

From: Maciej Żenczykowski <zenczykowski@...il.com>
Date: Wed, 27 Nov 2019 12:50:39 -0800

> 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.

I can see both sides of this argument, but anyways this is a new features and
thus net-next material.  It's nice to keep this discussion going, of course,
but if this trends in the positive you still need to resubmit this when
net-next opens back up.

Thanks.