Re: New skb extension for use by LSMs (skb "security blob")?

From: David Miller <davem@davemloft.net>
To: paul@paul-moore.com
Cc: netdev@vger.kernel.org, linux-security-module@vger.kernel.org,
	selinux@vger.kernel.org
Subject: Re: New skb extension for use by LSMs (skb "security blob")?
Date: Wed, 21 Aug 2019 20:54:54 -0700 (PDT)	[thread overview]
Message-ID: <20190821.205454.2103510420957943248.davem@davemloft.net> (raw)
In-Reply-To: <CAHC9VhRLexftb5mK8_izVQkv9w46m=aPukws2d2m+yrMvHUF_g@mail.gmail.com>

From: Paul Moore <paul@paul-moore.com>
Date: Wed, 21 Aug 2019 23:27:03 -0400

> On Wed, Aug 21, 2019 at 6:50 PM David Miller <davem@davemloft.net> wrote:
>> From: Paul Moore <paul@paul-moore.com>
>> Date: Wed, 21 Aug 2019 18:00:09 -0400
>>
>> > I was just made aware of the skb extension work, and it looks very
>> > appealing from a LSM perspective.  As some of you probably remember,
>> > we (the LSM folks) have wanted a proper security blob in the skb for
>> > quite some time, but netdev has been resistant to this idea thus far.
>> >
>> > If I were to propose a patchset to add a SKB_EXT_SECURITY skb
>> > extension (a single extension ID to be shared among the different
>> > LSMs), would that be something that netdev would consider merging, or
>> > is there still a philosophical objection to things like this?
>>
>> Unlike it's main intended user (MPTCP), it sounds like LSM's would use
>> this in a way such that it would be enabled on most systems all the
>> time.
>>
>> That really defeats the whole purpose of making it dynamic. :-/
> 
> I would be okay with only adding a skb extension when we needed it,
> which I'm currently thinking would only be when we had labeled
> networking actually configured at runtime and not just built into the
> kernel.  In SELinux we do something similar today when it comes to our
> per-packet access controls; if labeled networking is not configured we
> bail out of the LSM hooks early to improve performance (we would just
> be comparing unlabeled_t to unlabeled_t anyway).  I think the other
> LSMs would be okay with this usage as well.
> 
> While a number of distros due enable some form of LSM and the labeled
> networking bits at build time, vary few (if any?) provide a default
> configuration so I would expect no additional overhead in the common
> case.
> 
> Would that be acceptable?

I honestly don't know, I kinda feared that once the SKB extension went in
people would start dumping things there and that's exactly what's happening.
I just so happened to be reviewing:

	https://patchwork.ozlabs.org/patch/1150091/

while you were writing this email.

It's rediculous, the vultures are out.