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

[Casey Schaufler]

On 8/21/2019 8:54 PM, David Miller wrote:
> From: Paul Moore <paul at paul-moore.com>
> Date: Wed, 21 Aug 2019 23:27:03 -0400
>
>> On Wed, Aug 21, 2019 at 6:50 PM David Miller <davem at davemloft.net> wrote:
>>> From: Paul Moore <paul at 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.

Only SELinux and Smack use the networking hooks today,
although I understand that AppArmor has plans to do so
in the not too distant future. Smack enables labeled
networking at all times. While Smack doesn't have the
expansive use that SELinux does because of Android, it
is used extensively in embedded systems via Tizen and
Yocto Project deployments.

>>> That really defeats the whole purpose of making it dynamic. :-/

It argues that fulfilling the needs of LSMs ought to be a basic
feature of the skb, rather than a dynamic extension. When LSMs were
introduced 20 years ago it was assumed their use would be rare, and
it was. Today almost all Linux systems use LSMs, and once AppArmor
adds network labeling it will be quite difficult to find a major
distribution that doesn't need the support.

>> 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.

Smack uses labeled (CIPSO now, CALIPSO 'soon') networking by default
and depends on it heavily for basic system policy enforcement.

>> 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.

Tizen isn't a distro, but neither is Android.

>> 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.

As Paul has mentioned, the LSM community (Paul and me in particular)
have been looking for a better way to deal with the network stack
for a long time.

> 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.