linux-kernel - Re: [RFC] Turn lockdown into an LSM

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Date:   Thu, 23 May 2019 04:05:56 +1000 (AEST)
From:   James Morris <jmorris@...ei.org>
To:     Andy Lutomirski <luto@...nel.org>
cc:     Matthew Garrett <mjg59@...gle.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Turn lockdown into an LSM

On Wed, 22 May 2019, Andy Lutomirski wrote:

> And I still think it would be nice to have some credible use case for
> a more fine grained policy than just the tri-state.  Having a lockdown
> policy of "may not violate kernel confidentiality except using
> kprobes" may be convenient, but it's also basically worthless, since
> kernel confidentiality is gone.

This is an important point, but there's also "can't use any lockdown 
features because the admin might need to use kprobes".  I mention a 
use-case below.

I think it's fine (and probably preferred) to keep the default behavior 
tri-state and allow LSMs to implement finer-grained policies.

> All this being said, I do see one big benefit for LSM integration:
> SELinux or another LSM could allow certain privileged tasks to bypass
> lockdown.  

Some environments _need_ a "break glass" option, and a well-defined policy
(e.g. an SELinux domain which can only be entered via serial console, with
2FA or JIT credentials) to selectively un-lock the kernel lockdown in  
production would mean the difference between having a fleet of millions of
nodes 99.999% locked down vs 0%.

> This seems fine, except that there's potential nastiness
> where current->cred isn't actually a valid thing to look at in the
> current context.

Right.

Can we identify any such cases in the current patchset?

One option would be for the LSM to assign a default (untrusted/unknown) 
value for the subject and then apply policy as needed (e.g. allow or deny 
these).

> So I guess my proposal is: use LSM, but make the hook very coarse
> grained: int security_violate_confidentiality(const struct cred *) and
> int security_violate_integrity(const struct cred *).

Perhaps security_kernel_unlock_*

-- 
James Morris
<jmorris@...ei.org>