linux-kernel - Re: [PATCH 00/12] One more attempt at useful kernel lockdown

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Date:	Mon, 9 Sep 2013 16:19:11 -0700 (PDT)
From:	David Lang <david@...g.hm>
To:	Matthew Garrett <matthew.garrett@...ula.com>
cc:	"Valdis.Kletnieks@...edu" <Valdis.Kletnieks@...edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"jmorris@...ei.org" <jmorris@...ei.org>,
	"linux-security-module@...r.kernel.org" 
	<linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 00/12] One more attempt at useful kernel lockdown

On Mon, 9 Sep 2013, Matthew Garrett wrote:

> On Mon, 2013-09-09 at 11:25 -0700, David Lang wrote:
>
>> 1 lock down modules
>> 2 lock down kexec
>
> Having thought about this, the answer is no. It presents exactly the
> same problem as capabilities do - the set can never be meaningfully
> extended. If an application sets only the bits it knows about, and if a
> new security-sensitive feature is added to the kernel, the feature will
> be left enabled and the system will be insecure. Alternatively, if an
> application sets all the bits regardless of whether it knows them or
> not, it may enable a lockdown feature that it otherwise required.

In this case you are no less secure than you were before the feature was added, 
you just can't take advantage of the new feature without updating userspace.

That's a very common situation

> The only way this is useful is if all the bits are semantically
> equivalent, and in that case there's no point in having anything other
> than a single bit. Users who want a more fine-grained interface should
> use one of the existing mechanisms for doing so - leave the kernel open
> and impose the security policy from userspace using either capabilities
> or selinux.

so if you only have a single bit, how do you deal with the case where that bit 
locks down something that's required? (your reason for not just setting all bits 
in the first approach)

your arguments don't seem self consistent.

why should there only be one way to lock down a system? there are lots of 
different use cases.

If I'm building a kiosk PC (or voting machine), I want to disable a lot of 
things that I could not get away with disabling on a generic laptop. Are we 
going to have Securelevel, ReallySecurelevel, ReallyReallySecurelevel, etc? or 
can we accept that security is not binary and allow users to disable features 
in a more granualar way?

And if SELinux can do the job, what is the reason for creating this new option?

David Lang
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