linux-kernel - Re: [RFC] Second attempt at kernel secure boot support

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Date:	Thu, 01 Nov 2012 09:08:25 +0000
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	Chris Friesen <chris.friesen@...band.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Jiri Kosina <jkosina@...e.cz>, Josh Boyer <jwboyer@...il.com>,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, linux-efi@...r.kernel.org
Subject: Re: [RFC] Second attempt at kernel secure boot support

On Wed, 2012-10-31 at 23:19 +0100, Oliver Neukum wrote:
> On Wednesday 31 October 2012 15:58:05 Chris Friesen wrote:
> > On 10/31/2012 02:14 PM, Oliver Neukum wrote:
> 
> > > That would do it on my system.
> > > Maybe in theory you could solve this by the kernel invalidating images
> > > it hasn't written itself and forbidding to change the resume partition from the
> > > kernel command line, but that would break user space hibernation.
> > 
> > If the resuming kernel refuses to resume from images it didn't create 
> > itself, why do you need to forbid changing the resume partition from the 
> > kernel command line?
> 
> You don't. Signed images solve the problem.

I really don't think they do.  The proposed attack vector is to try to
prevent a local root exploit from running arbitrary in-kernel code,
because that would compromise the secure boot part of the kernel.

I really think that's mythical: a local privilege elevation attack
usually exploits some bug (classically a buffer overflow) which executes
arbitrary code in kernel context.  In that case, the same attack vector
can be used to compromise any in-kernel protection mechanism including
turning off the secure boot capability and reading the in-kernel private
signing key.

There have been one or two privilege elevation attacks that didn't
involve in-kernel code (usually by compromising a suid binary or other
cross domain scripting attack) that would only compromise local root and
thus be confined to the secure boot prison but they are, historically, a
minority.

The point I'm making is that given that the majority of exploits will
already be able to execute arbitrary code in-kernel, there's not much
point trying to consider features like this as attacker prevention.  We
should really be focusing on discussing why we'd want to prevent a
legitimate local root from writing to the suspend partition in a secure
boot environment.

James

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