linux-kernel - Re: [RFC V4 PATCH 00/15] Signature verification of hibernate snapshot

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Date:	Thu, 26 Sep 2013 16:56:33 +0200
From:	Vojtech Pavlik <vojtech@...e.cz>
To:	Jiri Kosina <jiri.kosina@...e.com>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Pavel Machek <pavel@....cz>,
	Alan Stern <stern@...land.harvard.edu>,
	David Howells <dhowells@...hat.com>,
	"Lee, Chun-Yi" <joeyli.kernel@...il.com>,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, linux-efi@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-crypto@...r.kernel.org,
	opensuse-kernel@...nsuse.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Len Brown <len.brown@...el.com>,
	Josh Boyer <jwboyer@...hat.com>,
	Matt Fleming <matt.fleming@...el.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	"David S. Miller" <davem@...emloft.net>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	Gary Lin <GLin@...e.com>, Vivek Goyal <vgoyal@...hat.com>,
	"Lee, Chun-Yi" <jlee@...e.com>
Subject: Re: [RFC V4 PATCH 00/15] Signature verification of hibernate snapshot

On Thu, Sep 26, 2013 at 04:48:00PM +0200, Jiri Kosina wrote:

> > The only two problems I see are
> > 
> >      1. The key isn't generational (any compromise obtains it).  This
> >         can be fixed by using a set of keys generated on each boot and
> >         passing in both K_{N-1} and K_N
> 
> I think this could be easily made optional, leaving the user with choice 
> of faster or "safer" boot.

Ideally, the key should be regenerated on each true reboot and kept the
same if it is just a resume. Unfortunately, I don't see a way to
distinguish those before we call ExitBootServices().

The reasoning behind that is that in the case of a kernel compromise, a
suspended-and-resumed kernel will still be compromised, so there is no
value in passing it a new key. A freshly booted kernel, though, should
get a new key, exactly because the attacker could have obtained a key
from the previous, compromised one.

This speeds up the ususal suspend-and-resume cycle, but provides full
security once the user performs a full reboot.

The question that remains is how to tell in advance.

> >      2. No external agency other than the next kernel can do the
> >         validation since the validating key has to be secret
> 
> This is true, but as you said, the relevance of this seems to be rather 
> questionable.

Indeed, it's hard to imagine a scenario that is also valid within the
secure boot threat model.

-- 
Vojtech Pavlik
Director SUSE Labs
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