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

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Date:	Thu, 26 Sep 2013 14:22:10 +0200
From:	Vojtech Pavlik <vojtech@...e.cz>
To:	Pavel Machek <pavel@....cz>
Cc:	joeyli <jlee@...e.com>, Alan Stern <stern@...land.harvard.edu>,
	David Howells <dhowells@...hat.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>,
	James Bottomley <james.bottomley@...senpartnership.com>,
	Greg KH <gregkh@...uxfoundation.org>, JKosina@...e.com,
	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>
Subject: Re: [RFC V4 PATCH 00/15] Signature verification of hibernate snapshot

On Thu, Sep 26, 2013 at 02:06:21PM +0200, Pavel Machek wrote:

> > For the symmetric key solution, I will try HMAC (Hash Message
> > Authentication Code). It's already used in networking, hope the
> > performance is not too bad to a big image.
> 
> Kernel already supports crc32 of the hibernation image, you may want
> to take a look how that is done.
> 
> Maybe you want to replace crc32 with cryptographics hash (sha1?) and
> then use only hash for more crypto? That way speed of whatever crypto
> you do should not be an issue.

Well, yes, one could skip the CRC when the signing is enabled to gain a
little speedup.

> Actually...
> 
> Is not it as simple as storing hash of hibernation image into NVRAM
> and then verifying the hash matches the value in NVRAM on next
> startup? No encryption needed. 

First, there is no encryption going on. Only doing a HMAC (digest (hash)
using a key) of the image.

Second, since NVRAM is accessible through efivarsfs, storing the hash in
NVRAM wouldn't prevent an attacker from modifying the hash to match a
modified image.

There is a reason why the key for the HMAC is stored in the NVRAM in a
BootServices variable that isn't accessible from the OS and is
write-protected on hardware level from the OS.

> And that may even be useful for non-secure-boot people, as it ensures
> you boot right image after resume, boot it just once, etc...

The HMAC approach isn't much more complicated, and it gives you all
these benefits even with secure boot disabled.

-- 
Vojtech Pavlik
Director SUSE Labs
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