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

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Date:	Mon, 5 Nov 2012 16:31:40 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	Chris Friesen <chris.friesen@...band.com>,
	Pavel Machek <pavel@....cz>,
	Eric Paris <eparis@...isplace.org>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Oliver Neukum <oneukum@...e.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	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 Mon, 5 Nov 2012, Jiri Kosina wrote:

> Do I understand you correctly that by the 'glue' stuff you actually mean 
> the division of the kexec image into segments?
> 
> Of course, when we are dividing the image into segments and then passing 
> those individually (even more so if some transformations are performed on 
> those segments, which I don't know whether that's the case or not), then 
> we can't do any signature verification of the image any more.
> 
> But I still don't fully understand what is so magical about taking the 
> kernel image as is, and passing the whole lot to the running kernel as-is, 
> allowing for signature verification.
> 
> Yes, it couldn't be sys_kexec_load(), as that would be ABI breakage, so 
> it'd mean sys_kexec_raw_load(), or whatever ... but I fail to see why that 
> would be problem in principle.
> 
> If you can point me to the code where all the magic that prevents this 
> easy handling is happening, I'd appreciate it.

OK, so after wandering through kexec-tools sources for a while, I am 
starting to get your point. I wasn't actually aware of the fact that it 
supports such a wide variety of binary formats etc. (multiboot, nbi, etc).

I had a naive idea of just putting in-kernel verification of a complete 
ELF binary passed to kernel by userspace, and if the signature matches, 
jumping to it.
Would work for elf-x86_64 nicely I guess, but we'd lose a lot of other 
functionality currently being provided by kexec-tools.

Bah. This is a real pandora's box.

-- 
Jiri Kosina
SUSE Labs
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