linux-kernel - Re: kexec_load(2) bypasses signature verification

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Message-ID: <20150615200115.GG5003@thunk.org>
Date:	Mon, 15 Jun 2015 16:01:15 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Josh Boyer <jwboyer@...oraproject.org>
Cc:	Eric Biederman <ebiederm@...ssion.com>,
	David Howells <dhowells@...hat.com>,
	kexec <kexec@...ts.infradead.org>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: Re: kexec_load(2) bypasses signature verification

On Mon, Jun 15, 2015 at 09:37:05AM -0400, Josh Boyer wrote:
> The bits that actually read Secure Boot state out of the UEFI
> variables, and apply protections to the machine to avoid compromise
> under the SB threat model.  Things like disabling the old kexec...

I don't have any real interest in using Secure Boot, but I *am*
interested in using CONFIG_KEXEC_VERIFY_SIG[1].  So perhaps we need to
have something similar to what we have with signed modules in terms of
CONFIG_MODULE_SIG_FORCE and module/sig_enforce, but for
KEXEC_VERIFY_SIG.  This would mean creating a separate flag
independent of the one Linus suggested for Secure Boot, but since we
have one for signed modules, we do have precedent for this sort of
thing.

						- Ted

[1] Yes, it doesn't buy all that much, since if the system is rooted
the adversary can just replace the kernel in /boot and force a normal,
slower reboot, but the same could be said for signed modules --- the
adversary could just replace all of /boot/vmlinux-<kver> and
/lib/modules/<kver>.  But both measures make it a tad more bit
difficult, especially for the adversary to do this replacement without
being noticed (for example linode will send me e-mail if the system
reboots normally, but not with a kexec-mediated reboot), and for cloud
systems where we don't have secure boot anyway, it's about the best we
can do.
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