linux-kernel - Re: [PATCH V34 09/29] kexec_file: Restrict at runtime if the kernel is locked down

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Date:   Thu, 27 Jun 2019 14:59:09 +1000 (AEST)
From:   James Morris <jmorris@...ei.org>
To:     Matthew Garrett <matthewgarrett@...gle.com>
cc:     linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Jiri Bohac <jbohac@...e.cz>,
        David Howells <dhowells@...hat.com>,
        Matthew Garrett <mjg59@...gle.com>, kexec@...ts.infradead.org
Subject: Re: [PATCH V34 09/29] kexec_file: Restrict at runtime if the kernel
 is locked down

On Fri, 21 Jun 2019, Matthew Garrett wrote:

> From: Jiri Bohac <jbohac@...e.cz>
> 
> When KEXEC_SIG is not enabled, kernel should not load images through
> kexec_file systemcall if the kernel is locked down.

This is not a criticism of the patch but a related issue which I haven't 
seen discussed (apologies if it has).

If signed code is loaded into ring 0, verified by the kernel, then 
executed, you still lose your secure/trusted/verified boot state. If the 
currently running kernel has been runtime-compromised, any signature 
verification performed by the kernel cannot be trusted.

This problem is out of scope for the lockdown threat model (which 
naturally cannot include a compromised kernel), but folk should be aware 
that signature-verified kexec does not provide equivalent assurance to a 
full reboot on a secure-boot system.

Potential mitigations here include runtime integrity verification of the
kernel via a separate security monitor (hypervisor, SMM, TEE etc.) or some
kind of platform support for kexec verification.

-- 
James Morris
<jmorris@...ei.org>