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

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Message-ID: <CACdnJusJeCYPKVFHiu6yn+mfqQe5k0RqZhbCUZEkxtXx_shMmw@mail.gmail.com>
Date:   Thu, 27 Jun 2019 08:28:08 -0700
From:   Matthew Garrett <mjg59@...gle.com>
To:     James Morris <jmorris@...ei.org>
Cc:     LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Jiri Bohac <jbohac@...e.cz>,
        David Howells <dhowells@...hat.com>, kexec@...ts.infradead.org
Subject: Re: [PATCH V34 09/29] kexec_file: Restrict at runtime if the kernel
 is locked down

On Wed, Jun 26, 2019 at 9:59 PM James Morris <jmorris@...ei.org> wrote:
> 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.

By that metric, on a secure boot system how do we determine that code
running in the firmware environment wasn't compromised before it
launched the initial signed kernel?