linux-kernel - Re: [PATCH v2] builddeb: Support signing kernels with the module signing key

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Message-ID: <20220208161259.inytmx6gm4w34gct@jak-t480s>
Date:   Tue, 8 Feb 2022 17:12:59 +0100
From:   Julian Andres Klode <julian.klode@...onical.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Ben Hutchings <ben@...adent.org.uk>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        efi@...ts.einval.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>, keyrings@...r.kernel.org,
        David Woodhouse <dwmw2@...radead.org>,
        debian-kernel <debian-kernel@...ts.debian.org>
Subject: Re: [PATCH v2] builddeb: Support signing kernels with the module
 signing key

On Tue, Feb 08, 2022 at 01:10:34PM +0000, Matthew Wilcox wrote:
> On Tue, Feb 08, 2022 at 12:01:22PM +0100, Julian Andres Klode wrote:
> > It's worth pointing out that in Ubuntu, the generated MOK key
> > is for module signing only (extended key usage 1.3.6.1.4.1.2312.16.1.2),
> > kernels signed with it will NOT be bootable.
> 
> Why should these be separate keys?  There's no meaningful security
> boundary between a kernel module and the ernel itself; a kernel
> modulecan, for example, write to CR3, and that's game over for
> any pretence at separation.

I don't really _know_, but I believe the difference is that the
kernel binaries runs in boot services, and calls ExitBootServices,
whereas modules are loaded after ExitBootServices.

But I don't know the full rationale why (a) the feature exists in
the first place and (b) why the Ubuntu security team chose to require
that constraint.

My goal is just to make people aware of that so they can make
informed decisions :)

-- 
debian developer - deb.li/jak | jak-linux.org - free software dev
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