linux-kernel - Re: [PATCH 4/4] module, KEYS: Make use of platform keyring for signature verification

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Date:   Tue, 15 Feb 2022 17:12:32 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Michal Suchánek <msuchanek@...e.de>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Philipp Rudo <prudo@...hat.com>, Baoquan He <bhe@...hat.com>,
        Alexander Egorenkov <egorenar@...ux.ibm.com>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        James Morse <james.morse@....com>,
        Dave Young <dyoung@...hat.com>,
        Kairui Song <kasong@...hat.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-modules@...r.kernel.org,
        keyrings@...r.kernel.org, linux-security-module@...r.kernel.org,
        stable@...nel.org, Eric Snowberg <eric.snowberg@...cle.com>
Subject: Re: [PATCH 4/4] module, KEYS: Make use of platform keyring for
 signature verification

On Tue, 2022-02-15 at 21:47 +0100, Michal Suchánek wrote:
> Hello,
> 
> On Tue, Feb 15, 2022 at 03:08:18PM -0500, Mimi Zohar wrote:
> > [Cc'ing Eric Snowberg]
> > 
> > Hi Michal,
> > 
> > On Tue, 2022-02-15 at 20:39 +0100, Michal Suchanek wrote:
> > > Commit 278311e417be ("kexec, KEYS: Make use of platform keyring for signature verify")
> > > adds support for use of platform keyring in kexec verification but
> > > support for modules is missing.
> > > 
> > > Add support for verification of modules with keys from platform keyring
> > > as well.
> > 
> > Permission for loading the pre-OS keys onto the "platform" keyring and
> > using them is limited to verifying the kexec kernel image, nothing
> > else.
> 
> Why is the platform keyring limited to kexec, and nothing else?
> 
> It should either be used for everything or for nothing. You have the
> option to compile it in and then it should be used, and the option to
> not compile it in and then it cannot be used.
> 
> There are two basic use cases:
> 
> (1) there is a vendor key which is very hard to use so you sign
> something small and simple like shim with the vendor key, and sign your
> kernel and modules with your own key that's typically enrolled with shim
> MOK, and built into the kernel.
> 
> (2) you import your key into the firmware, and possibly disable the
> vendor key. You can load the kernel directly without shim, and then your
> signing key is typically in the platform keyring and built into the
> kernel.
> 
> In neither case do I see any reason to use some keyrings for kexec and
> other keyrings for modules.

When building your own kernel there isn't a problem.  Additional keys
may be built into the kernel image, which are loaded onto the
".builtin_trusted_keys" keyring, and may be stored in MOK.  Normally
different keys are used for signing the kernel image and kernel
modules.  Kernel modules can be signed by the build time ephemeral
kernel module signing key, which is built into the kernel and
automatically loaded onto the ".builtin_trusted_keys" keyring.
 
Similarly distros build the kernel module signing key into the kernel,
which is built into the kernel and loaded onto the
".builtin_trusted_keys" keyring.  By loading the pre-OS keys onto the
".platform" keyring,  kexec may verify the distro or other signed
kernel images.

-- 
thanks,

Mimi