linux-kernel - Re: Read-protected UEFI variables

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Message-ID: <CAKv+Gu_V7BCihPhZf=pKrkBZwFWeEoupamXvYSqdS+k93Gme=g@mail.gmail.com>
Date:   Wed, 14 Feb 2018 13:09:59 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Benjamin Drung <benjamin.drung@...fitbricks.com>
Cc:     Matthew Garrett <matthew.garrett@...ula.com>,
        Jeremy Kerr <jk@...abs.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        linux-efi@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Read-protected UEFI variables

On 14 February 2018 at 12:52, Benjamin Drung
<benjamin.drung@...fitbricks.com> wrote:
> Hi,
>
> I am exploring the possibility to store SSH and other keys in UEFI
> variables for systems that do not have persistent storage. These
> systems boot via network and need individual SSH keys which ideally
> should not be distributed via network.
>
> The plan is to write a small daemon that starts at boot and gets the
> SSH keys from EFI variables to individualize the system with SSH keys.
> I plan to release the code as free software. Simple proof-of-concept
> code:
>
> mount -t efivarfs none /sys/firmware/efi/efivars
> for key in ssh_host_dsa_key ssh_host_ecdsa_key ssh_host_rsa_key; do
>   dd ibs=1 skip=4 if=/sys/firmware/efi/efivars/${key}-89df11f4-38e6-473e-ab43-b4406b76fba9 of=/etc/ssh/$key
> done
>
> I am not the first person having the idea to use UEFI variables to
> store keys:
> https://www.usenix.org/conference/srecon17asia/program/presentation/korgachin
>
> There is one problem: The keys should be readable only by root. When
> mounting efivarfs, all variables have the permission 644 which makes
> them readable by all users. I have different ideas how to solve it:
>
> 1) Hard-code a list of GUIDs that should be only readable by root in
> the kernel module. These modules would also be not set to immutable.
>
> 2) Instead of hard-coding GUIDs, add a kernel module parameter to
> specify the GUIDs. Maybe have a default list in the kernel module.
>
> 3) Add a mount option to specify the protected GUIDs.
>
> Feedback is welcome.
>

I'd consider a patch that makes the permissions a mount option for
efivarfs, applying to all variables. The reason is that these
variables shouldn't have been world readable in the first place, and I
am reluctant to make this overly complex.

On the other hand, you should realize that UEFI was never designed to
keep secrets, and so whether it is a good idea to put secrets in UEFI
variables to begin with is dubious IMHO.