Re: [PATCH 9/9] MODSIGN: Allow the "db" UEFI variable to be suppressed

Peter Jones
2016-11-21T19:18:46Z

On Mon, Nov 21, 2016 at 08:06:44PM +0100, Ard Biesheuvel wrote:
> On 21 November 2016 at 20:05, Peter Jones <pjones@redhat.com> wrote:
> > On Mon, Nov 21, 2016 at 04:42:45PM +0000, Ard Biesheuvel wrote:
> >> On 21 November 2016 at 16:26, Josh Boyer <jwboyer@fedoraproject.org> wrote:
> >> > On Mon, Nov 21, 2016 at 11:18 AM, Ard Biesheuvel
> >> > <ard.biesheuvel@linaro.org> wrote:
> >> >> On 16 November 2016 at 18:11, David Howells <dhowells@redhat.com> wrote:
> >> >>> From: Josh Boyer <jwboyer@fedoraproject.org>
> >> >>>
> >> >>> If a user tells shim to not use the certs/hashes in the UEFI db variable
> >> >>> for verification purposes, shim will set a UEFI variable called
> >> >>> MokIgnoreDB. Have the uefi import code look for this and ignore the db
> >> >>> variable if it is found.
> >> >>>
> >> >>
> >> >> Similar concern as in the previous patch: it appears to me that you
> >> >> can DoS a machine by setting MokIgnoreDB if, e.g., its modules are
> >> >> signed against a cert that resides in db, and shim/mokmanager are not
> >> >> being used.
> >> >
> >> > If shim/mokmanager aren't used, then you can't actually modify
> >> > MokIgnoreDB. Again, it requires physical access and a reboot into
> >> > mokmanager to actually take effect.
> >> >
> >>
> >> This does the trick as well
> >>
> >> printf "\x07\x00\x00\x00\x01" >
> >> /sys/firmware/efi/efivars/MokIgnoreDB-605dab50-e046-4300-abb6-3dd810dd8b23
> >
> > So that really means two things. First, kernel should only honor any of
> > the Mok* variables if they're Boot Services-only variables. Second, to
> > avoid the DoS, shim should create them all as Boot Services-only the
> > first time it boots. That'll prevent them from being created post-boot.
> >
>
> All of that assumes you are using shim and mokmanager in the first place.

No, it doesn't. If you're not using shim, there's no DoS problem,
because what would you be DoSing? And likewise, if you're not using
Secure Boot at all, you have no guarantee of anything about your boot
environment, starting with the idea that the boot loader isn't hostile.
If you're not using Secure Boot, a hostile pre-boot driver could easily
add DB entries just as easily as MokList entries, or any other
variables.

The fact that keys can be injected is true with or without this patch,
though it does make it easier. But making a boot loader that injects
keys into the kernel's built-in keyring isn't actually very difficult.

If you're not using firmware enforced SB and shim, you do not have
security against this.

--
Peter