Re: [PATCH 0/4] KEYS: validate key trust with owner and builtin keys only

Mimi Zohar
2014-06-10T14:53:52Z

On Tue, 2014-06-10 at 09:29 -0400, Josh Boyer wrote:
> On Tue, Jun 10, 2014 at 04:21:36PM +0300, Dmitry Kasatkin wrote:
> > On 10/06/14 15:52, Mimi Zohar wrote:
> > > On Tue, 2014-06-10 at 08:20 -0400, Josh Boyer wrote:
> > >> On Tue, Jun 10, 2014 at 11:48:14AM +0300, Dmitry Kasatkin wrote:
> > >>> Also I want to discuss here Fedora UEFI patches as they are the reason for
> > >>> the these original patchset.
> > >>>
> > >>> http://pkgs.fedoraproject.org/cgit/kernel.git/tree/modsign-uefi.patch
> > >>>
> > >>> They provide functionality to specify MokIgnoreDb variable to limit loading of
> > >>> UEFI keys only from MOK List, while ignoring DB. This is certainly a good
> > >>> functionality. But once MODULE_SIG_UEFI is enabled, it looks there is no way
> > >>> to prevent loading keys from UEFI at all. And this might not be a good default
> > >>> functionality. Someone might want not allow loading of keys from UEFI unless
> > >>> kernel parameter is specified to allow it without recompiling the kernel
> > >>> and disabling MODULE_SIG_UEFI.
> > >>>
> > >>> Josh, why such design decision was made?
> > >> IIRC, it's because kernel parameters can be added programmatically from a
> > >> remote user if they gain root access. Having a kernel parameter to
> > >> disable a key piece of secure boot isn't all that great. We disable
> > >> other kernel parameters like acpi_rspd as well.
> > > In this case, there shouldn't be a problem as the kernel parameters
> > > would further limit the keys usage.
> > >
> > > Mimi
> >
> > Josh probably means that it can be removed and restriction is lifted..
> > And after reboot, all keys come to the keyring..
>
> Right. Or if we went with your suggestion of the default being "do not
> load UEFI keys", then they could conversely add the parameter instead.
> This might not be an immediate threat to the SB attack vector itself
> (thought it could be if I thought about it harder), but it's certainly
> a change in system behavior that would catch a user unaware.

Agreed, it could catch the user unaware of the change, but always
allowing all the UEFI keys, is not a better alternative. Perhaps,
requiring the option, would at least prevent the user from being unaware
of the change.

Mimi

> At any rate, I'm likely not the best person to weigh in on this aspect.
> Matthew has certainly done more thinking about these kinds of problems.
>
> josh