Re: [PATCH 8/9] MODSIGN: Import certificates from UEFI Secure Boot

Mimi Zohar
2016-12-02T20:18:56Z

Since this discussion affects which keys can be added to trusted
keyrings, cc'ing linux-ima-devel.

On Fri, 2016-12-02 at 10:57 -0800, James Bottomley wrote:
> On Thu, 2016-11-24 at 11:17 -0800, James Bottomley wrote:
> > On Mon, 2016-11-21 at 16:16 +0000, Ard Biesheuvel wrote:
> > > On 16 November 2016 at 18:11, David Howells <dhowells@redhat.com>
> > > wrote:
> > > > From: Josh Boyer <jwboyer@fedoraproject.org>
> > > >
> > > > Secure Boot stores a list of allowed certificates in the 'db'
> > > > variable. This imports those certificates into the system trusted
> > > > keyring. This allows for a third party signing certificate to
> > > > be used in conjunction with signed modules. By importing the
> > > > public certificate into the 'db' variable, a user can allow a
> > > > module signed with that certificate to load. The shim UEFI
> > > > bootloader has a similar certificate list stored in the
> > > > 'MokListRT' variable. We import those as well.
> > > >
> > >
> > > This sounds like a bad idea to me. For the standard databases like
> > > db and dbx, we can rely on the firmware to ensure that they are
> > > what you expect.
> >
> > Actually, I think it's a bad idea for the opposite reason: Shim
> > explicitly pivots the root of trust away from the db keys to its own
> > Moklist keys. We have no choice and are forced to trust db for the
> > secure boot part, but once we're in the kernel proper, I'd argue that
> > we would only want to trust the pivoted root, i.e. Moklist.
> >
> > Trusting both could generate unwanted consequences, like pressure on
> > Microsoft to sign modules or worse, pressure on OEMs to include
> > module keys or hashes ... or worst of all OEMs signing external
> > modules.
> >
> > > For MokListRt, not so much: anyone with sufficient
> > > capabilities can generate such a variable from userland, and not
> > > every arch/distro combo will be using shim and/or mokmanager. (The
> > > debates are still ongoing, but my position is that there is no need
> > > for shim at all on ARM given that the M$ problem only exists on
> > > x86)
> >
> > OK, so on this point, I'm already not using Shim on my x86 box.
> > However, what you find if you're using grub is that because grub
> > doesn't do signature verification, you still have to use the shim
> > protocol callout, so you need something between UEFI and grub to load
> > at least this protocol. I suppose this would go away once we can
> > persuade grub to verify signatures.
>
> Hm, that got crickets.
>
> Let me propose an alternative mechanism then.
>
> My problem is that although I am forced to trust the secure boot keys
> for the UEFI security boundary, I don't necessarily want to trust them
> for signing things for my kernel, so I want to pivot (or at
> leastselectively weed out) keys. Shim already has this concept
> partially with MokIgnoreDB.
>
> For the purposes of the kernel, I think we simply need a variable, lets
> call it MokKernelCerts, that gives the list of certificates to import
> into the kernel keyring. I think this variable should be BS NV only
> (not RT) meaning we have to collect it before ExitBootServices(). The
> reason for this is to ensure it's populated by a trusted entity within
> the UEFI secure boot boundary. This will cause a kexec problem, so we
> might have to relax this and use a RT shadow as we already do for
> MokList. The idea is that we populate the kernel certificates only
> from this variable, so policy can be decided by the bootloader (or
> something else which runs within the secure boot environment).
>
> You can stop reading here if you're not interested in *how* this policy
> would work.
>
> To make it work, Shim or one of the other intermediates would set up
> the variable. we could communicate policy to it with the usual Foo,
> FooUpdate mechanism we already use for MokList. The default policy (if
> the variable doesn't exist on firstboot) can be whatever the distro
> wants, so if Fedora wants all the secure boot certs, it can do that and
> other distros can follow their own stricter or less strict policies.
> The user would be able to overwrite this using the Update process,
> which could be password verified like MokList already is.
>
> Does this sound acceptable to everyone?