Re: linux-next: UEFI Secure boot lockdown patchset

Mimi Zohar
2018-05-01T22:21:32Z

On Tue, 2018-05-01 at 21:59 +0000, Matthew Garrett wrote:
> On Tue, May 1, 2018 at 2:50 PM Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:
>
> > On Tue, 2018-05-01 at 21:02 +0000, Matthew Garrett wrote:
> > > Hm. My concern is that distributions are going to ship IMA in a
> > > configuration that allows users to add their own keys at boot time (it's
> > > difficult to use it in a generic way otherwise), and that's going to
> allow
> > > kexecing of arbitrary images without requiring physical access. I think
> > > kexec_file_load() needs to be relying on non-IMA signatures.
>
> > I don't see how. Unless the kernel was built with extra room for a
> > local CA public key (CONFIG_SYSTEM_EXTRA_CERTIFICATE), which would be
> > loaded onto the builtin keyring, there is no way of adding keys to the
> > IMA keyring. Adding the extra public key would require the kernel to
> > be resigned.
>
> Oh, is kexec verified off the _module keyring? We still end up with the
> problem that distributions don't have a mechanism to ship IMA signatures
> yet, but that avoids the user modification problem. I've just posted a
> patchset to debian-dpkg, we'll see how that goes.

I'm not aware of a _module keyring. With IMA-appraisal, the signature
verification of the kernel image (kexec_file_load) uses the trusted
IMA keyring. Nayna Jain posted a patch that defines a new platform
keyring[1], which would only be used to validate the kernel image and
initramfs signatures.

Your review would be much appreciated!

I really do hope that some version of including file signatures in
Debian packages will be upstreamed soon. Good luck!

[1] https://lkml.org/lkml/2018/2/28/1089

Mimi