Re: [PATCH] docs: UEFI Secure Boot security policy

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Fri, Jun 13, 2025 at 08:35:26AM +0200, Jan Beulich wrote:
> On 12.06.2025 23:32, Stefano Stabellini wrote:
> > On Thu, 12 Jun 2025, Andrew Cooper wrote:
> >> +Support in Xen
> >> +--------------
> >> +
> >> +There are multiple ways to achieve this security goal, with differing
> >> +tradeoffs for the eventual system.
> >> +
> >> +On one end of the spectrum is the Unified Kernel Image.  e.g. Xen is 
> >> bundled
> >> +with the dom0 kernel and init-ramdisk, with an embedded command line, and 
> >> with
> >> +livepatching and kexec compiled out, and suitably signed.  The signature 
> >> is
> >> +checked by the bootloader and, as this covers all the privileged code, Xen
> >> +doesn't need to perform further checks itself.
> >> +
> >> +On the other end of the spectrum is maintaining the features of existing
> >> +deployments.  e.g. Xen needs signature checking capabilities for the dom0
> >> +kernel, livepatches and kexec kernels, and needs to allow the use of safe
> >> +command line options while disallowing unsafe ones.
> > 
> > I just wanted to mention that there is one more option which I used in
> > the past: the firmware/bootloader loads Xen, the Dom0 kernel, and other
> > binaries, check their signatures, then boot Xen.
> > 
> > This is similar to the "Unified Kernel Image" approach in the sense that
> > Xen doesn't need to do any signature checking for the dom0 kernel, but
> > it doesn't require all the binaries to be glued together.
> > 
> > Assuming that the firmware/bootloader is capable of loading multiple
> > binaries and checking the signature of multiple binaries before booting
> > the next element, it works fine.
> 
> How would an initrd, a ucode blob, or an XSM policy blob be signed?

At least grub supports gpg detached signatures, and can be configured to
require them.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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