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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 13 Jun 2025, Marek Marczykowski-Górecki wrote:
> 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.

That's right. U-boot supports something similar as well.