Some questions regarding firmware handling in the qemu driver

[Jim Fehlig]

Hi All,

I received a bug report (private, sorry) about inability to "deploy uefi virtual 
machine with secureboot enabled on aarch64 kvm host". Indeed the qemu driver has 
some checks that would prohibit using secure boot with aarch64 virt machines, e.g.

https://gitlab.com/libvirt/libvirt/-/blob/master/src/qemu/qemu_validate.c#L571

However it appears qemu does not restrict booting a firmware with keys enrolled 
and secure boot enabled. E.g.

qemu-system-aarch64 -m 4096 -cpu host -accel kvm -smp 4 -M virt -drive 
if=pflash,format=raw,readonly=on,file=/usr/share/qemu/aavmf-aarch64-opensuse-code.bin 
-drive if=pflash,format=raw,file=/vm_images/jim/images/test/test-vars-store.bin ...

seems to work fine and within the guest I see db keys loaded by kernel

[    4.782777] integrity: Loading X.509 certificate: UEFI:db
[    4.789494] integrity: Loaded X.509 cert 'Build time autogenerated kernel 
key: 44e3470bd0c5eb190e3292dfc42db061521184ee'
[    4.789548] integrity: Loading X.509 certificate: UEFI:db
[    4.789701] integrity: Loaded X.509 cert 'openSUSE Secure Boot Signkey: 
0332fa9cbf0d88bf21924b0de82a09a54d5defc8'
[    4.789710] integrity: Loading X.509 certificate: UEFI:db
[    4.789841] integrity: Loaded X.509 cert 'SUSE Linux Enterprise Secure Boot 
Signkey: 3fb077b6cebc6ff2522e1c148c57c777c788e3e7'

Can we consider easing the secure boot restrictions in qemuValidateDomainDefBoot?

Experimenting with the behavior on x86 raised other questions:

libvirt requires the firmware to support SMM to enable secure boot. But is SMM a 
strict requirement for secure boot? IIUC, lack of SMM makes the securely booted 
stack less secure since it is easier to tamper with it, but it does not prevent 
securely booting the components.

When selecting firmwares manually and marking the loader secure, VM creation 
fails unless SMM is explicitly set in <features>. E.g. the following will fail 
with "unsupported configuration: Secure boot requires SMM feature enabled"

   <os>
     <type arch="x86_64" machine="q35">hvm</type>
     <loader readonly="yes" secure="yes" 
type="pflash">/usr/share/qemu/ovmf-x86_64-smm-code.bin</loader>
     <nvram template="/usr/share/qemu/ovmf-x86_64-smm-vars.bin"/>
     <boot dev="hd"/>
   </os>

even though the descriptor file for /usr/share/qemu/ovmf-x86_64-smm-code.bin 
advertises secure-boot and requires-smm. Is this just a case of trying to mix 
old style explicit firmware selection vs firmware auto-select? I.e., if 
selecting the firmware explicitly, the onus is on the user to also specify any 
related and required config?

Regards,
Jim