Re: [PATCH v8 0/4] Allow guest access to EFI confidential computing secret area

Dov Murik
2022-03-30T06:12:35Z

On 29/03/2022 23:28, Dov Murik wrote:
>
>
> On 29/03/2022 21:30, Borislav Petkov wrote:
>
>>
>> So now that I think of it, it would be even nicer if the fact whether
>> guest debugging is allowed, were available to the guest *very early*
>> during boot. Because I think the most important cases where you'd want
>> to singlestep a SEV* guest with the qemu gdbstub is early guest kernel
>> boot code. So it would be cool if we'd have access to the debugging
>> setting that early.
>>
>> Lemme have a look at your patches in detail to get an idea what's
>> happening there.
>

After a night's sleep I figured out that an SEV guest cannot tell if a
value it's reading was (a) encrypted by the host using
KVM_SEV_LAUNCH_UPDATE_DATA, or (b) added using secret injection using
KVM_SEV_LAUNCH_SECRET.

The only difference is that if the host is using
KVM_SEV_LAUNCH_UPDATE_DATA, then it changes the measurement. But maybe
for debugging scenarios we (= Guest Owner) don't care about the
measurement being correct.

If that's the case, we don't need a secure channel and secret injection.
You can use a simple "sev=debug" (or whatever) in the kernel
command-line to indicate your needs.

Did I miss something?

-Dov