Re: [PATCH] Allow RDTSC and RDTSCP from userspace

Dave Hansen
2020-04-24T22:53:12Z

On 4/24/20 2:27 PM, Tom Lendacky wrote:
> On 4/24/20 4:24 PM, Dave Hansen wrote:
>> On 4/24/20 2:03 PM, Mike Stunes wrote:
>>> I needed to allow RDTSC(P) from userspace and in early boot in order to
>>> get userspace started properly. Patch below.
>>>
>>> ---
>>> SEV-ES guests will need to execute rdtsc and rdtscp from userspace and
>>> during early boot. Move the rdtsc(p) #VC handler into common code and
>>> extend the #VC handlers.
>>
>> Do SEV-ES guests _always_ #VC on rdtsc(p)?
>
> Only if the hypervisor is intercepting those instructions.

Ahh, so any instruction that can have an instruction intercept set
potentially needs to be able to tolerate a #VC? Those instruction
intercepts are under the control of the (untrusted relative to the
guest) hypervisor, right?

From the main sev-es series:

+#ifdef CONFIG_AMD_MEM_ENCRYPT
+idtentry vmm_communication do_vmm_communication has_error_code=1
+#endif

Since this is set as non-paranoid, that both limits the instructions
that can be used in entry paths *and* limits the future architecture
from being able add instructions that a current SEV-ES guest doesn't
know about. Does SEV-ES have versioning so guests can tell if they
might be subject to new interrupt intercepts for which they are not
prepared? I didn't see anything obvious in section 15.35 of the manual.

There's also a nugget in the manual that says:

> Similarly, the hypervisor should avoid setting intercept bits for
> events that would occur in the #VC handler (such as IRET).

That's a fun point because it means that the (untrusted) hypervisor can
cause endless faults. I *guess* we have mitigation for this with our
stack guard pages, but it's still a bit nasty that the hypervisor can
arbitrarily land a guest in the double-fault handler.

It just all seems a bit weak for the hypervisor to be considered
untrusted. But, it's _certainly_ a steep in the right direction from SEV.