linux-kernel - Re: [PATCH] Allow RDTSC and RDTSCP from userspace

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Date:   Fri, 24 Apr 2020 15:53:09 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        Mike Stunes <mstunes@...are.com>, joro@...tes.org
Cc:     dan.j.williams@...el.com, dave.hansen@...ux.intel.com,
        hpa@...or.com, jgross@...e.com, jroedel@...e.de, jslaby@...e.cz,
        keescook@...omium.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, luto@...nel.org,
        peterz@...radead.org, thellstrom@...are.com,
        virtualization@...ts.linux-foundation.org, x86@...nel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH] Allow RDTSC and RDTSCP from userspace

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.