Re: [PATCH Part1 RFC v3 11/22] x86/sev: Add helper for validating pages in early enc attribute changes

Brijesh Singh
2021-06-16T11:00:21Z

On 6/16/21 5:07 AM, Borislav Petkov wrote:
> On Mon, Jun 14, 2021 at 04:01:44PM -0500, Brijesh Singh wrote:
>> Now that we have to defined a Linux specific reason set, we could
>> potentially define a new error code "Invalid response code" and return
> I don't understand - you have a WARN for the following check:
>
> if (WARN(GHCB_MSR_PSC_RESP_VAL(val),
> "Failed to change page state to '%s' paddr 0x%lx error 0x%llx\n",

This WARN indicates that command execution failed but it does not mean
that hypervisor violated the GHCB protocol.

>
> what's wrong with doing:
>
> if (WARN(GHCB_RESP_CODE(val) != GHCB_MSR_PSC_RESP,
> "Wrong PSC response code: 0x%x\n",
> (unsigned int)GHCB_RESP_CODE(val)))
> goto e_term;

There are around 6 MSR based VMGEXITs and every VMEXIT are paired with
request and response. Guest uses the request code while submitting the
command, and hypervisor provides result through the response code. If
the guest sees hypervisor didn't use the response code (i.e violates the
GHCB protocol) then it terminates the guest with reason code set to
"General termination".  e.g look at the do_vc_no_ghcb()

https://elixir.bootlin.com/linux/v5.13-rc6/source/arch/x86/kernel/sev-shared.c#L157

I am trying to be consistent with previous VMGEXIT implementations. If
the command itself failed then use the command specific error code to
tell hypervisor why we terminated but if the hypervisor violated the
GHCB specification then use the "general request termination".

>
> above it too?
>