RE: [PATCH 3/9] x86/hyperv: Mark Hyper-V vp assist page unencrypted in SEV-SNP enlightened guest

Michael Kelley (LINUX)
2023-06-08T13:26:04Z

From: Vitaly Kuznetsov <vkuznets@redhat.com> Sent: Tuesday, June 6, 2023 8:49 AM
>
> Tianyu Lan <ltykernel@gmail.com> writes:
>
> > On 6/5/2023 8:13 PM, Vitaly Kuznetsov wrote:
> >>> @@ -113,6 +114,11 @@ static int hv_cpu_init(unsigned int cpu)
> >>>
> >>> }
> >>> if (!WARN_ON(!(*hvp))) {
> >>> + if (hv_isolation_type_en_snp()) {
> >>> + WARN_ON_ONCE(set_memory_decrypted((unsigned long)(*hvp), 1));
> >>> + memset(*hvp, 0, PAGE_SIZE);
> >>> + }
> >> Why do we need to set the page as decrypted here and not when we
> >> allocate the page (a few lines above)?
> >
> > If Linux root partition boots in the SEV-SNP guest, the page still needs
> > to be decrypted.

We have code in place that prevents this scenario. We don't allow Linux
in the root partition to run in SEV-SNP mode. See commit f8acb24aaf89.

> >
>
> I'd suggest we add a flag to indicate that VP assist page was actually
> set (on the first invocation of hv_cpu_init() for guest partitions and
> all invocations for root partition) and only call
> set_memory_decrypted()/memset() then: that would both help with the
> potential issue with KVM using enlightened vmcs and avoid the unneeded
> hypercall.
>

I think there's actually a more immediate problem with the code as
written. The VP assist page for a CPU is not re-encrypted or freed when
a CPU goes offline (for reasons that have been discussed elsewhere). So
if a CPU in an SEV-SNP VM goes offline and then comes back online, the
originally allocated and already decrypted VP assist page will be reused.
But bad things will happen if we try to decrypt the page again.

Given that we disallow the root partition running in SEV-SNP mode,
can we avoid the complexity of a flag, and just do the decryption and
zero'ing when the page is allocated?

Michael