Re: [RFC Part1 PATCH 07/13] x86/compressed: register GHCB memory when SNP is active

Borislav Petkov
2021-04-07T12:00:03Z

On Wed, Mar 24, 2021 at 11:44:18AM -0500, Brijesh Singh wrote:
> The SEV-SNP guest is required to perform GHCB GPA registration. This is

Why does it need to do that? Some additional security so as to not allow
changing the GHCB once it is established?

I'm guessing that's enforced by the SNP fw and we cannot do that
retroactively for SEV...? Because it sounds like a nice little thing we
could do additionally.

> because the hypervisor may prefer that a guest use a consistent and/or
> specific GPA for the GHCB associated with a vCPU. For more information,
> see the GHCB specification section 2.5.2.

I think you mean

"2.3.2 GHCB GPA Registration"

Please use the section name too because that doc changes from time to
time.

Also, you probably should update it here:

https://bugzilla.kernel.org/show_bug.cgi?id=206537

> diff --git a/arch/x86/boot/compressed/sev-snp.c b/arch/x86/boot/compressed/sev-snp.c
> index 5c25103b0df1..a4c5e85699a7 100644
> --- a/arch/x86/boot/compressed/sev-snp.c
> +++ b/arch/x86/boot/compressed/sev-snp.c
> @@ -113,3 +113,29 @@ void sev_snp_set_page_shared(unsigned long paddr)
> {
> sev_snp_set_page_private_shared(paddr, SNP_PAGE_STATE_SHARED);
> }
> +
> +void sev_snp_register_ghcb(unsigned long paddr)

Right and let's prefix SNP-specific functions with "snp_" only so that
it is clear which is wcich when looking at the code.

> +{
> + u64 pfn = paddr >> PAGE_SHIFT;
> + u64 old, val;
> +
> + if (!sev_snp_enabled())
> + return;
> +
> + /* save the old GHCB MSR */
> + old = sev_es_rd_ghcb_msr();
> +
> + /* Issue VMGEXIT */

No need for that comment.

> + sev_es_wr_ghcb_msr(GHCB_REGISTER_GPA_REQ_VAL(pfn));
> + VMGEXIT();
> +
> + val = sev_es_rd_ghcb_msr();
> +
> + /* If the response GPA is not ours then abort the guest */
> + if ((GHCB_SEV_GHCB_RESP_CODE(val) != GHCB_REGISTER_GPA_RESP) ||
> + (GHCB_REGISTER_GPA_RESP_VAL(val) != pfn))
> + sev_es_terminate(GHCB_SEV_ES_REASON_GENERAL_REQUEST);

Yet another example where using a specific termination reason could help
with debugging guests. Looking at the GHCB spec, I hope GHCBData[23:16]
is big enough for all reasons. I'm sure it can be extended ofc ...

:-)

> + /* Restore the GHCB MSR value */
> + sev_es_wr_ghcb_msr(old);
> +}
> diff --git a/arch/x86/include/asm/sev-snp.h b/arch/x86/include/asm/sev-snp.h
> index f514dad276f2..0523eb21abd7 100644
> --- a/arch/x86/include/asm/sev-snp.h
> +++ b/arch/x86/include/asm/sev-snp.h
> @@ -56,6 +56,13 @@ struct __packed snp_page_state_change {
> struct snp_page_state_entry entry[SNP_PAGE_STATE_CHANGE_MAX_ENTRY];
> };
>
> +/* GHCB GPA register */
> +#define GHCB_REGISTER_GPA_REQ 0x012UL
> +#define GHCB_REGISTER_GPA_REQ_VAL(v) (GHCB_REGISTER_GPA_REQ | ((v) << 12))
> +
> +#define GHCB_REGISTER_GPA_RESP 0x013UL

Let's append "UL" to the other request numbers for consistency.

Thx.

--
Regards/Gruss,
Boris.