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

Brijesh Singh
2021-04-07T17:35:07Z

On 4/7/21 6:59 AM, Borislav Petkov wrote:
> 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.

The feature is part of the GHCB version 2 and is enforced by the
hypervisor. I guess it can be extended for the ES. Since this feature
was not available in GHCB version 1 (base ES) so it should be presented
as an optional for the ES ?

>
>> 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.kernel.org%2Fshow_bug.cgi%3Fid%3D206537&amp;data=04%7C01%7Cbrijesh.singh%40amd.com%7Ce8ae7574ecc742be6c1a08d8f9bcac94%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637533936070042328%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=NaHJ5R9Dfo%2FPnci%2B%2B6xK9ecpV0%2F%2FYbsdGl25%2BFj3TaU%3D&amp;reserved=0
>

Yes, the section may have changed since I wrote the description. Noted.
I will refer the section name.

>> 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 ...

Maybe we can request the GHCB version 3 to add the extended error code.

> :-)
>
>> + /* 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.
>