Re: [RFC Part1 PATCH 06/13] x86/compressed: rescinds and validate the memory used for the GHCB

Brijesh Singh
2021-04-07T17:15:48Z

On 4/7/21 9:21 AM, Tom Lendacky wrote:
> On 4/7/21 8:35 AM, Brijesh Singh wrote:
>> On 4/7/21 6:16 AM, Borislav Petkov wrote:
>>> On Tue, Apr 06, 2021 at 10:47:18AM -0500, Brijesh Singh wrote:
>>>> Before the GHCB is established the caller does not need to save and
>>>> restore MSRs. The page_state_change() uses the GHCB MSR protocol and it
>>>> can be called before and after the GHCB is established hence I am saving
>>>> and restoring GHCB MSRs.
>>> I think you need to elaborate on that, maybe with an example. What the
>>> other sites using the GHCB MSR currently do is:
>>>
>>> 1. request by writing it
>>> 2. read the response
>>>
>>> None of them save and restore it.
>>>
>>> So why here?
>> GHCB provides two ways to exit from the guest to the hypervisor. The MSR
>> protocol and NAEs. The MSR protocol is generally used before the GHCB is
>> established. After the GHCB is established the guests typically uses the
>> NAEs. All of the current call sites uses the MSR protocol before the
>> GHCB is established so they do not need to save and restore the GHCB.
>> The GHCB is established on the first #VC -
>> arch/x86/boot/compressed/sev-es.c early_setup_sev_es(). The GHCB page
>> must a shared page:
>>
>> early_setup_sev_es()
>>
>>   set_page_decrypted()
>>
>>    sev_snp_set_page_shared()
>>
>> The sev_snp_set_page_shared() called before the GHCB is established.
>> While exiting from the decompression the sev_es_shutdown_ghcb() is
>> called to deinit the GHCB.
>>
>> sev_es_shutdown_ghcb()
>>
>>   set_page_encrypted()
>>
>>     sev_snp_set_page_private()
>>
>> Now that sev_snp_set_private() is called after the GHCB is established.
> I believe the current SEV-ES code always sets the GHCB address in the GHCB
> MSR before invoking VMGEXIT, so I think you're safe either way. Worth
> testing at least.

Ah, I didn;t realize that the sev_es_ghcb_hv_call() helper sets the GHCB
MSR before invoking VMGEXIT. I should be able to drop the save and
restore during the page state change. Thanks Tom.