Re: [PATCH v15 06/13] x86/sev: Prevent GUEST_TSC_FREQ MSR interception for Secure TSC enabled guests

Tom Lendacky
2024-12-10T17:22:17Z

On 12/10/24 11:13, Nikunj A Dadhania wrote:
> On 12/10/2024 5:41 PM, Borislav Petkov wrote:
>> On Tue, Dec 03, 2024 at 02:30:38PM +0530, Nikunj A Dadhania wrote:
>>> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
>>> index af28fb962309..59c5e716fdd1 100644
>>> --- a/arch/x86/coco/sev/core.c
>>> +++ b/arch/x86/coco/sev/core.c
>>> @@ -1473,6 +1473,14 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
>>> if (regs->cx == MSR_IA32_TSC && (sev_status & MSR_AMD64_SNP_SECURE_TSC))
>>> return __vc_handle_msr_tsc(regs, write);
>>>
>>> + /*
>>> + * GUEST_TSC_FREQ should not be intercepted when Secure TSC is
>>> + * enabled. Terminate the SNP guest when the interception is enabled.
>>> + */
>>> + if (regs->cx == MSR_AMD64_GUEST_TSC_FREQ && (sev_status & MSR_AMD64_SNP_SECURE_TSC))
>>> + return ES_VMM_ERROR;
>>> +
>>> +
>>
>> If you merge this logic into the switch-case, the patch becomes even easier
>> and the code cleaner:
>
> This is incorrect, for a non-Secure TSC guest, a read of intercepted
> MSR_AMD64_GUEST_TSC_FREQ will return value of rdtsc_ordered(). This is an invalid
> MSR when SecureTSC is not enabled.

For the non-Secure TSC guest, I still think that we should continue to
use the GHCB MSR NAE event instead of switching to using rdtsc_ordered().

Thanks,
Tom

>
>>
>> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
>> index 050170eb28e6..35d9a3bb4b06 100644
>> --- a/arch/x86/coco/sev/core.c
>> +++ b/arch/x86/coco/sev/core.c
>> @@ -1446,6 +1446,13 @@ static enum es_result __vc_handle_msr_tsc(struct pt_regs *regs, bool write)
>> if (!(sev_status & MSR_AMD64_SNP_SECURE_TSC))
>> goto read_tsc;
>>
>> + /*
>> + * GUEST_TSC_FREQ should not be intercepted when Secure TSC is
>> + * enabled. Terminate the SNP guest when the interception is enabled.
>> + */
>> + if (regs->cx == MSR_AMD64_GUEST_TSC_FREQ)
>> + return ES_VMM_ERROR;
>> +
>> if (write) {
>> WARN_ONCE(1, "TSC MSR writes are verboten!\n");
>> return ES_UNSUPPORTED;
>> @@ -1472,6 +1479,7 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
>> case MSR_SVSM_CAA:
>> return __vc_handle_msr_caa(regs, write);
>> case MSR_IA32_TSC:
>> + case MSR_AMD64_GUEST_TSC_FREQ:
>> return __vc_handle_msr_tsc(regs, write);
>> default:
>> break;
>>
>
> Regards
> Nikunj