Re: [PATCH] x86/sev: Check whether SEV or SME is supported first

[Pu Wen]

On 2021/5/27 1:27, Sean Christopherson wrote:
> On Wed, May 26, 2021, Pu Wen wrote:
>> The first two bits of the CPUID leaf 0x8000001F EAX indicate whether
>> SEV or SME is supported respectively. It's better to check whether
>> SEV or SME is supported before checking the SEV MSR(0xc0010131) to
>> see whether SEV or SME is enabled.
>>
>> This also avoid the MSR reading failure on the first generation Hygon
>> Dhyana CPU which does not support SEV or SME.
>>
>> Fixes: eab696d8e8b9 ("x86/sev: Do not require Hypervisor CPUID bit for SEV guests")
>> Cc: <stable@xxxxxxxxxxxxxxx> # v5.10+
>> Signed-off-by: Pu Wen <puwen@xxxxxxxx>
>> ---
>>   arch/x86/mm/mem_encrypt_identity.c | 11 ++++++-----
>>   1 file changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
>> index a9639f663d25..470b20208430 100644
>> --- a/arch/x86/mm/mem_encrypt_identity.c
>> +++ b/arch/x86/mm/mem_encrypt_identity.c
>> @@ -504,10 +504,6 @@ void __init sme_enable(struct boot_params *bp)
>>   #define AMD_SME_BIT	BIT(0)
>>   #define AMD_SEV_BIT	BIT(1)
>>   
>> -	/* Check the SEV MSR whether SEV or SME is enabled */
>> -	sev_status   = __rdmsr(MSR_AMD64_SEV);
>> -	feature_mask = (sev_status & MSR_AMD64_SEV_ENABLED) ? AMD_SEV_BIT : AMD_SME_BIT;
>> -
>>   	/*
>>   	 * Check for the SME/SEV feature:
>>   	 *   CPUID Fn8000_001F[EAX]
>> @@ -519,11 +515,16 @@ void __init sme_enable(struct boot_params *bp)
>>   	eax = 0x8000001f;
>>   	ecx = 0;
>>   	native_cpuid(&eax, &ebx, &ecx, &edx);
>> -	if (!(eax & feature_mask))
>> +	/* Check whether SEV or SME is supported */
>> +	if (!(eax & (AMD_SEV_BIT | AMD_SME_BIT)))
> 
> Hmm, checking CPUID at all before MSR_AMD64_SEV is flawed for SEV, e.g. the VMM
> doesn't need to pass-through CPUID to attack the guest, it can lie directly.
> 
> SEV-ES is protected by virtue of CPUID interception being reflected as #VC, which
> effectively tells the guest that it's (probably) an SEV-ES guest and also gives
> the guest the opportunity to sanity check the emulated CPUID values provided by
> the VMM.
> 
> In other words, this patch is flawed, but commit eab696d8e8b9 was also flawed by
> conditioning the SEV path on CPUID.0x80000000.

Yes, so I think we'd better admit that the VMM is still trusted for SEV guests
as you mentioned below.

> 
> Given that #VC can be handled cleanly, the kernel should be able to handle a #GP
> at this point.  So I think the proper fix is to change __rdmsr() to
> native_read_msr_safe(), or an open coded variant if necessary, and drop the CPUID

Reading MSR_AMD64_SEV which is not implemented on Hygon Dhyana CPU will cause
the kernel reboot, and native_read_msr_safe() has no help.

> checks for SEV.
> 
> The other alternative is to admit that the VMM is still trusted for SEV guests

Agree with that.

-- 
Regards,
Pu Wen

> and take this patch as is (with a reworded changelog).  This probably has my
> vote, I don't see much value in pretending that the VMM can't exfiltrate data
> from an SEV guest.  In fact, a malicious VMM is probably more likely to get
> access to interesting data by _not_ lying about SEV being enabled, because lying
> about SEV itself will hose the guest sooner than later.
> 
>>   		return;
>>   
>>   	me_mask = 1UL << (ebx & 0x3f);
>>   
>> +	/* Check the SEV MSR whether SEV or SME is enabled */
>> +	sev_status   = __rdmsr(MSR_AMD64_SEV);
>> +	feature_mask = (sev_status & MSR_AMD64_SEV_ENABLED) ? AMD_SEV_BIT : AMD_SME_BIT;
>> +
>>   	/* Check if memory encryption is enabled */
>>   	if (feature_mask == AMD_SME_BIT) {
>>   		/*
>> -- 
>> 2.23.0
>>