Re: [PATCH] KVM: SVM: Assume a 64-bit hypercall for guests with protected state

[Tom Lendacky <thomas.lendacky@amd.com>]

On 5/24/21 6:53 AM, Vitaly Kuznetsov wrote:
> Tom Lendacky <thomas.lendacky@amd.com> writes:
> 
>> When processing a hypercall for a guest with protected state, currently
>> SEV-ES guests, the guest CS segment register can't be checked to
>> determine if the guest is in 64-bit mode. For an SEV-ES guest, it is
>> expected that communication between the guest and the hypervisor is
>> performed to shared memory using the GHCB. In order to use the GHCB, the
>> guest must have been in long mode, otherwise writes by the guest to the
>> GHCB would be encrypted and not be able to be comprehended by the
>> hypervisor. Given that, assume that the guest is in 64-bit mode when
>> processing a hypercall from a guest with protected state.
>>
>> Fixes: f1c6366e3043 ("KVM: SVM: Add required changes to support intercepts under SEV-ES")
>> Reported-by: Sean Christopherson <seanjc@google.com>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>  arch/x86/kvm/x86.c | 7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 9b6bca616929..e715c69bb882 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -8403,7 +8403,12 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>>  
>>  	trace_kvm_hypercall(nr, a0, a1, a2, a3);
>>  
>> -	op_64_bit = is_64_bit_mode(vcpu);
>> +	/*
>> +	 * If running with protected guest state, the CS register is not
>> +	 * accessible. The hypercall register values will have had to been
>> +	 * provided in 64-bit mode, so assume the guest is in 64-bit.
>> +	 */
>> +	op_64_bit = is_64_bit_mode(vcpu) || vcpu->arch.guest_state_protected;
>>  	if (!op_64_bit) {
>>  		nr &= 0xFFFFFFFF;
>>  		a0 &= 0xFFFFFFFF;
> 
> While this is might be a very theoretical question, what about other
> is_64_bit_mode() users? Namely, a very similar to the above check exists
> in kvm_hv_hypercall() and kvm_xen_hypercall().

Xen doesn't support SEV, so I think this one is ok until they do. Although
I guess we could be preemptive and hit all those call sites. The other
ones are in arch/x86/kvm/hyperv.c.

Thoughts?

Thanks,
Tom

>