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

[Tom Lendacky]

On 5/24/21 8:49 AM, Vitaly Kuznetsov wrote:
> Tom Lendacky <thomas.lendacky@xxxxxxx> writes:
> 
>> On 5/24/21 6:53 AM, Vitaly Kuznetsov wrote:
>>> Tom Lendacky <thomas.lendacky@xxxxxxx> 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@xxxxxxxxxx>
>>>> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
>>>> ---
>>>>  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?
> 
> Would it hurt if we just move 'vcpu->arch.guest_state_protected' check
> to is_64_bit_mode() itself? It seems to be too easy to miss this
> peculiar detail about SEV in review if new is_64_bit_mode() users are to
> be added.

I thought about that, but wondered if is_64_bit_mode() was to be used in
other places in the future, if it would be a concern. I think it would be
safe since anyone adding it to a new section of code is likely to look at
what that function is doing first.

I'm ok with this. Paolo, I know you already queued this, but would you
prefer moving the check into is_64_bit_mode()?

Thanks,
Tom

>