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?
Thanks,
Tom
>
