KVM: SVM: Update EFER software model on CR0 trap for SEV-ES

In general, activating long mode involves setting the EFER_LME bit in
the EFER register and then enabling the X86_CR0_PG bit in the CR0
register. At this point, the EFER_LMA bit will be set automatically by
hardware.

In the case of SVM/SEV guests where writes to CR0 are intercepted, it's
necessary for the host to set EFER_LMA on behalf of the guest since
hardware does not see the actual CR0 write.

In the case of SEV-ES guests where writes to CR0 are trapped instead of
intercepted, the hardware *does* see/record the write to CR0 before
exiting and passing the value on to the host, so as part of enabling
SEV-ES support commit f1c6366e ("KVM: SVM: Add required changes to
support intercepts under SEV-ES") dropped special handling of the
EFER_LMA bit with the understanding that it would be set automatically.

However, since the guest never explicitly sets the EFER_LMA bit, the
host never becomes aware that it has been set. This becomes problematic
when userspace tries to get/set the EFER values via
KVM_GET_SREGS/KVM_SET_SREGS, since the EFER contents tracked by the host
will be missing the EFER_LMA bit, and when userspace attempts to pass
the EFER value back via KVM_SET_SREGS it will fail a sanity check that
asserts that EFER_LMA should always be set when X86_CR0_PG and EFER_LME
are set.

Fix this by always inferring the value of EFER_LMA based on X86_CR0_PG
and EFER_LME, regardless of whether or not SEV-ES is enabled.

Fixes: f1c6366e ("KVM: SVM: Add required changes to support intercepts under SEV-ES")
Reported-by: Peter Gonda <pgonda@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210507165947.2502412-2-seanjc@google.com>
[A two year old patch that was revived after we noticed the failure in
 KVM_SET_SREGS and a similar patch was posted by Michael Roth.  This is
 Sean's patch, but with Michael's more complete commit message. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/svm.c

@@ -1855,15 +1855,17 @@ void svm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
 	bool old_paging = is_paging(vcpu);
 
 #ifdef CONFIG_X86_64
-	if (vcpu->arch.efer & EFER_LME && !vcpu->arch.guest_state_protected) {
+	if (vcpu->arch.efer & EFER_LME) {
 		if (!is_paging(vcpu) && (cr0 & X86_CR0_PG)) {
 			vcpu->arch.efer |= EFER_LMA;
-			svm->vmcb->save.efer |= EFER_LMA | EFER_LME;
+			if (!vcpu->arch.guest_state_protected)
+				svm->vmcb->save.efer |= EFER_LMA | EFER_LME;
 		}
 
 		if (is_paging(vcpu) && !(cr0 & X86_CR0_PG)) {
 			vcpu->arch.efer &= ~EFER_LMA;
-			svm->vmcb->save.efer &= ~(EFER_LMA | EFER_LME);
+			if (!vcpu->arch.guest_state_protected)
+				svm->vmcb->save.efer &= ~(EFER_LMA | EFER_LME);
 		}
 	}
 #endif