KVM: x86: Skip EFER vs. guest CPUID checks for host-initiated writes

commit 11988499e62b310f3bf6f6d0a807a06d3f9ccc96 upstream.

KVM allows userspace to violate consistency checks related to the
guest's CPUID model to some degree.  Generally speaking, userspace has
carte blanche when it comes to guest state so long as jamming invalid
state won't negatively affect the host.

Currently this is seems to be a non-issue as most of the interesting
EFER checks are missing, e.g. NX and LME, but those will be added
shortly.  Proactively exempt userspace from the CPUID checks so as not
to break userspace.

Note, the efer_reserved_bits check still applies to userspace writes as
that mask reflects the host's capabilities, e.g. KVM shouldn't allow a
guest to run with NX=1 if it has been disabled in the host.

Fixes: d80174745ba39 ("KVM: SVM: Only allow setting of EFER_SVME when CPUID SVM is set")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 22 insertions, 11 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d830a0d60ba4..6e8c4ed2cc24 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -990,11 +990,8 @@ static u32 emulated_msrs[] = {
 
 static unsigned num_emulated_msrs;
 
-bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer)
+static bool __kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer)
 {
-	if (efer & efer_reserved_bits)
-		return false;
-
 	if (efer & EFER_FFXSR) {
 		struct kvm_cpuid_entry2 *feat;
 
@@ -1012,19 +1009,33 @@ bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer)
 	}
 
 	return true;
+
+}
+bool kvm_valid_efer(struct kvm_vcpu *vcpu, u64 efer)
+{
+	if (efer & efer_reserved_bits)
+		return false;
+
+	return __kvm_valid_efer(vcpu, efer);
 }
 EXPORT_SYMBOL_GPL(kvm_valid_efer);
 
-static int set_efer(struct kvm_vcpu *vcpu, u64 efer)
+static int set_efer(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 {
 	u64 old_efer = vcpu->arch.efer;
+	u64 efer = msr_info->data;
 
-	if (!kvm_valid_efer(vcpu, efer))
-		return 1;
+	if (efer & efer_reserved_bits)
+		return false;
 
-	if (is_paging(vcpu)
-	    && (vcpu->arch.efer & EFER_LME) != (efer & EFER_LME))
-		return 1;
+	if (!msr_info->host_initiated) {
+		if (!__kvm_valid_efer(vcpu, efer))
+			return 1;
+
+		if (is_paging(vcpu) &&
+		    (vcpu->arch.efer & EFER_LME) != (efer & EFER_LME))
+			return 1;
+	}
 
 	efer &= ~EFER_LMA;
 	efer |= vcpu->arch.efer & EFER_LMA;
@@ -2055,7 +2066,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		break;
 
 	case MSR_EFER:
-		return set_efer(vcpu, data);
+		return set_efer(vcpu, msr_info);
 	case MSR_K7_HWCR:
 		data &= ~(u64)0x40;	/* ignore flush filter disable */
 		data &= ~(u64)0x100;	/* ignore ignne emulation enable */