[PATCH v4 5/9] KVM: SVM: Check EFER.SVME and CPL on #GP intercept of SVM instructions

[Yosry Ahmed]

When KVM intercepts #GP on an SVM instruction from L2, it checks the
legality of RAX, and injects a #GP if RAX is illegal, or otherwise
synthesizes a #VMEXIT to L1. However, checking EFER.SVME and CPL takes
precedence over both the RAX check and the intercept. Call
nested_svm_check_permissions() first to cover both.

Note that if #GP is intercepted on SVM instruction in L1, the intercept
handlers of VMRUN/VMLOAD/VMSAVE already perform these checks.

Note #2, if KVM does not intercept #GP, the check for EFER.SVME is not
done in the correct order, because KVM handles it by intercepting the
instructions when EFER.SVME=0 and injecting #UD.  However, a #GP
injected by hardware would happen before the instruction intercept,
leading to #GP taking precedence over #UD from the guest's perspective.
Opportunistically add a FIXME for this.

Fixes: 82a11e9c6fa2 ("KVM: SVM: Add emulation support for #GP triggered by SVM instructions")
Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
---
 arch/x86/kvm/svm/svm.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index a511ee1139725..bb0bb0f9c858f 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1052,6 +1052,11 @@ static void svm_recalc_instruction_intercepts(struct kvm_vcpu *vcpu)
 	 * No need to toggle any of the vgif/vls/etc. enable bits here, as they
 	 * are set when the VMCB is initialized and never cleared (if the
 	 * relevant intercepts are set, the enablements are meaningless anyway).
+	 *
+	 * FIXME: When #GP is not intercepted, a #GP on these instructions (e.g.
+	 * due to CPL > 0) could be injected by hardware before the instruction
+	 * is intercepted, leading to #GP taking precedence over #UD from the
+	 * guest's perspective.
 	 */
 	if (!(vcpu->arch.efer & EFER_SVME)) {
 		svm_set_intercept(svm, INTERCEPT_VMLOAD);
@@ -2289,6 +2294,9 @@ static int gp_interception(struct kvm_vcpu *vcpu)
 		unsigned long rax = kvm_register_read(vcpu, VCPU_REGS_RAX);
 
 		if (is_guest_mode(vcpu)) {
+			if (nested_svm_check_permissions(vcpu))
+				return 1;
+
 			if (!page_address_valid(vcpu, rax))
 				goto reinject;
 
-- 
2.53.0.851.ga537e3e6e9-goog