KVM: Reenter guest after emulation failure if due to access to non-mmio address

When shadow pages are in use sometimes KVM try to emulate an instruction
when it accesses a shadowed page. If emulation fails KVM un-shadows the
page and reenter guest to allow vcpu to execute the instruction. If page
is not in shadow page hash KVM assumes that this was attempt to do MMIO
and reports emulation failure to userspace since there is no way to fix
the situation. This logic has a race though. If two vcpus tries to write
to the same shadowed page simultaneously both will enter emulator, but
only one of them will find the page in shadow page hash since the one who
founds it also removes it from there, so another cpu will report failure
to userspace and will abort the guest.

Fix this by checking (in addition to checking shadowed page hash) that
page that caused the emulation belongs to valid memory slot. If it is
then reenter the guest to allow vcpu to reexecute the instruction.
Signed-off-by: NGleb Natapov <gleb@redhat.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>

arch/x86/kvm/x86.c

@@ -3930,6 +3930,29 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu)
 	return EMULATE_FAIL;
 }
 
+static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t gva)
+{
+	gpa_t gpa;
+
+	/*
+	 * if emulation was due to access to shadowed page table
+	 * and it failed try to unshadow page and re-entetr the
+	 * guest to let CPU execute the instruction.
+	 */
+	if (kvm_mmu_unprotect_page_virt(vcpu, gva))
+		return true;
+
+	gpa = kvm_mmu_gva_to_gpa_system(vcpu, gva, NULL);
+
+	if (gpa == UNMAPPED_GVA)
+		return true; /* let cpu generate fault */
+
+	if (!kvm_is_error_hva(gfn_to_hva(vcpu->kvm, gpa >> PAGE_SHIFT)))
+		return true;
+
+	return false;
+}
+
 int emulate_instruction(struct kvm_vcpu *vcpu,
 			unsigned long cr2,
 			u16 error_code,
@@ -3998,7 +4021,7 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 
 		++vcpu->stat.insn_emulation;
 		if (r)  {
-			if (kvm_mmu_unprotect_page_virt(vcpu, cr2))
+			if (reexecute_instruction(vcpu, cr2))
 				return EMULATE_DONE;
 			if (emulation_type & EMULTYPE_SKIP)
 				return EMULATE_FAIL;
@@ -4019,12 +4042,7 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 	r = x86_emulate_insn(&vcpu->arch.emulate_ctxt, &emulate_ops);
 
 	if (r) { /* emulation failed */
-		/*
-		 * if emulation was due to access to shadowed page table
-		 * and it failed try to unshadow page and re-entetr the
-		 * guest to let CPU execute the instruction.
-		 */
-		if (kvm_mmu_unprotect_page_virt(vcpu, cr2))
+		if (reexecute_instruction(vcpu, cr2))
 			return EMULATE_DONE;
 
 		return handle_emulation_failure(vcpu);