KVM: selftests: Manage CPUID array in Hyper-V CPUID test's core helper

Allocate, get, and free the CPUID array in the Hyper-V CPUID test in the
test's core helper, instead of copy+pasting code at each call site.  In
addition to deduplicating a small amount of code, restricting visibility
of the array to a single invocation of the core test prevents "leaking" an
array across test cases.  Passing in @vcpu to the helper will also allow
pivoting on VM-scoped information without needing to pass more booleans,
e.g. to conditionally assert on features that require an in-kernel APIC.

To avoid use-after-free bugs due to overzealous and careless developers,
opportunstically add a comment to explain that the system-scoped helper
caches the Hyper-V CPUID entries, i.e. that the caller is not responsible
for freeing the memory.

Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20250118003454.2619573-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/x86/hyperv_cpuid.c

@@ -41,13 +41,18 @@ static bool smt_possible(void)
 	return res;
 }
 
-static void test_hv_cpuid(const struct kvm_cpuid2 *hv_cpuid_entries,
-			  bool evmcs_expected)
+static void test_hv_cpuid(struct kvm_vcpu *vcpu, bool evmcs_expected)
 {
+	const struct kvm_cpuid2 *hv_cpuid_entries;
 	int i;
 	int nent_expected = 10;
 	u32 test_val;
 
+	if (vcpu)
+		hv_cpuid_entries = vcpu_get_supported_hv_cpuid(vcpu);
+	else
+		hv_cpuid_entries = kvm_get_supported_hv_cpuid();
+
 	TEST_ASSERT(hv_cpuid_entries->nent == nent_expected,
 		    "KVM_GET_SUPPORTED_HV_CPUID should return %d entries"
 		    " (returned %d)",
@@ -109,6 +114,13 @@ static void test_hv_cpuid(const struct kvm_cpuid2 *hv_cpuid_entries,
 		 *	entry->edx);
 		 */
 	}
+
+	/*
+	 * Note, the CPUID array returned by the system-scoped helper is a one-
+	 * time allocation, i.e. must not be freed.
+	 */
+	if (vcpu)
+		free((void *)hv_cpuid_entries);
 }
 
 static void test_hv_cpuid_e2big(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
@@ -129,7 +141,6 @@ static void test_hv_cpuid_e2big(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
 int main(int argc, char *argv[])
 {
 	struct kvm_vm *vm;
-	const struct kvm_cpuid2 *hv_cpuid_entries;
 	struct kvm_vcpu *vcpu;
 
 	TEST_REQUIRE(kvm_has_cap(KVM_CAP_HYPERV_CPUID));
@@ -138,20 +149,15 @@ int main(int argc, char *argv[])
 
 	/* Test vCPU ioctl version */
 	test_hv_cpuid_e2big(vm, vcpu);
-
-	hv_cpuid_entries = vcpu_get_supported_hv_cpuid(vcpu);
-	test_hv_cpuid(hv_cpuid_entries, false);
-	free((void *)hv_cpuid_entries);
+	test_hv_cpuid(vcpu, false);
 
 	if (!kvm_cpu_has(X86_FEATURE_VMX) ||
 	    !kvm_has_cap(KVM_CAP_HYPERV_ENLIGHTENED_VMCS)) {
 		print_skip("Enlightened VMCS is unsupported");
 		goto do_sys;
 	}
 	vcpu_enable_evmcs(vcpu);
-	hv_cpuid_entries = vcpu_get_supported_hv_cpuid(vcpu);
-	test_hv_cpuid(hv_cpuid_entries, true);
-	free((void *)hv_cpuid_entries);
+	test_hv_cpuid(vcpu, true);
 
 do_sys:
 	/* Test system ioctl version */
@@ -161,9 +167,7 @@ int main(int argc, char *argv[])
 	}
 
 	test_hv_cpuid_e2big(vm, NULL);
-
-	hv_cpuid_entries = kvm_get_supported_hv_cpuid();
-	test_hv_cpuid(hv_cpuid_entries, kvm_cpu_has(X86_FEATURE_VMX));
+	test_hv_cpuid(NULL, kvm_cpu_has(X86_FEATURE_VMX));
 
 out:
 	kvm_vm_free(vm);