KVM: selftests: Test max vCPU IDs corner cases

The KVM_CREATE_VCPU ioctl ABI had an implicit integer truncation bug,
allowing 2^32 aliases for a vCPU ID by setting the upper 32 bits of a 64
bit ioctl() argument.

It also allowed excluding a once set boot CPU ID.

Verify this no longer works and gets rejected with an error.
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lore.kernel.org/r/20240614202859.3597745-5-minipli@grsecurity.net
[sean: tweak assert message+comment for 63:32!=0 testcase]
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/x86_64/max_vcpuid_cap_test.c

@@ -26,19 +26,37 @@ int main(int argc, char *argv[])
 	TEST_ASSERT(ret < 0,
 		    "Setting KVM_CAP_MAX_VCPU_ID beyond KVM cap should fail");
 
+	/* Test BOOT_CPU_ID interaction (MAX_VCPU_ID cannot be lower) */
+	if (kvm_has_cap(KVM_CAP_SET_BOOT_CPU_ID)) {
+		vm_ioctl(vm, KVM_SET_BOOT_CPU_ID, (void *)MAX_VCPU_ID);
+
+		/* Try setting KVM_CAP_MAX_VCPU_ID below BOOT_CPU_ID */
+		ret = __vm_enable_cap(vm, KVM_CAP_MAX_VCPU_ID, MAX_VCPU_ID - 1);
+		TEST_ASSERT(ret < 0,
+			    "Setting KVM_CAP_MAX_VCPU_ID below BOOT_CPU_ID should fail");
+	}
+
 	/* Set KVM_CAP_MAX_VCPU_ID */
 	vm_enable_cap(vm, KVM_CAP_MAX_VCPU_ID, MAX_VCPU_ID);
 
-
 	/* Try to set KVM_CAP_MAX_VCPU_ID again */
 	ret = __vm_enable_cap(vm, KVM_CAP_MAX_VCPU_ID, MAX_VCPU_ID + 1);
 	TEST_ASSERT(ret < 0,
 		    "Setting KVM_CAP_MAX_VCPU_ID multiple times should fail");
 
-	/* Create vCPU with id beyond KVM_CAP_MAX_VCPU_ID cap*/
+	/* Create vCPU with id beyond KVM_CAP_MAX_VCPU_ID cap */
 	ret = __vm_ioctl(vm, KVM_CREATE_VCPU, (void *)MAX_VCPU_ID);
 	TEST_ASSERT(ret < 0, "Creating vCPU with ID > MAX_VCPU_ID should fail");
 
+	/* Create vCPU with bits 63:32 != 0, but an otherwise valid id */
+	ret = __vm_ioctl(vm, KVM_CREATE_VCPU, (void *)(1L << 32));
+	TEST_ASSERT(ret < 0, "Creating vCPU with ID[63:32] != 0 should fail");
+
+	/* Create vCPU with id within bounds */
+	ret = __vm_ioctl(vm, KVM_CREATE_VCPU, (void *)0);
+	TEST_ASSERT(ret >= 0, "Creating vCPU with ID 0 should succeed");
+
+	close(ret);
 	kvm_vm_free(vm);
 	return 0;
 }