KVM: selftests: Verify stats fd is usable after VM fd has been closed

Verify that VM and vCPU binary stats files are usable even after userspace
has put its last direct reference to the VM.  This is a regression test
for a UAF bug where KVM didn't gift the stats files a reference to the VM.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230711230131.648752-8-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: sean-jc

tools/testing/selftests/kvm/kvm_binary_stats_test.c

@@ -252,15 +252,21 @@ int main(int argc, char *argv[])
 			stats_test(vcpu_get_stats_fd(vcpus[i * max_vcpu + j]));
 		}
 
+		/*
+		 * Close the VM fd and redo the stats tests.  KVM should gift a
+		 * reference (to the VM) to each stats fd, i.e. stats should
+		 * still be accessible even after userspace has put its last
+		 * _direct_ reference to the VM.
+		 */
+		kvm_vm_free(vms[i]);
+
 		stats_test(vm_stats_fds);
 		for (j = 0; j < max_vcpu; ++j)
 			stats_test(vcpu_stats_fds[j]);
 
 		ksft_test_result_pass("vm%i\n", i);
 	}
 
-	for (i = 0; i < max_vm; ++i)
-		kvm_vm_free(vms[i]);
 	free(vms);
 	free(vcpus);
 	free(vcpu_stats_fds);