KVM: Don't create VM debugfs files outside of the VM directory

Unfortunately, there is no guarantee that KVM was able to instantiate a
debugfs directory for a particular VM. To that end, KVM shouldn't even
attempt to create new debugfs files in this case. If the specified
parent dentry is NULL, debugfs_create_file() will instantiate files at
the root of debugfs.

For arm64, it is possible to create the vgic-state file outside of a
VM directory, the file is not cleaned up when a VM is destroyed.
Nonetheless, the corresponding struct kvm is freed when the VM is
destroyed.

Nip the problem in the bud for all possible errant debugfs file
creations by initializing kvm->debugfs_dentry to -ENOENT. In so doing,
debugfs_create_file() will fail instead of creating the file in the root
directory.

Cc: stable@kernel.org
Fixes: 929f45e ("kvm: no need to check return value of debugfs_create functions")
Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220406235615.1447180-2-oupton@google.com

virt/kvm/kvm_main.c

@@ -932,7 +932,7 @@ static void kvm_destroy_vm_debugfs(struct kvm *kvm)
 	int kvm_debugfs_num_entries = kvm_vm_stats_header.num_desc +
 				      kvm_vcpu_stats_header.num_desc;
 
-	if (!kvm->debugfs_dentry)
+	if (IS_ERR(kvm->debugfs_dentry))
 		return;
 
 	debugfs_remove_recursive(kvm->debugfs_dentry);
@@ -955,6 +955,12 @@ static int kvm_create_vm_debugfs(struct kvm *kvm, int fd)
 	int kvm_debugfs_num_entries = kvm_vm_stats_header.num_desc +
 				      kvm_vcpu_stats_header.num_desc;
 
+	/*
+	 * Force subsequent debugfs file creations to fail if the VM directory
+	 * is not created.
+	 */
+	kvm->debugfs_dentry = ERR_PTR(-ENOENT);
+
 	if (!debugfs_initialized())
 		return 0;
 
@@ -5479,7 +5485,7 @@ static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm)
 	}
 	add_uevent_var(env, "PID=%d", kvm->userspace_pid);
 
-	if (kvm->debugfs_dentry) {
+	if (!IS_ERR(kvm->debugfs_dentry)) {
 		char *tmp, *p = kmalloc(PATH_MAX, GFP_KERNEL_ACCOUNT);
 
 		if (p) {