[PATCH 4.19 25/32] KVM: Dont create VM debugfs files outside of the VM directory

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Oliver Upton <oupton@google.com>

commit a44a4cc1c969afec97dbb2aedaf6f38eaa6253bb upstream.

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: 929f45e32499 ("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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 virt/kvm/kvm_main.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -610,7 +610,7 @@ static void kvm_destroy_vm_debugfs(struc
 {
 	int i;
 
-	if (!kvm->debugfs_dentry)
+	if (IS_ERR(kvm->debugfs_dentry))
 		return;
 
 	debugfs_remove_recursive(kvm->debugfs_dentry);
@@ -628,6 +628,12 @@ static int kvm_create_vm_debugfs(struct
 	struct kvm_stat_data *stat_data;
 	struct kvm_stats_debugfs_item *p;
 
+	/*
+	 * 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;