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: 9224dbb621bc ("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

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 70e05af5ebead64cc03218fa60f2f83d7d907941..e39a6f56fc47e5d94b349c4d04c30af2a7461691 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/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) {