KVM creates a debugfs directory for each VM in order to store statistics about the virtual machine. The directory name is built from the process pid and a VM fd. While generally unique, it is possible to keep a file descriptor alive in a way that causes duplicate directories, which manifests as these messages:
[ 471.846235] debugfs: Directory '20245-4' with parent 'kvm' already present!
Even though this should not happen in practice, it is more or less expected in the case of KVM for testcases that call KVM_CREATE_VM and close the resulting file descriptor repeatedly and in parallel.
When this happens, debugfs_create_dir() returns an error but kvm_create_vm_debugfs() goes on to allocate stat data structs which are later leaked. The slow memory leak was spotted by syzkaller, where it caused OOM reports.
Since the issue only affects debugfs, do a lookup before calling debugfs_create_dir, so that the message is downgraded and rate-limited. While at it, ensure kvm->debugfs_dentry is NULL rather than an error if it is not created. This fixes kvm_destroy_vm_debugfs, which was not checking IS_ERR_OR_NULL correctly.
Cc: stable@vger.kernel.org Fixes: 536a6f88c49d ("KVM: Create debugfs dir and stat files for each VM") Reported-by: Alexey Kardashevskiy aik@ozlabs.ru Suggested-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Paolo Bonzini pbonzini@redhat.com --- virt/kvm/kvm_main.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index d20fba0fc290..b50dbe269f4b 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -892,6 +892,8 @@ static void kvm_destroy_vm_debugfs(struct kvm *kvm)
static int kvm_create_vm_debugfs(struct kvm *kvm, int fd) { + static DEFINE_MUTEX(kvm_debugfs_lock); + struct dentry *dent; char dir_name[ITOA_MAX_LEN * 2]; struct kvm_stat_data *stat_data; const struct _kvm_stats_desc *pdesc; @@ -903,8 +905,20 @@ static int kvm_create_vm_debugfs(struct kvm *kvm, int fd) return 0;
snprintf(dir_name, sizeof(dir_name), "%d-%d", task_pid_nr(current), fd); - kvm->debugfs_dentry = debugfs_create_dir(dir_name, kvm_debugfs_dir); + mutex_lock(&kvm_debugfs_lock); + dent = debugfs_lookup(dir_name, kvm_debugfs_dir); + if (dent) { + pr_warn_ratelimited("KVM: debugfs: duplicate directory %s\n", dir_name); + dput(dent); + mutex_unlock(&kvm_debugfs_lock); + return 0; + } + dent = debugfs_create_dir(dir_name, kvm_debugfs_dir); + mutex_unlock(&kvm_debugfs_lock); + if (IS_ERR(dent)) + return 0;
+ kvm->debugfs_dentry = dent; kvm->debugfs_stat_data = kcalloc(kvm_debugfs_num_entries, sizeof(*kvm->debugfs_stat_data), GFP_KERNEL_ACCOUNT); @@ -5201,7 +5215,7 @@ static void kvm_uevent_notify_change(unsigned int type, struct kvm *kvm) } add_uevent_var(env, "PID=%d", kvm->userspace_pid);
- if (!IS_ERR_OR_NULL(kvm->debugfs_dentry)) { + if (kvm->debugfs_dentry) { char *tmp, *p = kmalloc(PATH_MAX, GFP_KERNEL_ACCOUNT);
if (p) {