Re: [PATCH] KVM: Do not leak memory for duplicate debugfs directories

From: Alexey Kardashevskiy
Date: Wed Aug 04 2021 - 19:32:50 EST

On 04/08/2021 19:37, Paolo Bonzini wrote:
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.

Rather the already allocated srructs leak, no?

The slow memory leak was spotted by syzkaller, where it
caused OOM reports.

I gave it a try and almost replied with "tested-by" but after running it over night I got 1 of these with followed OOM:

[ 1104.951394][ T544] debugfs: Directory '544-4' with parent 'kvm' already present!
[ 1104.951600][ T544] Failed to create 544-4

This is definitely an improvement as this used to happen a few times every hour but still puzzling :-/

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@xxxxxxxxxxxxxxx
Fixes: 536a6f88c49d ("KVM: Create debugfs dir and stat files for each VM")
Reported-by: Alexey Kardashevskiy <aik@xxxxxxxxx>
Suggested-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
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,
@@ -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) {