Vulnerability: Race condition in the ion_ioctl function in drivers/staging/android/ion/ion.c in the Linux kernel before 4.6 allows local users to gain privileges or cause a denial of service (use-after-free) by calling ION_IOC_FREE on two CPUs at the same time.
Vulnerability ID: CVE-2016-9120
CWE: CWE-264
Severity score: 10
Project: https://github.com/torvalds/linux
Published: 2016-12-08
File: drivers/staging/android/ion/ion.c
Commit message: staging/android/ion : fix a race condition in the ion driver

There is a use-after-free problem in the ion driver.
This is caused by a race condition in the ion_ioctl()
function.

A handle has ref count of 1 and two tasks on different
cpus calls ION_IOC_FREE simultaneously.

cpu 0                                   cpu 1
-------------------------------------------------------
ion_handle_get_by_id()
(ref == 2)
                            ion_handle_get_by_id()
                            (ref == 3)

ion_free()
(ref == 2)

ion_handle_put()
(ref == 1)

                            ion_free()
                            (ref == 0 so ion_handle_destroy() is
                            called
                            and the handle is freed.)

                            ion_handle_put() is called and it
                            decreases the slub's next free pointer

The problem is detected as an unaligned access in the
spin lock functions since it uses load exclusive
 instruction. In some cases it corrupts the slub's
free pointer which causes a mis-aligned access to the
next free pointer.(kmalloc returns a pointer like
ffffc0745b4580aa). And it causes lots of other
hard-to-debug problems.

This symptom is caused since the first member in the
ion_handle structure is the reference count and the
ion driver decrements the reference after it has been
freed.

To fix this problem client->lock mutex is extended
to protect all the codes that uses the handle.

Signed-off-by: Eun Taik Lee <eun.taik.lee@samsung.com>
Reviewed-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Commit author: EunTaik Lee

Vulnerable version (before fix):
 static void ion_handle_get(struct ion_handle *handle) 	kref_get(&handle->ref); }  static int ion_handle_put(struct ion_handle *handle) { 	struct ion_client *client = handle->client; 	int ret;  	mutex_lock(&client->lock); 	ret = kref_put(&handle->ref, ion_handle_destroy); 	mutex_unlock(&client->lock);  	return ret; static struct ion_handle *ion_handle_lookup(struct ion_client *client, 	return ERR_PTR(-EINVAL); }  static struct ion_handle *ion_handle_get_by_id(struct ion_client *client, 						int id) { 	struct ion_handle *handle;  	mutex_lock(&client->lock); 	handle = idr_find(&client->idr, id); 	if (handle) 		ion_handle_get(handle); 	mutex_unlock(&client->lock);  	return handle ? handle : ERR_PTR(-EINVAL); }  static bool ion_handle_validate(struct ion_client *client, 				struct ion_handle *handle) { struct ion_handle *ion_alloc(struct ion_client *client, size_t len, } EXPORT_SYMBOL(ion_alloc);  void ion_free(struct ion_client *client, struct ion_handle *handle) { 	bool valid_handle;  	BUG_ON(client != handle->client);  	mutex_lock(&client->lock); 	valid_handle = ion_handle_validate(client, handle);  	if (!valid_handle) { 		WARN(1, "%s: invalid handle passed to free.\n", __func__); 		mutex_unlock(&client->lock); 		return; 	} 	mutex_unlock(&client->lock); 	ion_handle_put(handle); } EXPORT_SYMBOL(ion_free);  static long ion_ioctl(struct file *filp, unsigned int cmd, unsigned long arg) 	{ 		struct ion_handle *handle;  		handle = ion_handle_get_by_id(client, data.handle.handle); 		if (IS_ERR(handle)) 			return PTR_ERR(handle); 		ion_free(client, handle); 		ion_handle_put(handle); 		break; 	} 	case ION_IOC_SHARE:

Fixed version (after fix):
 static void ion_handle_get(struct ion_handle *handle) 	kref_get(&handle->ref); }  static int ion_handle_put_nolock(struct ion_handle *handle) { 	int ret;  	ret = kref_put(&handle->ref, ion_handle_destroy);  	return ret; }  int ion_handle_put(struct ion_handle *handle) { 	struct ion_client *client = handle->client; 	int ret;  	mutex_lock(&client->lock); 	ret = ion_handle_put_nolock(handle); 	mutex_unlock(&client->lock);  	return ret; static struct ion_handle *ion_handle_lookup(struct ion_client *client, 	return ERR_PTR(-EINVAL); }  static struct ion_handle *ion_handle_get_by_id_nolock(struct ion_client *client, 						int id) { 	struct ion_handle *handle;  	handle = idr_find(&client->idr, id); 	if (handle) 		ion_handle_get(handle);  	return handle ? handle : ERR_PTR(-EINVAL); }  struct ion_handle *ion_handle_get_by_id(struct ion_client *client, 						int id) { 	struct ion_handle *handle;  	mutex_lock(&client->lock); 	handle = ion_handle_get_by_id_nolock(client, id); 	mutex_unlock(&client->lock);  	return handle; }  static bool ion_handle_validate(struct ion_client *client, 				struct ion_handle *handle) { struct ion_handle *ion_alloc(struct ion_client *client, size_t len, } EXPORT_SYMBOL(ion_alloc);  static void ion_free_nolock(struct ion_client *client, struct ion_handle *handle) { 	bool valid_handle;  	BUG_ON(client != handle->client);  	valid_handle = ion_handle_validate(client, handle);  	if (!valid_handle) { 		WARN(1, "%s: invalid handle passed to free.\n", __func__); 		return; 	} 	ion_handle_put_nolock(handle); }  void ion_free(struct ion_client *client, struct ion_handle *handle) { 	BUG_ON(client != handle->client);  	mutex_lock(&client->lock); 	ion_free_nolock(client, handle); 	mutex_unlock(&client->lock); } EXPORT_SYMBOL(ion_free);  static long ion_ioctl(struct file *filp, unsigned int cmd, unsigned long arg) 	{ 		struct ion_handle *handle;  		mutex_lock(&client->lock); 		handle = ion_handle_get_by_id_nolock(client, data.handle.handle); 		if (IS_ERR(handle)) { 			mutex_unlock(&client->lock); 			return PTR_ERR(handle); 		} 		ion_free_nolock(client, handle); 		ion_handle_put_nolock(handle); 		mutex_unlock(&client->lock); 		break; 	} 	case ION_IOC_SHARE:

Patch:
patch: @@ -387,13 +387,22 @@ static void ion_handle_get(struct ion_handle *handle)
 	kref_get(&handle->ref);
 }
 
-static int ion_handle_put(struct ion_handle *handle)
+static int ion_handle_put_nolock(struct ion_handle *handle)
+{
+	int ret;
+
+	ret = kref_put(&handle->ref, ion_handle_destroy);
+
+	return ret;
+}
+
+int ion_handle_put(struct ion_handle *handle)
 {
 	struct ion_client *client = handle->client;
 	int ret;
 
 	mutex_lock(&client->lock);
-	ret = kref_put(&handle->ref, ion_handle_destroy);
+	ret = ion_handle_put_nolock(handle);
 	mutex_unlock(&client->lock);
 
 	return ret;
@@ -417,20 +426,30 @@ static struct ion_handle *ion_handle_lookup(struct ion_client *client,
 	return ERR_PTR(-EINVAL);
 }
 
-static struct ion_handle *ion_handle_get_by_id(struct ion_client *client,
+static struct ion_handle *ion_handle_get_by_id_nolock(struct ion_client *client,
 						int id)
 {
 	struct ion_handle *handle;
 
-	mutex_lock(&client->lock);
 	handle = idr_find(&client->idr, id);
 	if (handle)
 		ion_handle_get(handle);
-	mutex_unlock(&client->lock);
 
 	return handle ? handle : ERR_PTR(-EINVAL);
 }
 
+struct ion_handle *ion_handle_get_by_id(struct ion_client *client,
+						int id)
+{
+	struct ion_handle *handle;
+
+	mutex_lock(&client->lock);
+	handle = ion_handle_get_by_id_nolock(client, id);
+	mutex_unlock(&client->lock);
+
+	return handle;
+}
+
 static bool ion_handle_validate(struct ion_client *client,
 				struct ion_handle *handle)
 {
@@ -532,22 +551,28 @@ struct ion_handle *ion_alloc(struct ion_client *client, size_t len,
 }
 EXPORT_SYMBOL(ion_alloc);
 
-void ion_free(struct ion_client *client, struct ion_handle *handle)
+static void ion_free_nolock(struct ion_client *client, struct ion_handle *handle)
 {
 	bool valid_handle;
 
 	BUG_ON(client != handle->client);
 
-	mutex_lock(&client->lock);
 	valid_handle = ion_handle_validate(client, handle);
 
 	if (!valid_handle) {
 		WARN(1, "%s: invalid handle passed to free.\n", __func__);
-		mutex_unlock(&client->lock);
 		return;
 	}
+	ion_handle_put_nolock(handle);
+}
+
+void ion_free(struct ion_client *client, struct ion_handle *handle)
+{
+	BUG_ON(client != handle->client);
+
+	mutex_lock(&client->lock);
+	ion_free_nolock(client, handle);
 	mutex_unlock(&client->lock);
-	ion_handle_put(handle);
 }
 EXPORT_SYMBOL(ion_free);
 
@@ -1332,11 +1357,15 @@ static long ion_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 	{
 		struct ion_handle *handle;
 
-		handle = ion_handle_get_by_id(client, data.handle.handle);
-		if (IS_ERR(handle))
+		mutex_lock(&client->lock);
+		handle = ion_handle_get_by_id_nolock(client, data.handle.handle);
+		if (IS_ERR(handle)) {
+			mutex_unlock(&client->lock);
 			return PTR_ERR(handle);
-		ion_free(client, handle);
-		ion_handle_put(handle);
+		}
+		ion_free_nolock(client, handle);
+		ion_handle_put_nolock(handle);
+		mutex_unlock(&client->lock);
 		break;
 	}
 	case ION_IOC_SHARE:
path: drivers/staging/android/ion/ion.c