video: tegra: nvmap: Fix data race between create and destroy client

nvmap uses pid of group_leader task to indicate a client process. During
create_client operation, whenever any client with the same group_leader
pid already exists in clients list of nvmap_device, then nvmap
increments the count field of nvmap_client struct. Otherwise, create a
new nvmap_client. Both of the operations i.e. checking the list for
client and incrementing the counter happen inside lock. On the other
hand, during nvmap_release, first the counter is decremented and checked
if it's zero or not. If it's zero then the lock is taken and client is
removed from client list of nvmap_device. As both the operations i.e.
decrementing the counter value and removing client from list (if the
counter becomes 0) are not happening inside a lock, it's resulting into
the following data race scenario.
1) nvmap_release on existing client process 1
   - decrement client's counter
   - counter value has become zero
   - client is yet to be removed from the dev->clients list
   - context switch happen to __nvmap_create_client as another
namespace/thread with same with same group_leader pid is created.
2) __nvmap_create_client
   - as the client with same pid exists in dev->client list, it
increments counter value to 1, instead of creating a new client struct.
   - context switch happen to nvmap_release from step 1
3) nvmap_release
   - It calls destroy_client and remove the client from dev->client
list.
   - Completes rest of the operations in destroy_client and returns.
   - Context switch to remaining operations from step 2
4) nvmap_release
   - Now, when the nvmap_release will be called for the thread/namespace
which was created in step 2, then list_del operation would fail as the
client struct was already removed from dev->client list.

Fix the above issue by doing both operations i.e. decrementing the
counter value and removing the client struct from dev->client list in a
single lock.

Bug 4829958

Change-Id: I87ebbcb45b18114d0ec75520443bee010f88d59e
Signed-off-by: Ketan Patil <ketanp@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/linux-nv-oot/+/3209794
Reviewed-by: Brad Griffis <bgriffis@nvidia.com>
Reviewed-by: Pritesh Raithatha <praithatha@nvidia.com>
GVS: buildbot_gerritrpt <buildbot_gerritrpt@nvidia.com>
Reviewed-by: Ashish Mhetre <amhetre@nvidia.com>

drivers/video/tegra/nvmap/nvmap_dev.c

@@ -256,12 +256,20 @@ static void destroy_client(struct nvmap_client *client)
 	if (!client)
 		return;
 
+	mutex_lock(&nvmap_dev->clients_lock);
+	/*
+	 * count field tracks the number of namespaces within a process.
+	 * Destroy the client only after all namespaces close the /dev/nvmap node.
+	 */
+	if (atomic_dec_return(&client->count)) {
+		mutex_unlock(&nvmap_dev->clients_lock);
+		return;
+	}
+
 	nvmap_id_array_exit(&client->id_array);
 #ifdef NVMAP_CONFIG_HANDLE_AS_ID
 	client->ida = NULL;
 #endif
-
-	mutex_lock(&nvmap_dev->clients_lock);
 	if (!IS_ERR_OR_NULL(nvmap_dev->handles_by_pid)) {
 		pid_t pid = nvmap_client_pid(client);
 		nvmap_pid_put_locked(nvmap_dev, pid);
@@ -328,13 +336,7 @@ static int nvmap_release(struct inode *inode, struct file *filp)
 		return 0;
 
 	trace_nvmap_release(priv, priv->name);
-	/*
-	 * count field tracks the number of namespaces within a process.
-	 * Destroy the client only after all namespaces close the /dev/nvmap node.
-	 */
-	if (!atomic_dec_return(&priv->count))
 	destroy_client(priv);
-
 	return 0;
 }