ASoC: SOF: ipc4-mtrace: protect per-core nodes against multiple open

Add protection against multiple open of the mtrace/coreN debugfs
nodes. This is not supported in the implementation, and this will
show up as unexpected behaviour of the interface, and potential
use of already freed memory.

Fixes: f4ea22f7aa75 ("ASoC: SOF: ipc4: Add support for mtrace log extraction")
Signed-off-by: Kai Vehmanen <[e-mail]>
Reviewed-by: Péter Ujfalusi <[e-mail]>
Reviewed-by: Pierre-Louis Bossart <[e-mail]>
Reviewed-by: Ranjani Sridharan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

sound/soc/sof/ipc4-mtrace.c

···
 	int id;
 	u32 slot_offset;
 	void *log_buffer;
+	struct mutex buffer_lock; /* for log_buffer alloc/free */
 	u32 host_read_ptr;
 	u32 dsp_write_ptr;
 	/* pos update IPC arrived before the slot offset is known, queried */
···
 	struct sof_mtrace_core_data *core_data = inode->i_private;
 	int ret;
 
+	mutex_lock(&core_data->buffer_lock);
+
+	if (core_data->log_buffer) {
+		ret = -EBUSY;
+		goto out;
+	}
+
 	ret = debugfs_file_get(file->f_path.dentry);
 	if (unlikely(ret))
-		return ret;
+		goto out;
 
 	core_data->log_buffer = kmalloc(SOF_MTRACE_SLOT_SIZE, GFP_KERNEL);
 	if (!core_data->log_buffer) {
 		debugfs_file_put(file->f_path.dentry);
-		return -ENOMEM;
+		ret = -ENOMEM;
+		goto out;
 	}
 
 	ret = simple_open(inode, file);
···
 		kfree(core_data->log_buffer);
 		debugfs_file_put(file->f_path.dentry);
 	}
+
+out:
+	mutex_unlock(&core_data->buffer_lock);
 
 	return ret;
 }
···
 
 	debugfs_file_put(file->f_path.dentry);
 
+	mutex_lock(&core_data->buffer_lock);
 	kfree(core_data->log_buffer);
+	core_data->log_buffer = NULL;
+	mutex_unlock(&core_data->buffer_lock);
 
 	return 0;
 }
···
 		struct sof_mtrace_core_data *core_data = &priv->cores[i];
 
 		init_waitqueue_head(&core_data->trace_sleep);
+		mutex_init(&core_data->buffer_lock);
 		core_data->sdev = sdev;
 		core_data->id = i;
 	}