ASoC: SOF: ipc3-dtrace: uninitialized data in dfsentry_trace_filter_write()

This doesn't check how many bytes the simple_write_to_buffer() writes to
the buffer.  The only thing that we know is that the first byte is
initialized and the last byte of the buffer is set to NUL.  However
the middle bytes could be uninitialized.

There is no need to use simple_write_to_buffer().  This code does not
support partial writes but instead passes "pos = 0" as the starting
offset regardless of what the user passed as "*ppos".  Just use the
copy_from_user() function and initialize the whole buffer.

Fixes: 671e0b9 ("ASoC: SOF: Clone the trace code to ipc3-dtrace as fw_tracing implementation")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/74148292-ce4d-4e01-a1a7-921e6767da14@moroto.mountain
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: Dan Carpenter

sound/soc/sof/ipc3-dtrace.c

@@ -186,7 +186,6 @@ static ssize_t dfsentry_trace_filter_write(struct file *file, const char __user
 	struct snd_sof_dfsentry *dfse = file->private_data;
 	struct sof_ipc_trace_filter_elem *elems = NULL;
 	struct snd_sof_dev *sdev = dfse->sdev;
-	loff_t pos = 0;
 	int num_elems;
 	char *string;
 	int ret;
@@ -201,11 +200,11 @@ static ssize_t dfsentry_trace_filter_write(struct file *file, const char __user
 	if (!string)
 		return -ENOMEM;
 
-	/* assert null termination */
-	string[count] = 0;
-	ret = simple_write_to_buffer(string, count, &pos, from, count);
-	if (ret < 0)
+	if (copy_from_user(string, from, count)) {
+		ret = -EFAULT;
 		goto error;
+	}
+	string[count] = '\0';
 
 	ret = trace_filter_parse(sdev, string, &num_elems, &elems);
 	if (ret < 0)