firewire: nosy: ensure user_length is taken into account when fetching packet contents

Ensure that packet_buffer_get respects the user_length provided. If
the length of the head packet exceeds the user_length, packet_buffer_get
will now return 0 to signify to the user that no data were read
and a larger buffer size is required. Helps prevent user space overflows.

Signed-off-by: Thanassis Avgerinos <thanassis.avgerinos@gmail.com>
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>

Author: ethan42

drivers/firewire/nosy.c

@@ -148,10 +148,12 @@ packet_buffer_get(struct client *client, char __user *data, size_t user_length)
 	if (atomic_read(&buffer->size) == 0)
 		return -ENODEV;
 
-	/* FIXME: Check length <= user_length. */
+	length = buffer->head->length;
+
+	if (length > user_length)
+		return 0;
 
 	end = buffer->data + buffer->capacity;
-	length = buffer->head->length;
 
 	if (&buffer->head->data[length] < end) {
 		if (copy_to_user(data, buffer->head->data, length))