firewire: fix NULL pointer deref. and resource leak

By supplying ioctl()s in the wrong order, a userspace client was able to
trigger NULL pointer dereferences.  Furthermore, by calling
ioctl_create_iso_context more than once, new contexts could be created
without ever freeing the previously created contexts.

Thanks to Anders Blomdell for the report.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/fw-cdev.c

@@ -646,6 +646,10 @@ static int ioctl_create_iso_context(struct client *client, void *buffer)
 	struct fw_cdev_create_iso_context *request = buffer;
 	struct fw_iso_context *context;
 
+	/* We only support one context at this time. */
+	if (client->iso_context != NULL)
+		return -EBUSY;
+
 	if (request->channel > 63)
 		return -EINVAL;
 
@@ -792,8 +796,9 @@ static int ioctl_start_iso(struct client *client, void *buffer)
 {
 	struct fw_cdev_start_iso *request = buffer;
 
-	if (request->handle != 0)
+	if (client->iso_context == NULL || request->handle != 0)
 		return -EINVAL;
+
 	if (client->iso_context->type == FW_ISO_CONTEXT_RECEIVE) {
 		if (request->tags == 0 || request->tags > 15)
 			return -EINVAL;
@@ -810,7 +815,7 @@ static int ioctl_stop_iso(struct client *client, void *buffer)
 {
 	struct fw_cdev_stop_iso *request = buffer;
 
-	if (request->handle != 0)
+	if (client->iso_context == NULL || request->handle != 0)
 		return -EINVAL;
 
 	return fw_iso_context_stop(client->iso_context);