firewire: cdev: check write quadlet request length to avoid buffer overflow

Check that the data length of a write quadlet request actually is large
enough for a quadlet.  Otherwise, fw_fill_request could access the four
bytes after the end of the outbound_transaction_event structure.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

Modification of Clemens' change:  Consolidate the check into
init_request() which is used by the affected ioctl_send_request() and
ioctl_send_broadcast_request() and the unaffected
ioctl_send_stream_packet(), to save a few lines of code.

Note, since struct outbound_transaction_event *e is slab-allocated, such
an out-of-bounds access won't hit unallocated memory but may result in a
(virtually impossible to exploit) information disclosure.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/core-cdev.c

@@ -563,6 +563,10 @@ static int init_request(struct client *client,
 	    (request->length > 4096 || request->length > 512 << speed))
 		return -EIO;
 
+	if (request->tcode == TCODE_WRITE_QUADLET_REQUEST &&
+	    request->length < 4)
+		return -EINVAL;
+
 	e = kmalloc(sizeof(*e) + request->length, GFP_KERNEL);
 	if (e == NULL)
 		return -ENOMEM;