firewire: cdev: prevent kernel stack leaking into ioctl arguments

Found by the UC-KLEE tool:  A user could supply less input to
firewire-cdev ioctls than write- or write/read-type ioctl handlers
expect.  The handlers used data from uninitialized kernel stack then.

This could partially leak back to the user if the kernel subsequently
generated fw_cdev_event_'s (to be read from the firewire-cdev fd)
which notably would contain the _u64 closure field which many of the
ioctl argument structures contain.

The fact that the handlers would act on random garbage input is a
lesser issue since all handlers must check their input anyway.

The fix simply always null-initializes the entire ioctl argument buffer
regardless of the actual length of expected user input.  That is, a
runtime overhead of memset(..., 40) is added to each firewirew-cdev
ioctl() call.  [Comment from Clemens Ladisch:  This part of the stack is
most likely to be already in the cache.]

Remarks:
  - There was never any leak from kernel stack to the ioctl output
    buffer itself.  IOW, it was not possible to read kernel stack by a
    read-type or write/read-type ioctl alone; the leak could at most
    happen in combination with read()ing subsequent event data.
  - The actual expected minimum user input of each ioctl from
    include/uapi/linux/firewire-cdev.h is, in bytes:
    [0x00] = 32, [0x05] =  4, [0x0a] = 16, [0x0f] = 20, [0x14] = 16,
    [0x01] = 36, [0x06] = 20, [0x0b] =  4, [0x10] = 20, [0x15] = 20,
    [0x02] = 20, [0x07] =  4, [0x0c] =  0, [0x11] =  0, [0x16] =  8,
    [0x03] =  4, [0x08] = 24, [0x0d] = 20, [0x12] = 36, [0x17] = 12,
    [0x04] = 20, [0x09] = 24, [0x0e] =  4, [0x13] = 40, [0x18] =  4.
Reported-by: David Ramos <daramos@stanford.edu>
Cc: <stable@vger.kernel.org>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/core-cdev.c

@@ -1637,8 +1637,7 @@ static int dispatch_ioctl(struct client *client,
 	    _IOC_SIZE(cmd) > sizeof(buffer))
 		return -ENOTTY;
 
-	if (_IOC_DIR(cmd) == _IOC_READ)
-		memset(&buffer, 0, _IOC_SIZE(cmd));
+	memset(&buffer, 0, sizeof(buffer));
 
 	if (_IOC_DIR(cmd) & _IOC_WRITE)
 		if (copy_from_user(&buffer, arg, _IOC_SIZE(cmd)))