Fix pktcdvd ioctl dev_minor range check

The PKT_CTRL_CMD_STATUS device ioctl retrieves a pointer to a
pktcdvd_device from the global pkt_devs array. The index into this
array is provided directly by the user and is a signed integer, so the
comparison to ensure that it falls within the bounds of this array will
fail when provided with a negative index.

This can be used to read arbitrary kernel memory or cause a crash due to
an invalid pointer dereference. This can be exploited by users with
permission to open /dev/pktcdvd/control (on many distributions, this is
readable by group "cdrom").

Signed-off-by: Dan Rosenberg <[e-mail]>
[ Rather than add a cast, just make the function take the right type -Linus ]
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/block/pktcdvd.c

···
 	pkt_shrink_pktlist(pd);
 }
 
-static struct pktcdvd_device *pkt_find_dev_from_minor(int dev_minor)
+static struct pktcdvd_device *pkt_find_dev_from_minor(unsigned int dev_minor)
 {
 	if (dev_minor >= MAX_WRITERS)
 		return NULL;