TTY: drop driver reference in tty_open fail path

commit c290f835 upstream.

When tty_driver_lookup_tty fails in tty_open, we forget to drop a
reference to the tty driver. This was added by commit 4a2b5fdd (Move
tty lookup/reopen to caller).

Fix that by adding tty_driver_kref_put to the fail path.

I will refactor the code later. This is for the ease of backporting to
stable.

Introduced-in: v2.6.28-rc2
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Acked-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
[bwh: Backported to 2.6.32: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

CVE-2011-5321
Signed-off-by: Willy Tarreau <w@1wt.eu>

drivers/char/tty_io.c

@@ -1779,6 +1779,7 @@ static int __tty_open(struct inode *inode, struct file *filp)
 
 		if (IS_ERR(tty)) {
 			mutex_unlock(&tty_mutex);
+			tty_driver_kref_put(driver);
 			return PTR_ERR(tty);
 		}
 	}