BugLink: http://bugs.launchpad.net/bugs/1627730

commit 3b8767a8 upstream.

It should check access_ok().  Otherwise a bunch of places turn into
trivially exploitable rootholes.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>