mpls, nospec: Sanitize array index in mpls_label_ok()

mpls, nospec: Sanitize array index in mpls_label_ok()

BugLink: http://bugs.launchpad.net/bugs/1751131
commit 3968523f855050b8195134da951b87c20bd66130 upstream.

mpls_label_ok() validates that the 'platform_label' array index from a
userspace netlink message payload is valid. Under speculation the
mpls_label_ok() result may not resolve in the CPU pipeline until after
the index is used to access an array element. Sanitize the index to zero
to prevent userspace-controlled arbitrary out-of-bounds speculation, a
precursor for a speculative execution side channel vulnerability.

Cc: <stable@vger.kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>