x86, mpx: Strictly enforce empty prctl() args

Description from Michael Kerrisk.  He suggested an identical patch
to one I had already coded up and tested.

commit fe3d197f "x86, mpx: On-demand kernel allocation of bounds
tables" added two new prctl() operations, PR_MPX_ENABLE_MANAGEMENT and
PR_MPX_DISABLE_MANAGEMENT.  However, no checks were included to ensure
that unused arguments are zero, as is done in many existing prctl()s
and as should be done for all new prctl()s. This patch adds the
required checks.
Suggested-by: NAndy Lutomirski <luto@amacapital.net>
Suggested-by: NMichael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: NDave Hansen <dave.hansen@linux.intel.com>
Cc: Dave Hansen <dave@sr71.net>
Link: http://lkml.kernel.org/r/20150108223022.7F56FD13@viggo.jf.intel.comSigned-off-by: NThomas Gleixner <tglx@linutronix.de>

kernel/sys.c

@@ -2210,9 +2210,13 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		up_write(&me->mm->mmap_sem);
 		break;
 	case PR_MPX_ENABLE_MANAGEMENT:
+		if (arg2 || arg3 || arg4 || arg5)
+			return -EINVAL;
 		error = MPX_ENABLE_MANAGEMENT(me);
 		break;
 	case PR_MPX_DISABLE_MANAGEMENT:
+		if (arg2 || arg3 || arg4 || arg5)
+			return -EINVAL;
 		error = MPX_DISABLE_MANAGEMENT(me);
 		break;
 	default: