Testcase:
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
#include <sys/mman.h>
#include <stdio.h>

void main ()
{
	void *ptr = mmap (NULL, getpagesize () * 16, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, 0, 0);

	printf ("D: %p\n", ptr);
	mprotect (ptr, getpagesize (), PROT_READ|PROT_WRITE);

	*(int*)ptr = 18;
	printf ("%d\n", *(int*)ptr);
}
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Actual results:

valgrind ./a.out
==25441== Memcheck, a memory error detector
==25441== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==25441== Using Valgrind-3.5.0-Debian and LibVEX; rerun with -h for copyright info
==25441== Command: ./a.out
==25441==
D: 0x4020000
==25441== Invalid write of size 4
==25441==    at 0x400620: main (in /raid/home/zovarga/a.out)
==25441==  Address 0x4020000 is not stack'd, malloc'd or (recently) free'd
==25441==
==25441== Invalid read of size 4
==25441==    at 0x40062A: main (in /raid/home/zovarga/a.out)
==25441==  Address 0x4020000 is not stack'd, malloc'd or (recently) free'd
==25441==
18