libdislocator.so.c

/*

   american fuzzy lop - dislocator, an abusive allocator
   -----------------------------------------------------

   Written and maintained by Michal Zalewski <lcamtuf@google.com>

   Copyright 2016 Google Inc. All rights reserved.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at:

     http://www.apache.org/licenses/LICENSE-2.0

   This is a companion library that can be used as a drop-in replacement
   for the libc allocator in the fuzzed binaries. It improves the odds of
   bumping into heap-related security bugs in several ways:

     - It allocates all buffers so that they are immediately adjacent to a
       subsequent PROT_NONE page, causing most off-by-one reads and writes
       to immediately segfault,

     - It adds a canary immediately below the allocated buffer, to catch
       writes to negative offsets (won't catch reads, though),

     - It sets the memory returned by malloc() to garbage values, improving
       the odds of crashing when the target accesses uninitialized data,

     - It sets freed memory to PROT_NONE and does not actually reuse it,
       causing most use-after-free bugs to segfault right away,

     - It forces all realloc() calls to return a new address - and sets
       PROT_NONE on the original block. This catches use-after-realloc bugs,

     - It checks for calloc() overflows and can cause soft or hard failures
       of alloc requests past a configurable memory limit (AFL_LD_LIMIT_MB,
       AFL_LD_HARD_FAIL).

   Basically, it is inspired by some of the non-default options available
   for the OpenBSD allocator - see malloc.conf(5) on that platform for
   reference. It is also somewhat similar to several other debugging
   libraries, such as gmalloc and DUMA, but is simple, plug-and-play, and
   designed specifically for fuzzing jobs.

   Note that it does nothing for stack-based memory handling errors. The
   -fstack-protector-all setting for GCC / clang, enabled when using
   AFL_HARDEN, can catch some subset of that.

   The allocator is slow and memory-intensive (even the tiniest allocation
   uses up 4 kB of physical memory and 8 kB of virtual mem), making it
   completely unsuitable for "production" uses; but it can be faster and more
   hassle-free than ASAN / MSAN when fuzzing small, self-contained binaries.

   To use this library, run AFL like so:

   AFL_LD_PRELOAD=/path/to/libdislocator.so ./afl-fuzz [...other params...]

   You *have* to specify path, even if it's just ./libdislocator.so or
   $PWD/libdislocator.so.

   Similarly to afl-tmin, the library is not "proprietary" and can be
   used with other fuzzers or testing tools without the need for any code
   tweaks.

   Note that the LD_PRELOAD approach will work only if the target binary is
   dynamically linked.

 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <sys/mman.h>

#include "config.h"
#include "types.h"

#ifndef PAGE_SIZE
#  define PAGE_SIZE 4096
#endif /* !PAGE_SIZE */

#ifndef MAP_ANONYMOUS
#  define MAP_ANONYMOUS MAP_ANON
#endif /* !MAP_ANONYMOUS */

/* Error / message handling: */

#define DEBUGF(_x...) do { \
    if (alloc_verbose) { \
      if (++call_depth == 1) { \
        fprintf(stderr, "[AFL] " _x); \
        fprintf(stderr, "\n"); \
      } \
      call_depth--; \
    } \
  } while (0)

#define FATAL(_x...) do { \
    if (++call_depth == 1) { \
      fprintf(stderr, "*** [AFL] " _x); \
      fprintf(stderr, " ***\n"); \
      abort(); \
    } \
    call_depth--; \
  } while (0)

/* Macro to count the number of pages needed to store a buffer: */

#define PG_COUNT(_l) (((_l) + (PAGE_SIZE - 1)) / PAGE_SIZE)

/* Canary & clobber bytes: */

#define ALLOC_CANARY  0xAACCAACC
#define ALLOC_CLOBBER 0x41

#define PTR_C(_p) (((u32*)(_p))[-1])
#define PTR_L(_p) (((u32*)(_p))[-2])

/* Configurable stuff (use AFL_DISLOC_* to set): */

static u32 max_mem = MAX_ALLOC;         /* Max heap usage to permit         */
static u8  alloc_verbose,               /* Additional debug messages        */
           hard_fail;                   /* abort() when max_mem exceeded?   */

static __thread size_t total_mem;       /* Currently allocated mem          */

static __thread u32 call_depth;         /* To avoid recursion via fprintf() */

/* This is the main alloc function. It allocates one page more than necessary,
   sets that tailing page to PROT_NONE, and then increments the return address
   so that it is right-aligned to that boundary. Since it always uses mmap(),
   the returned memory will be zeroed. */

static void* __dislocator_alloc(size_t len) {

  void* ret;

  if (total_mem + len > max_mem) {

    if (hard_fail)
      FATAL("total allocs exceed %u MB", max_mem / 1024 / 1024);

    DEBUGF("total allocs exceed %u MB, returning NULL",
           max_mem / 1024 / 1024);

    return NULL;

  }

  /* We will also store buffer length and a canary below the actual buffer, so
     let's add 8 bytes for that. */

  ret = mmap(NULL, (1 + PG_COUNT(len + 8)) * PAGE_SIZE, PROT_READ | PROT_WRITE,
             MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

  if (ret == (void*)-1) {

    if (hard_fail) FATAL("mmap() failed on alloc (OOM?)");

    DEBUGF("mmap() failed on alloc (OOM?)");

    return NULL;

  }

  /* Set PROT_NONE on the last page. */

  if (mprotect(ret + PG_COUNT(len + 8) * PAGE_SIZE, PAGE_SIZE, PROT_NONE))
    FATAL("mprotect() failed when allocating memory");

  /* Offset the return pointer so that it's right-aligned to the page
     boundary. */

  ret += PAGE_SIZE * PG_COUNT(len + 8) - len - 8;

  /* Store allocation metadata. */

  ret += 8;

  PTR_L(ret) = len;
  PTR_C(ret) = ALLOC_CANARY;

  total_mem += len;

  return ret;

}


/* The "user-facing" wrapper for calloc(). This just checks for overflows and
   displays debug messages if requested. */

void* calloc(size_t elem_len, size_t elem_cnt) {

  void* ret;

  size_t len = elem_len * elem_cnt;

  /* Perform some sanity checks to detect obvious issues... */

  if (elem_cnt && len / elem_cnt != elem_len)
    FATAL("calloc(%zu, %zu) would overflow", elem_len, elem_cnt);

  ret = __dislocator_alloc(len);

  DEBUGF("calloc(%zu, %zu) = %p [%zu total]", elem_len, elem_cnt, ret,
         total_mem);

  return ret;

}


/* The wrapper for malloc(). Roughly the same, also clobbers the returned
   memory (unlike calloc(), malloc() is not guaranteed to return zeroed
   memory). */

void* malloc(size_t len) {

  void* ret;

  ret = __dislocator_alloc(len);

  DEBUGF("malloc(%zu) = %p [%zu total]", len, ret, total_mem);

  if (ret && len) memset(ret, ALLOC_CLOBBER, len);

  return ret;

}


/* The wrapper for free(). This simply marks the entire region as PROT_NONE.
   If the region is already freed, the code segfault during the attempt to
   read the canary. Not very graceful, but works, right? */

void free(void* ptr) {

  u32 len;

  DEBUGF("free(%p)", ptr);

  if (!ptr) return;

  if (PTR_C(ptr) != ALLOC_CANARY) FATAL("bad allocator canary on free()");

  len = PTR_L(ptr);

  total_mem -= len;

  /* Protect everything. Note that the extra page at the end is already
     set as PROT_NONE, so we don't need to touch that. */

  ptr -= PAGE_SIZE * PG_COUNT(len + 8) - len - 8;

  if (mprotect(ptr - 8, PG_COUNT(len + 8) * PAGE_SIZE, PROT_NONE))
    FATAL("mprotect() failed when freeing memory");

  /* Keep the mapping; this is wasteful, but prevents ptr reuse. */

}


/* Realloc is pretty straightforward, too. We forcibly reallocate the buffer,
   move data, and then free (aka mprotect()) the original one. */

void* realloc(void* ptr, size_t len) {

  void* ret;

  ret = malloc(len);

  if (ret && ptr) {

    if (PTR_C(ptr) != ALLOC_CANARY) FATAL("bad allocator canary on realloc()");

    memcpy(ret, ptr, MIN(len, PTR_L(ptr)));
    free(ptr);

  }

  DEBUGF("realloc(%p, %zu) = %p [%zu total]", ptr, len, ret, total_mem);

  return ret;

}


__attribute__((constructor)) void __dislocator_init(void) {

  u8* tmp = getenv("AFL_LD_LIMIT_MB");

  if (tmp) {

    max_mem = atoi(tmp) * 1024 * 1024;
    if (!max_mem) FATAL("Bad value for AFL_LD_LIMIT_MB");

  }

  alloc_verbose = !!getenv("AFL_LD_VERBOSE");
  hard_fail = !!getenv("AFL_LD_HARD_FAIL");

}