AFL/libdislocator.so.c

/*
  Copyright 2016 Google LLC 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

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.
*/

/*

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

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

   This is a companion library that can be used as a drop-in replacement
   for the libc allocator in the fuzzed binaries. See README.dislocator for
   more info.
*/

#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 0xCC

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

/* Configurable stuff (use AFL_LD_* 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?   */
           no_calloc_over;              /* abort() on calloc() overflows?   */

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 || total_mem + len < total_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) {

    if (no_calloc_over) {
      DEBUGF("calloc(%zu, %zu) would overflow, returning NULL", elem_len, elem_cnt);
      return NULL;
    }

    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 will 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");
  no_calloc_over = !!getenv("AFL_LD_NO_CALLOC_OVER");

}