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/*
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Copyright 2016 Google LLC All rights reserved.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at:
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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/*
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american fuzzy lop - dislocator, an abusive allocator
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-----------------------------------------------------
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Written and maintained by Michal Zalewski <lcamtuf@google.com>
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This is a companion library that can be used as a drop-in replacement
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for the libc allocator in the fuzzed binaries. See README.dislocator for
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more info.
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <limits.h>
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#include <sys/mman.h>
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#include "../config.h"
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#include "../types.h"
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#ifndef PAGE_SIZE
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# define PAGE_SIZE 4096
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#endif /* !PAGE_SIZE */
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#ifndef MAP_ANONYMOUS
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# define MAP_ANONYMOUS MAP_ANON
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#endif /* !MAP_ANONYMOUS */
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/* Error / message handling: */
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#define DEBUGF(_x...) do { \
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if (alloc_verbose) { \
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if (++call_depth == 1) { \
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fprintf(stderr, "[AFL] " _x); \
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fprintf(stderr, "\n"); \
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} \
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call_depth--; \
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} \
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} while (0)
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#define FATAL(_x...) do { \
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if (++call_depth == 1) { \
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fprintf(stderr, "*** [AFL] " _x); \
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fprintf(stderr, " ***\n"); \
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abort(); \
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} \
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call_depth--; \
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} while (0)
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/* Macro to count the number of pages needed to store a buffer: */
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#define PG_COUNT(_l) (((_l) + (PAGE_SIZE - 1)) / PAGE_SIZE)
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/* Canary & clobber bytes: */
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#define ALLOC_CANARY 0xAACCAACC
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#define ALLOC_CLOBBER 0xCC
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#define PTR_C(_p) (((u32*)(_p))[-1])
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#define PTR_L(_p) (((u32*)(_p))[-2])
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/* Configurable stuff (use AFL_LD_* to set): */
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static u32 max_mem = MAX_ALLOC; /* Max heap usage to permit */
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static u8 alloc_verbose, /* Additional debug messages */
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hard_fail, /* abort() when max_mem exceeded? */
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no_calloc_over; /* abort() on calloc() overflows? */
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static __thread size_t total_mem; /* Currently allocated mem */
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static __thread u32 call_depth; /* To avoid recursion via fprintf() */
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/* This is the main alloc function. It allocates one page more than necessary,
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sets that tailing page to PROT_NONE, and then increments the return address
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so that it is right-aligned to that boundary. Since it always uses mmap(),
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the returned memory will be zeroed. */
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static void* __dislocator_alloc(size_t len) {
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void* ret;
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if (total_mem + len > max_mem || total_mem + len < total_mem) {
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if (hard_fail)
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FATAL("total allocs exceed %u MB", max_mem / 1024 / 1024);
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DEBUGF("total allocs exceed %u MB, returning NULL",
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max_mem / 1024 / 1024);
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return NULL;
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}
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/* We will also store buffer length and a canary below the actual buffer, so
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let's add 8 bytes for that. */
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ret = mmap(NULL, (1 + PG_COUNT(len + 8)) * PAGE_SIZE, PROT_READ | PROT_WRITE,
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MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
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if (ret == (void*)-1) {
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if (hard_fail) FATAL("mmap() failed on alloc (OOM?)");
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DEBUGF("mmap() failed on alloc (OOM?)");
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return NULL;
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}
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/* Set PROT_NONE on the last page. */
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if (mprotect(ret + PG_COUNT(len + 8) * PAGE_SIZE, PAGE_SIZE, PROT_NONE))
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FATAL("mprotect() failed when allocating memory");
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/* Offset the return pointer so that it's right-aligned to the page
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boundary. */
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ret += PAGE_SIZE * PG_COUNT(len + 8) - len - 8;
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/* Store allocation metadata. */
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ret += 8;
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PTR_L(ret) = len;
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PTR_C(ret) = ALLOC_CANARY;
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total_mem += len;
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return ret;
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}
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/* The "user-facing" wrapper for calloc(). This just checks for overflows and
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displays debug messages if requested. */
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void* calloc(size_t elem_len, size_t elem_cnt) {
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void* ret;
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size_t len = elem_len * elem_cnt;
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/* Perform some sanity checks to detect obvious issues... */
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if (elem_cnt && len / elem_cnt != elem_len) {
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if (no_calloc_over) {
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DEBUGF("calloc(%zu, %zu) would overflow, returning NULL", elem_len, elem_cnt);
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return NULL;
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}
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FATAL("calloc(%zu, %zu) would overflow", elem_len, elem_cnt);
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}
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ret = __dislocator_alloc(len);
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DEBUGF("calloc(%zu, %zu) = %p [%zu total]", elem_len, elem_cnt, ret,
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total_mem);
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return ret;
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}
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/* The wrapper for malloc(). Roughly the same, also clobbers the returned
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memory (unlike calloc(), malloc() is not guaranteed to return zeroed
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memory). */
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void* malloc(size_t len) {
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void* ret;
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ret = __dislocator_alloc(len);
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DEBUGF("malloc(%zu) = %p [%zu total]", len, ret, total_mem);
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if (ret && len) memset(ret, ALLOC_CLOBBER, len);
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return ret;
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}
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/* The wrapper for free(). This simply marks the entire region as PROT_NONE.
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If the region is already freed, the code will segfault during the attempt to
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read the canary. Not very graceful, but works, right? */
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void free(void* ptr) {
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u32 len;
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DEBUGF("free(%p)", ptr);
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if (!ptr) return;
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if (PTR_C(ptr) != ALLOC_CANARY) FATAL("bad allocator canary on free()");
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len = PTR_L(ptr);
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total_mem -= len;
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/* Protect everything. Note that the extra page at the end is already
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set as PROT_NONE, so we don't need to touch that. */
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ptr -= PAGE_SIZE * PG_COUNT(len + 8) - len - 8;
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if (mprotect(ptr - 8, PG_COUNT(len + 8) * PAGE_SIZE, PROT_NONE))
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FATAL("mprotect() failed when freeing memory");
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/* Keep the mapping; this is wasteful, but prevents ptr reuse. */
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}
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/* Realloc is pretty straightforward, too. We forcibly reallocate the buffer,
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move data, and then free (aka mprotect()) the original one. */
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void* realloc(void* ptr, size_t len) {
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void* ret;
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ret = malloc(len);
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if (ret && ptr) {
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if (PTR_C(ptr) != ALLOC_CANARY) FATAL("bad allocator canary on realloc()");
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memcpy(ret, ptr, MIN(len, PTR_L(ptr)));
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free(ptr);
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}
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DEBUGF("realloc(%p, %zu) = %p [%zu total]", ptr, len, ret, total_mem);
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return ret;
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}
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__attribute__((constructor)) void __dislocator_init(void) {
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u8* tmp = getenv("AFL_LD_LIMIT_MB");
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if (tmp) {
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max_mem = atoi(tmp) * 1024 * 1024;
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if (!max_mem) FATAL("Bad value for AFL_LD_LIMIT_MB");
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}
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alloc_verbose = !!getenv("AFL_LD_VERBOSE");
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hard_fail = !!getenv("AFL_LD_HARD_FAIL");
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no_calloc_over = !!getenv("AFL_LD_NO_CALLOC_OVER");
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}
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