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/*
american fuzzy lop++ - postprocessor for PNG
------------------------------------------
Originally written by Michal Zalewski
Copyright 2015 Google Inc. All rights reserved.
Adapted to the new API, 2020 by Dominik Maier
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
See post_library.so.c for a general discussion of how to implement
postprocessors. This specific postprocessor attempts to fix up PNG
checksums, providing a slightly more complicated example than found
in post_library.so.c.
Compile with:
gcc -shared -Wall -O3 post_library_png.so.c -o post_library_png.so -lz
*/
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <zlib.h>
#include <arpa/inet.h>
/* A macro to round an integer up to 4 kB. */
#define UP4K(_i) ((((_i) >> 12) + 1) << 12)
typedef struct post_state {
unsigned char *buf;
size_t size;
} post_state_t;
void *afl_custom_init(void *afl) {
post_state_t *state = malloc(sizeof(post_state_t));
if (!state) {
perror("malloc");
return NULL;
}
state->buf = calloc(sizeof(unsigned char), 4096);
if (!state->buf) {
free(state);
perror("calloc");
return NULL;
}
return state;
}
size_t afl_custom_post_process(post_state_t *data, const unsigned char *in_buf,
unsigned int len,
const unsigned char **out_buf) {
unsigned char *new_buf = (unsigned char *)in_buf;
unsigned int pos = 8;
/* Don't do anything if there's not enough room for the PNG header
(8 bytes). */
if (len < 8) {
*out_buf = in_buf;
return len;
}
/* Minimum size of a zero-length PNG chunk is 12 bytes; if we
don't have that, we can bail out. */
while (pos + 12 <= len) {
unsigned int chunk_len, real_cksum, file_cksum;
/* Chunk length is the first big-endian dword in the chunk. */
chunk_len = ntohl(*(uint32_t *)(in_buf + pos));
/* Bail out if chunk size is too big or goes past EOF. */
if (chunk_len > 1024 * 1024 || pos + 12 + chunk_len > len) break;
/* Chunk checksum is calculated for chunk ID (dword) and the actual
payload. */
real_cksum = htonl(crc32(0, in_buf + pos + 4, chunk_len + 4));
/* The in-file checksum is the last dword past the chunk data. */
file_cksum = *(uint32_t *)(in_buf + pos + 8 + chunk_len);
/* If the checksums do not match, we need to fix the file. */
if (real_cksum != file_cksum) {
/* First modification? Make a copy of the input buffer. Round size
up to 4 kB to minimize the number of reallocs needed. */
if (new_buf == in_buf) {
if (len <= data->size) {
new_buf = data->buf;
} else {
new_buf = realloc(data->buf, UP4K(len));
if (!new_buf) {
*out_buf = in_buf;
return len;
}
data->buf = new_buf;
data->size = UP4K(len);
memcpy(new_buf, in_buf, len);
}
}
*(uint32_t *)(new_buf + pos + 8 + chunk_len) = real_cksum;
}
/* Skip the entire chunk and move to the next one. */
pos += 12 + chunk_len;
}
*out_buf = new_buf;
return len;
}
/* Gets called afterwards */
void afl_custom_deinit(post_state_t *data) {
free(data->buf);
free(data);
}
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