Tuesday, December 8, 2009

iCTF 2009 - C++ Binary Review (1)

It was last Friday. We (Plaid parliament of Pwning) took 4th place in iCTF 2009. This year's iCTF was novel. Thousands of bots were running on UCSB, and they were connecting to us according to the search rank in the web search engine they provided. All the bots were using more than 15 different versions of browsers including Perl, Python, Erlang, Java, and C++.

Since my main role was to do binary analysis, I could read almost every browser code. Especially, C++ browsers were interesting to me, and we were the only team who found all the c++ browsers' vulnerabilities and crafted exploits for all of them during the competition.

Here, I will present a walk-through for crefox-1.0 problem, which is the first-level C++ problem.

The most interesting part in this problem is that it uses dlopen function. At first glance, we thought the program uses "safe_printf" function from a certain library. However, this was just a trick! Let's look at the source code below.

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int
print_func(const char *fmt, ...)
{

    void *h;

    /* lets retrieve the safe printf implementation from the library */
    int(*f)(const char *, ...);

    if (!strncmp("USESAFEPRINTFUNCTIONA", fmt, 21)) {
        h = dlopen(NULL, RTLD_LAZY);
        if (!h) {
            errnonf("dlopen: %s\n", dlerror());
            return -1;
        }
        f = (int(*)(const char *, ...))dlsym(h, "safe_printf");
        f("%s\n", fmt);
    }
    else
        /* ok, lets follow the user will and revert to the unsafe printf :-( */
        printf("%s\n", fmt);

    return 0;
}

The line of strncmp function is the most tricky part in this problem. Note that they first check a weird string and if it matches, it will load a function called "safe_printf". However, the returned function from dlsym is NULL here.

So what will happen when the function pointer f is called? The instruction pointer will go to the address of 0x00000000. So, here, we expect the segfault. Right?

However, the program will not terminate. Why? Let's look at the previous part of the source code before the print_func function is called. The most important part is shown below.
    ptr = mmap(NULL,
               size,
               PROT_READ | PROT_WRITE | PROT_EXEC,
               MAP_PRIVATE | MAP_ANON | MAP_FIXED, -1, 0);
So, the program uses mmap to allocate memory at the address zero! In addition, our page URL is copied to the memory. Therefore, we can execute arbitrary code we provide. Note that mmap used PROT_EXEC option to run the code.

The only problem is that our page URL should start with the string USESAFEPRINTFUNCTIONA. We need to know what will be the instruction for the string. Fortunately, the string represents a valid sequence of instructions as follows.

0x0:    push   %ebp
0x1:    push   %ebx
0x2:    inc    %ebp
0x3:    push   %ebx
0x4:    inc    %ecx
0x5:    inc    %esi
0x6:    inc    %ebp
0x7:    push   %eax
0x8:    push   %edx
...
They are just push and increment instructions. So the next step is really simple. We only need to put our shell code right after the string USESAFEPRINTFUNCTIONA. In this way, a web browser who visits our website (including the string USESAFEPRINTFUNCTIONA and shellcode in the page) will run our shellcode, and connect to our web server.

It was really fun to play iCTF, and all the binary problems were really intriguing. The next two problems are more tricky, and I will explain them later if I have time. :D


Full source code:
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/*
* iCTF Crefox browser
*
* Lorenzo ``Gigi Sullivan'' Cavallaro <sullivan@cs.ucsb.edu>
*
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <getopt.h>
#include <stdarg.h>
#include <errno.h>
#include <dlfcn.h>
#include <sys/mman.h>
#include <curl/curl.h>

#include "list.h"

#include <iostream>
#include <string>
using namespace std;

#include <htmlcxx/html/ParserDom.h>
using namespace htmlcxx;

#define MAXINPUT 8192
#define PROMPT "COMMAND>"
#define COOKIEFILE "./.cookiejar.txt"

#define VERSION "1.0.1"
#define UA "Crefox-" VERSION " (+http://ictf.cs.ucsb.edu/)"

extern char *optarg;
extern int optind, opterr, optopt;
extern char *__progname;

struct opts
{
    char *proxy;
    char *prompt;
    char plugins;
};

struct page
{
    char *memory;
    size_t size;
    void *mmap;
    size_t mmap_size;
};

struct plugins
{
    void *h;
    char *name;
    struct list_head list;
};

CURL *browser_init(void);
void browser_shutdown(int, CURL *) __attribute__((noreturn));
void parse_options(int, char **, struct opts *, void *);
void split(char **, char *, int);
int uniq(char **, char *, char);
char **parse_input(char *, int *);
size_t map_page(void *, size_t, size_t, void *);
void free_args(char **);
struct plugins *loadlib(const char *);
int load_plugins(struct plugins *);
void unload_plugins(struct plugins *);
int print_func(const char *, ...);
int safe_move(struct page *);

int extract_tags(char *, char *, char *, int(*)(void *));
int print_href(void *);

void __errf(int, const char *, ...) __attribute__((noreturn));
void __errnonf(const char *, ...);

#define errf(x, fmt, args...) \
do { \
 fprintf(stderr, "ERROR:%s:(fatal):", __progname), __errf(x, fmt, ##args); \
} while (0)

#define errnonf(fmt, args...) \
do { \
 fprintf(stderr, "ERROR:%s:(non fatal):", __progname), __errnonf(fmt, ##args); \
} while (0)

#define debug(args...) \
do { \
 fprintf(stderr, "DEBUG:%s:", __progname), fprintf(stderr, ##args); \
} while (0)

int
main(int argc, char **argv)
{

    char errbuf[CURL_ERROR_SIZE], quit;
    CURL *browser;
    int res;

    struct opts opts;
    struct page page;
    struct stat sbuf;
    struct plugins plugins;

    int(*output_page)(const char *, ...);

    browser = browser_init();
    if (!browser)
        errf(1, "browser_init()\n");

    memset(&page, 0, sizeof(page));
    memset(&opts, 0, sizeof(opts));
    parse_options(argc, argv, &opts, browser);

    INIT_LIST_HEAD(&plugins.list);

    res = load_plugins(&plugins);
    if (res != -1) {

        struct list_head *l;
        struct plugins *p;
        int i = 0;

        opts.plugins++;

        list_for_each(l, &plugins.list) {

            p = list_entry(l, struct plugins, list);
            debug("plugin[%i]:%s\n", i++, p->name);

        }
    }
    else {
        errnonf("plugins not successfully loaded\n");
    }

    if (opts.proxy) {
        curl_easy_setopt(browser, CURLOPT_PROXY, opts.proxy);
        debug("proxy: %s\n", opts.proxy);
    }

    curl_easy_setopt(browser, CURLOPT_USERAGENT, UA);
    curl_easy_setopt(browser, CURLOPT_REFERER, "http://ictf.cs.ucsb.edu");
    curl_easy_setopt(browser, CURLOPT_FOLLOWLOCATION, 1);
    curl_easy_setopt(browser, CURLOPT_ERRORBUFFER, errbuf);

    if (!stat(COOKIEFILE, &sbuf))
        curl_easy_setopt(browser, CURLOPT_COOKIEFILE, COOKIEFILE);
    curl_easy_setopt(browser, CURLOPT_COOKIEJAR, COOKIEFILE);

    curl_easy_setopt(browser, CURLOPT_WRITEFUNCTION, map_page);
    curl_easy_setopt(browser, CURLOPT_WRITEDATA, &page);

    output_page = &print_func;

    quit = 0;
    while (!quit) {

        char in[MAXINPUT];
        int num, i;
        char **args;

        memset(in, 0, MAXINPUT);
        memset(errbuf, 0, sizeof(errbuf));
        printf("%s", opts.prompt);

        if (!fgets(in, MAXINPUT, stdin)) {
            if (!errno) {
                fprintf(stderr, "\n");
                errnonf("Please use 'q' to quit the browser\n");
                continue;
            }
            errnonf("fgets(): error: %d %s\n", errno, strerror(errno));
            break;
        }

        args = parse_input(in, &num);
        if (!args) {
            errnonf("parse_input()\n");
            continue;
        }

        if (num > 1)
            curl_easy_setopt(browser, CURLOPT_URL, args[1]);

        if (!strcmp(args[0], "u")) {

            long code, redir;

            if (num == 2 && args[1]) {

                for (i = 0; i < num; i++)
                    debug("args[%d]: %s\n", i, args[i]);

                if (page.memory) {
                    free(page.memory);
                    memset(&page, 0, sizeof(page));
                }

                debug("Getting URL: %s\n", args[1]);

                curl_easy_setopt(browser, CURLOPT_POST, 0);

                if (curl_easy_perform(browser))
                    errnonf("URL retrieval: %s\n", errbuf);
                else {
                    curl_easy_getinfo(browser, CURLINFO_RESPONSE_CODE, &code);
                    debug("HTTP status: %ld\n", code);
                    curl_easy_getinfo(browser, CURLINFO_REDIRECT_COUNT, &redir);
                    debug("HTTP redirect #: %ld\n", redir);
                }

                if (page.memory && code == 200) {

                    if (safe_move(&page) == -1) {
                        errnonf("safe_move failed (no output can be generated)\n");
                        continue;
                    }

                    if (output_page((const char *)page.mmap) == -1)
                        printf("%s\n", (const char *)page.mmap);

                    memset(page.mmap, 0, page.mmap_size);
                    munmap(page.mmap, page.mmap_size);

                }
            }
            else
                errnonf("malformed 'u' request\n");

            free_args(args);
            continue;
        }

        if (!strcmp(args[0], "p")) {

            memset(errbuf, 0, sizeof(errbuf));

            for (i = 0; i < num; i++)
                debug("args[%d]: %s\n", i, args[i]);

            if (num == 3 && args[1] && args[2]) {

                unsigned long code;

                if (page.memory) {
                    free(page.memory);
                    memset(&page, 0, sizeof(page));
                }

                curl_easy_setopt(browser, CURLOPT_POST, 1);
                curl_easy_setopt(browser, CURLOPT_POSTFIELDS, args[2]);

                debug("Posting to URL: %s with data: %s\n", args[1], args[2]);

                if (curl_easy_perform(browser))
                    errnonf("URL retrieval: %s\n", errbuf);
                else {
                    curl_easy_getinfo(browser, CURLINFO_RESPONSE_CODE, &code);
                    debug("HTTP status: %ld\n", code);
                }

                if (page.memory && code == 200)
                    printf("%s\n", page.memory);
            }
            else
                errnonf("malformed 'p' request\n");

            free_args(args);
            continue;
        }

        if (!strcmp(args[0], "l")) {

            int res;

            res = extract_tags(page.memory, (char *)"a", (char *)"href", print_href);
            if (res == -1)
                errnonf("error while parsing/retrieving for <a> tags\n");

            free_args(args);
            continue;
        }

        if (!strcmp(args[0], "q")) {

            if (num != 1)
                errnonf("malformed 'q' request\n");
            else
                quit = 1;

            free_args(args);
            continue;
        }

        errnonf("malformed input\n");
    }

    unload_plugins(&plugins);

    exit(0);
}

int
extract_tags(char *page, char *tagstr, char *attr, int(*callback)(void *))
{

    string s_page;
    HTML::ParserDom parser;
    tree<HTML::Node> dom;
    tree<HTML::Node>::iterator it;
    tree<HTML::Node>::iterator end;
    std::pair<bool, std::string> tag;
    char lcase_tag[strlen(tagstr) + 1];
    unsigned int i;

    if (page)
        s_page = string(page);
    else {
        errnonf("zero-length or non existing page\n");
        return -1;
    }

    dom = parser.parseTree(s_page);
    it = dom.begin();
    end = dom.end();

    memset(lcase_tag, 0, sizeof(lcase_tag));

    for (i = 0; i < strlen(tagstr); i++)
        lcase_tag[i] = tolower(tagstr[i]);

    for (; it != end; ++it) {

        if (it->tagName() == lcase_tag) {

            it->parseAttributes();
            tag = it->attribute(attr);

            if (tag.first)
                (void)callback(&tag.second);
        }
    }

    return 0;
}

int
print_href(void *value)
{
    cout << *(std::string *)value << endl;
    return 0;
}

int
safe_move(struct page *page)
{

    void *ptr;
    unsigned int size = (page->size + 4096) & ~4095;
    int serrno = errno;

    errno = 0;
    ptr = mmap(NULL,
               size,
               PROT_READ | PROT_WRITE | PROT_EXEC,
               MAP_PRIVATE | MAP_ANON | MAP_FIXED, -1, 0);
    if (errno != 0) {
        errno = serrno;
        printf("errno: %d %s\n", errno, strerror(errno));
        return -1;
    }
    errno = serrno;

    memset(ptr, 0, size);
    memcpy(ptr, page->memory, strlen(page->memory));

    page->mmap = ptr;
    page->mmap_size = size;

    return 0;
}

int
print_func(const char *fmt, ...)
{

    void *h;

    /* lets retrieve the safe printf implementation from the library */
    int(*f)(const char *, ...);

    if (!strncmp("USESAFEPRINTFUNCTIONA", fmt, 21)) {
        h = dlopen(NULL, RTLD_LAZY);
        if (!h) {
            errnonf("dlopen: %s\n", dlerror());
            return -1;
        }
        f = (int(*)(const char *, ...))dlsym(h, "safe_printf");
        f("%s\n", fmt);
    }
    else
        /* ok, lets follow the user will and revert to the unsafe printf :-( */
        printf("%s\n", fmt);

    return 0;
}

struct plugins *
    loadlib(const char *n)
{

    struct plugins *p;

    p = (struct plugins *)calloc(1, sizeof(*p));
    if (!p)
        return NULL;

    p->name = strdup(n);
    p->h = dlopen(n, RTLD_NOW);
    if (!p->h) {
        free(p);
        return NULL;
    }

    return p;
}

int
load_plugins(struct plugins *phead)
{

    struct plugins *p;
    int res = 0;

    p = loadlib("libget.so");
    if (p)
        list_add(&(p->list), &(phead->list));
    else
        res = -1;

    p = loadlib("libpost.so");
    if (p)
        list_add(&(p->list), &(phead->list));
    else
        res = -1;

    p = loadlib("liblink.so");
    if (p)
        list_add(&(p->list), &(phead->list));
    else
        res = -1;

    return res;
}

void
unload_plugins(struct plugins *phead)
{

    struct list_head *l, *n;
    struct plugins *p;

    list_for_each_safe(l, n, &(phead->list)) {
        p = list_entry(l, struct plugins, list);
        list_del(l);
        (void)dlclose(p->h);
        free(p->name);
        free(p);
    }

    return;
}

void
free_args(char **args)
{

    char **p = args;

    for (; *p; p++)
        free(*p);
    free(args);
}

CURL *
browser_init(void)
{

    setbuf(stdout, NULL);
    setbuf(stderr, NULL);

    if (curl_global_init(CURL_GLOBAL_SSL))
        return NULL;
    return curl_easy_init();
}

void
browser_shutdown(int exitcode, void *arg)
{
    CURL *br = arg;

    curl_easy_cleanup(br);
    curl_global_cleanup();
    exit(exitcode);
}

void
parse_options(int argc, char **argv, struct opts *opts, void *br)
{

    char opt;

    if (on_exit(browser_shutdown, br))
        return;

    while ((opt = getopt(argc, argv, "x:p:")) != -1) {
        switch (opt) {
        case 'p':
            opts->prompt = strdup(optarg);
            break;
        case 'x':
            opts->proxy = strdup(optarg);
            break;
        default: /* '?' */
            fprintf(stderr, "Usage: %s [-p prompt] [-x proxy]\n", argv[0]);
            exit(2);
        }
    }

    if (!opts->prompt)
        opts->prompt = strdup(PROMPT);

    return;
}

void
__errf(int code, const char *fmt, ...)
{

    va_list va;

    va_start(va, fmt);
    vfprintf(stderr, fmt, va);
    va_end(va);

    exit(code);
}

void
__errnonf(const char *fmt, ...)
{

    va_list va;

    va_start(va, fmt);
    vfprintf(stderr, fmt, va);
    va_end(va);

    return;
}

int
uniq(char **out, char *inarg, char delim)
{

    int len, s = 0, i = 0;
    char *in, *p, *start, *end;

    if (!inarg)
        return 0;

    start = inarg;
    end = inarg + strlen(inarg) - 1;
    /* skip heading delim */
    while (*start == delim) start++;
    /* skip trailing ones */
    while (*end == delim) *end-- = 0;

    in = start;
    len = strlen(start) + 1;

    *out = (char *)malloc(len * sizeof(char));
    if (!*out)
        return -1;

    memset(*out, 0, len * sizeof(char));

    p = *out;

    while (*in) {

        if ((*in == delim)) {  /* found a delim? */
            if (!s) {                  /* never seen it so far? */
                i++;                    /* keep the real count */
                *p++ = *in++;
                s = 1;                  /* record it */
            }
            else
                in++;                   /* already seen? skip ahead */
        }
        else {
            /* no delim so straight copy */
            *p++ = *in++;
            s = 0;
        }

    }

    /* counts args[0] as well */
    return ++i;
}

void
split(char **arg, char *buf, int num)
{

    char *tmp, *p;
    int i = 0;

    /* make a safe copy of buf since strsep will mangle it */
    tmp = (char *)strdup(buf);

    do {
        p = (char *)strsep(&tmp, " ");
        if (!tmp && !i)
            p = buf;
        arg[i++] = (char *)strdup(p);
        p = tmp;
    } while (tmp);

    arg[i] = NULL;

    free(tmp);
    return;
}

char **
parse_input(char *in, int *num)
{

    char **args, *stripbuf;
    int x;

    while (strlen(in) > 0 && (in[strlen(in) - 1] == '\n' || in[strlen(in) - 1] == '\r'))
        in[strlen(in) - 1] = 0;

    *num = uniq(&stripbuf, in, ' ');
    x = *num;

    args = (char **)malloc((x + /* nil term */1) * sizeof(char *));
    memset(args, 0, (x + 1) * sizeof(char *));

    /*
    * we split buf into chunks delimited by ' ': these represent cmd
    * and cmd args. Due to the nature of split, we don't care about
    * ' " ` ; and so on
    */

    split(args, stripbuf, x + 1);

    free(stripbuf);

    return args;
}

size_t
map_page(void *ptr, size_t size, size_t nmemb, void *data)
{
    size_t realsize = size * nmemb;
    struct page *mem = (struct page *)data;
    char *tmp;

    if (realsize) {
        tmp = (char *)malloc((mem->size + realsize + 1) * sizeof(char));
        if (!tmp)
            return CURLE_WRITE_ERROR;

        memset(tmp, 0, mem->size + realsize + 1);

        if (mem->size) {
            memcpy(tmp, mem->memory, mem->size);
            free(mem->memory);
        }

        memcpy(&tmp[mem->size], ptr, realsize);
        mem->size += realsize;
        tmp[mem->size] = 0;

        mem->memory = tmp;
    }

    return realsize;
}

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Hi, I am a PhD candidate at CMU. I was one of the founding members of PPP (Plaid Parliament of Pwning). I like programming in OCaml, F#, Haskell, and C++.