Commit message: Drop privileges by default when opening user-related files

The module is typically executed as root and would sometimes
open files or follow symlinks that could be controlled from the
outside.

Drop privileges to the target user before opening any files.

Fixes CVE-2019-12209.

Thanks to Matthias Gerstner of the SUSE Security Team for reporting
the issue.
CVE: CVE-2019-12209
CWE: CWE-200
Vulnerability description: Yubico pam-u2f 1.0.7 attempts parsing of the configured authfile (default $HOME/.config/Yubico/u2f_keys) as root (unless openasuser was enabled), and does not properly verify that the path lacks symlinks pointing to other files on the system owned by root. If the debug option is enabled in the PAM configuration, part of the file contents of a symlink target will be logged, possibly revealing sensitive information.

Project: pam-u2f

int pam_sm_authenticate(pam_handle_t *pamh, int flags, int argc,
                        const char **argv) {

  struct passwd *pw = NULL, pw_s;
  const char *user = NULL;

  cfg_t cfg_st;
  cfg_t *cfg = &cfg_st;
  char buffer[BUFSIZE];
  char *buf = NULL;
  char *authfile_dir;
  size_t authfile_dir_len;
  int pgu_ret, gpn_ret;
   int retval = PAM_IGNORE;
   device_t *devices = NULL;
   unsigned n_devices = 0;
  int openasuser;
   int should_free_origin = 0;
   int should_free_appid = 0;
   int should_free_auth_file = 0;
   int should_free_authpending_file = 0;
 
   parse_cfg(flags, argc, argv, cfg);
 
  if (!cfg->origin) {
    strcpy(buffer, DEFAULT_ORIGIN_PREFIX);

    if (gethostname(buffer + strlen(DEFAULT_ORIGIN_PREFIX),
                    BUFSIZE - strlen(DEFAULT_ORIGIN_PREFIX)) == -1) {
      DBG("Unable to get host name");
      goto done;
    }
    DBG("Origin not specified, using \"%s\"", buffer);
    cfg->origin = strdup(buffer);
    if (!cfg->origin) {
      DBG("Unable to allocate memory");
      goto done;
    } else {
      should_free_origin = 1;
    }
  }

  if (!cfg->appid) {
    DBG("Appid not specified, using the same value of origin (%s)",
         cfg->origin);
    cfg->appid = strdup(cfg->origin);
    if (!cfg->appid) {
      DBG("Unable to allocate memory")
      goto done;
    } else {
      should_free_appid = 1;
    }
  }

  if (cfg->max_devs == 0) {
    DBG("Maximum devices number not set. Using default (%d)", MAX_DEVS);
    cfg->max_devs = MAX_DEVS;
  }

  devices = malloc(sizeof(device_t) * cfg->max_devs);
  if (!devices) {
    DBG("Unable to allocate memory");
    retval = PAM_IGNORE;
    goto done;
  }

  pgu_ret = pam_get_user(pamh, &user, NULL);
  if (pgu_ret != PAM_SUCCESS || user == NULL) {
    DBG("Unable to access user %s", user);
    retval = PAM_CONV_ERR;
    goto done;
  }

  DBG("Requesting authentication for user %s", user);

  gpn_ret = getpwnam_r(user, &pw_s, buffer, sizeof(buffer), &pw);
  if (gpn_ret != 0 || pw == NULL || pw->pw_dir == NULL ||
      pw->pw_dir[0] != '/') {
    DBG("Unable to retrieve credentials for user %s, (%s)", user,
         strerror(errno));
    retval = PAM_USER_UNKNOWN;
    goto done;
  }

  DBG("Found user %s", user);
  DBG("Home directory for %s is %s", user, pw->pw_dir);

  if (!cfg->auth_file) {
    buf = NULL;
    authfile_dir = secure_getenv(DEFAULT_AUTHFILE_DIR_VAR);
    if (!authfile_dir) {
      DBG("Variable %s is not set. Using default value ($HOME/.config/)",
           DEFAULT_AUTHFILE_DIR_VAR);
      authfile_dir_len =
        strlen(pw->pw_dir) + strlen("/.config") + strlen(DEFAULT_AUTHFILE) + 1;
      buf = malloc(sizeof(char) * (authfile_dir_len));

      if (!buf) {
        DBG("Unable to allocate memory");
        retval = PAM_IGNORE;
         goto done;
       }
 
       snprintf(buf, authfile_dir_len,
                "%s/.config%s", pw->pw_dir, DEFAULT_AUTHFILE);
     } else {
      DBG("Variable %s set to %s", DEFAULT_AUTHFILE_DIR_VAR, authfile_dir);
      authfile_dir_len = strlen(authfile_dir) + strlen(DEFAULT_AUTHFILE) + 1;
      buf = malloc(sizeof(char) * (authfile_dir_len));

      if (!buf) {
        DBG("Unable to allocate memory");
        retval = PAM_IGNORE;
        goto done;
      }
 
       snprintf(buf, authfile_dir_len,
                "%s%s", authfile_dir, DEFAULT_AUTHFILE);
     }
 
    DBG("Using default authentication file %s", buf);
 
     cfg->auth_file = buf; /* cfg takes ownership */
     should_free_auth_file = 1;
    buf = NULL;
  } else {
     DBG("Using authentication file %s", cfg->auth_file);
   }
 
  openasuser = geteuid() == 0 && cfg->openasuser;
   if (openasuser) {
    if (seteuid(pw_s.pw_uid)) {
      DBG("Unable to switch user to uid %i", pw_s.pw_uid);
       retval = PAM_IGNORE;
       goto done;
     }
    DBG("Switched to uid %i", pw_s.pw_uid);
   }
   retval = get_devices_from_authfile(cfg->auth_file, user, cfg->max_devs,
                                      cfg->debug, cfg->debug_file,
                                      devices, &n_devices);
   if (openasuser) {
    if (seteuid(0)) {
      DBG("Unable to switch back to uid 0");
       retval = PAM_IGNORE;
       goto done;
     }
    DBG("Switched back to uid 0");
   }
 
   if (retval != 1) {
    n_devices = 0;
  }

  if (n_devices == 0) {
    if (cfg->nouserok) {
      DBG("Found no devices but nouserok specified. Skipping authentication");
      retval = PAM_SUCCESS;
      goto done;
    } else if (retval != 1) {
      DBG("Unable to get devices from file %s", cfg->auth_file);
      retval = PAM_AUTHINFO_UNAVAIL;
      goto done;
    } else {
      DBG("Found no devices. Aborting.");
      retval = PAM_AUTHINFO_UNAVAIL;
      goto done;
    }
  }

  if (!cfg->authpending_file) {
    int actual_size = snprintf(buffer, BUFSIZE, DEFAULT_AUTHPENDING_FILE_PATH, getuid());
    if (actual_size >= 0 && actual_size < BUFSIZE) {
      cfg->authpending_file = strdup(buffer);
    }
    if (!cfg->authpending_file) {
      DBG("Unable to allocate memory for the authpending_file, touch request notifications will not be emitted");
    } else {
      should_free_authpending_file = 1;
    }
  } else {
    if (strlen(cfg->authpending_file) == 0) {
      DBG("authpending_file is set to an empty value, touch request notifications will be disabled");
      cfg->authpending_file = NULL;
    }
  }

  int authpending_file_descriptor = -1;
  if (cfg->authpending_file) {
    DBG("Using file '%s' for emitting touch request notifications", cfg->authpending_file);

    authpending_file_descriptor =
      open(cfg->authpending_file, O_RDONLY | O_CREAT | O_CLOEXEC | O_NOFOLLOW | O_NOCTTY, 0664);
    if (authpending_file_descriptor < 0) {
      DBG("Unable to emit 'authentication started' notification by opening the file '%s', (%s)",
          cfg->authpending_file, strerror(errno));
    }
  }

  if (cfg->manual == 0) {
    if (cfg->interactive) {
      converse(pamh, PAM_PROMPT_ECHO_ON,
               cfg->prompt != NULL ? cfg->prompt : DEFAULT_PROMPT);
    }

    retval = do_authentication(cfg, devices, n_devices, pamh);
  } else {
    retval = do_manual_authentication(cfg, devices, n_devices, pamh);
  }

  if (authpending_file_descriptor >= 0) {
    if (close(authpending_file_descriptor) < 0) {
      DBG("Unable to emit 'authentication stopped' notification by closing the file '%s', (%s)",
          cfg->authpending_file, strerror(errno));
    }
  }

  if (retval != 1) {
    DBG("do_authentication returned %d", retval);
    retval = PAM_AUTH_ERR;
    goto done;
  }

  retval = PAM_SUCCESS;

done:
  free_devices(devices, n_devices);

  if (buf) {
    free(buf);
    buf = NULL;
  }

  if (should_free_origin) {
    free((char *) cfg->origin);
    cfg->origin = NULL;
  }

  if (should_free_appid) {
    free((char *) cfg->appid);
    cfg->appid = NULL;
  }

  if (should_free_auth_file) {
    free((char *) cfg->auth_file);
    cfg->auth_file = NULL;
  }

  if (should_free_authpending_file) {
    free((char *) cfg->authpending_file);
    cfg->authpending_file = NULL;
  }

  if (cfg->alwaysok && retval != PAM_SUCCESS) {
    DBG("alwaysok needed (otherwise return with %d)", retval);
    retval = PAM_SUCCESS;
  }
  DBG("done. [%s]", pam_strerror(pamh, retval));

  if (cfg->is_custom_debug_file) {
    fclose(cfg->debug_file);
  }

  return retval;
}