#!/usr/bin/perl -T

# checkperms: ensure as best we can that a given file can only be
# modified by the given user (or the superuser, naturally).  This
# means checking file ownership and permissions all the way back to
# the root directory.  Pass the file by its absolute path.

# example invocation:

# checkperms dkg /home/dkg/.monkeysphere/authorized_user_ids

# return values: zero if we believe the file and path can only be
# modified by the user.  non-zero otherwise.

# see StrictModes in sshd_config(5) (and its implementation in
# OpenSSH's secure_filename() in auth.c) for the initial
# inspiration/rationale for this code.

# Author:
#  Daniel Kahn Gillmor <dkg@fifthhorseman.net>

# Started on: 2009-07-31 11:10:16-0400

# License: GPL v3 or later

use strict;

use Cwd qw(realpath); # found in debian in perl-base
use File::stat; # found in debian in perl-modules
use User::pwent; # found in debian in perl-modules
use Fcntl qw(:mode); # for S_IS* functions (in perl-base)
use File::Basename; # for dirname (in perl-modules)

my $username = shift;
my $path = shift;

defined($username) or die "You must pass a username and an absolute path.\n";
defined($path) or die "You must pass a username and an absolute path.\n";

my $pw = getpwnam($username) or die "no such user $username\n";
$path =~ m#^/# or die "path was not absolute (did not start with /)\n";

sub debug {
  if ($ENV{LOG_LEVEL} eq 'DEBUG') {
    # FIXME: prefix with $ENV{LOG_PREFIX}
    printf STDERR @_;
  }
}

## return undef if permissions are OK.  otherwise return an error string
sub permissions_ok {
  my $user = shift;
  my $path = shift;

  # if we can't even stat the path, the permissions are not ok:
  my $stat = lstat($path) or return "cannot stat '$path'";

  while (S_ISLNK($stat->mode)) {
    my $newpath = realpath($path) or return "cannot trace symlink '$path'";
    debug("tracing link %s to %s\n", $path, $newpath);
    $path = $newpath;
    $stat = lstat($path) or return "cannot stat '$path'";
  }
  debug("checking '%s'\n", $path);

  if (($stat->uid != $user->uid) &&
      ($stat->uid != 0)) {
    return sprintf("improper ownership on '%s': owner ID %d is neither %s (ID %d) nor the superuser",
		   $path, $stat->uid, $user->name, $user->uid);
  }

  if (S_IWGRP & $stat->mode) {
    return sprintf("improper group writability on '%s'", $path);
  }

  if (S_IWGRP & $stat->mode) {
    return sprintf("improper group writability on '%s'", $path);
  }

  if (S_IWOTH & $stat->mode) {
    return sprintf("improper other writability on '%s'", $path);
  }

  # see the rationalization in secure_filename() in auth.c in the
  # OpenSSH sources for an explanation of this bailout (see also
  # monkeysphere #675):
  if ($path eq $user->dir) {
    debug("stopping at the %s's home directory '%s'\n", $user->name, $path);
    return undef;
  }

  my $nextlevel = dirname($path);
  if ($path eq $nextlevel) { # we bottom out at the root (/ in UNIX)
    return undef;
  }
  return permissions_ok($user, $nextlevel);
}

my $err = permissions_ok($pw, $path);

if (defined($err)) {
  printf(STDERR "%s%s\n", $ENV{LOG_PREFIX}, $err);

  exit(1);
} else {
  exit(0);
}