Re: Safe /tmp cleanup

dsiebert@ICAEN.UIOWA.EDU
Thu, 13 Nov 1997 10:48:43 -0600

>
> Delete all files that haven't been accessed in 1.5 days in /dir and /ect:
>
> find2perl /dir /ect -eval '-A > 1.5 and unlink' | perl
>
> Steven> The source for this program is 294 lines of C (including comments).
>
> And completely unnecessary, given the above perl command-line. :-)
>
> The output of this find2perl run is 17 lines of Perl, by the way.
>
> Steven> Enough care seems to have been taken to avoid race hazards
> Steven> and my limited examination of code satisfied me that there are
> Steven> no security problems with it. Specfically, the program does
> Steven> everything itself, it does not rely on an external program for
> Steven> any function which should eliminate problems associated with
> Steven> special characters and/or buffer overflows due to deep paths.
>
> Ditto on the find2perl solution.
>
> "find2perl" comes with all modern Perl releases.
>
> Perl is your friend. Use Perl.
>

Wrong. Check out this snippet from find.pl (from perl 5.003):

# Get link count and check for directoriness.

($dev,$ino,$mode,$nlink) = lstat($_) unless $nlink;

if (-d _) {

# It really is a directory, so do it recursively.

if (!$prune && chdir $_) {
&finddir($name,$nlink);
chdir '..';
}
--$subcount;
}

It "checks for directoriness", and if it is a directory it chdir's into it.
This does not do anything at all to prevent someone changing the name which
used to be a directory into a link to somewhere else in the meantime. You
have to assume an attacker can make your Perl script run arbitrarily slow
(not like this is hard with Perl in the first place) and therefore can do
bad things in between the lstat and the chdir. The modification to the
GNU find I wrote (hopefully) catches any such possible attack. I have not
looked at the RedHat thing Steven mentions, so I can't comment on how well
it does in this regard.

--
Douglas Siebert                Director of Computing Facilities
douglas-siebert@uiowa.edu      Division of Mathematical Sciences, U of Iowa


If you let the system beat you long enough, eventually it'll get tired.