Re: hard links create local DoS vulnerability and security problems

[mru@kth.se (Måns Rullgård)]

Jakob Lell <jlell@JakobLell.de> writes:

> on Linux it is possible for any user to create a hard link to a file
> belonging to another user. This hard link continues to exist even if
> the original file is removed by the owner. However, as the link
> still belongs to the original owner, it is still counted to his
> quota. If a malicious user creates hard links for every temp file
> created by another user, this can make the victim run out of quota
> (or even fill up the hard disk). This makes a local DoS attack
> possible.

This only make for a DoS attack if the there are user-writable
directories on a filesystem that the system depends on being writable.

> Furthermore, users can even create links to a setuid binary. If
> there is a security whole like a buffer overflow in any setuid
> binary, a cracker can create a hard link to this file in his home
> directory. This link still exists when the administrator has fixed
> the security whole by removing or replacing the insecure
> program. This makes it possible for a cracker to keep a security
> whole open until an exploit is available. It is even possible to
> create links to every setuid program on the system. This doesn't
> create new security holes but makes it more likely that they are
> exploited.

If any SUID user writable directories (/tmp, /var/various, /home) are
kept on separate partitions from any SUID executables (belonging in
/bin and /usr/bin), creating such links becomes impossible, and the
problem vanishes.

If a SUID program is found to have a security hole, the administrator
should make sure he removes all links to it when deleting it, just to
be safe.

-- 
Måns Rullgård
mru@kth.se