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From: steven at enel.ucalgary.ca (Steven Leikeim)
Subject: Re: hard links on Linux create local DoS vulnerability and security problems

On Mon, Nov 24, 2003 at 05:36:29PM +0100, Jakob Lell wrote:
> Hello,
> 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.
> 

Actually, this is a problem with ALL UNIX/UNIX like systems. And has been
since the beginning.

> 
> To solve the problem, the kernel shouldn't allow users to create hard links to 
> files belonging to someone else.

There is a simpler solution. Place user files on a separate filesystem
from system files. This includes putting all temporary files on separate
filesystems of their own. (Both /tmp and /var/tmp.) Since hard links
cannot cross filesystems the problem disappears. Mounting user filesystems
nosuid and nodev will prevent security problems should a setuid binary
appear in that filesystem.

Of course, this does not eliminate the first "DoS" problem noted above, but
it is simple for an administrator to find where the extraneous links are
and deal with the offending party.

-- 
Steven Leikeim                        |
University of Calgary                 |   There are lies, damned lies,
Department of Electrical Engineering  |        and statistics.
Internet: steven@...l.ucalgary.ca     |

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