Re: Very large font size crashing X Font Server and Grounding Server to a Halt (was: remote DoS in Mozilla 1.0)

From: Security Coordinator (
Date: Fri Jun 14 2002 - 08:50:57 EST

On Thursday 13 June 2002 12:26, wrote:

Exactly, any user without limits can arbitrarily "fork bomb" a system too. A
shell script and newbie level programming talent is all you need... That
whole class of DoS are hard to stop. You can do 100 things, like starve a
system of file handles, open 50,000 listen ports, whatever. You can set
limits, but there are not even standard APIs for limiting every conceivable
exhaustible resource systems allocate.

> On 13 Jun, Federico Sevilla III wrote:
> > Suggestions on how to work around this on multiple levels would
> > definitely be appreciated. I'll be starting by removing the X font server
> > from our file and authentication server onto some high-powered
> > workstation, but I'm sure this won't be enough, and knowing that a user
> > process like xfs-daemon can drag the Linux kernel down to knees is not
> > very comforting. :(
> The protection that you need is provided by "ulimit" on most Unixes.
> There are facilities to limit maximum real memory used, maximum virtual
> memory, maximum number of processes, etc. This specific bug in XFree is
> one of a general case of inescapable user process bugs. It resulted in
> an almost infinite size malloc() request. You can acheive the same
> effect in any userspace program by just putting malloc() inside an
> infinite loop.
> If you allow users to run with unlimited memory permission, you are
> vulnerable. The XFree bug will hit more people than usual because it is
> common to put the ulimit on regular user logins and forget to place a
> limit on the automatically started processes. The default configuration
> from RedHat, SuSE, and others is to start XFree outside the login
> system. You can also place limits on these processes but you need to
> examine the startup scripts to install the limits in the right places.
> This would then result in a different DoS. Whenever XFree hits the
> memory limit, the malloc's will fail, and XFree will decide what to do
> about it. Depending on the circumstances, XFree may shut down, thus
> killing all the X window dependent processes.
> R Horn
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