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

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

[Federico Sevilla III]

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(Note: bug originally posted on BugTraq, this response is cross-posted to
the LKML because of the way the server hosting the X font server ground to
a halt.)

On Mon, 10 Jun 2002 at 10:20, Tom wrote:
> Affected
> ========
> Mozilla 1.0 and earlier
> verified on Linux and Solaris, other Unixes most likely affected as well.

Re-verified on Debian GNU/Linux running kernel 2.4.18-xfs with Mozilla
1.0.0 and XFree86 4.1.0 with the font server running on a separate server
also running on Debian GNU/Linux with kernel 2.4.18-xfs and the X font
server from XFree86 4.1.0.

The server running the X font server has 512MB RAM, 1GB swap, has one ext2
partition (/boot), one ReiserFS partition (Squid cache) and the rest are
XFS partitions. It also runs the kernel-mode NFS server v1.0 and Samba
2.2.4, among other things, and is typically headless.

> Effect
> ======
> System becomes unuseable or X windows crashes
> (varies depending on system configuration)

Here I had a workstation (that I thought 'hey, I can reboot this after
I've felt how the problem works') that used a remote font server, which
was company-wide. Yes, I'm the administrator so I killed myself. I've got
a personal note to myself, as well. "Don't try stuff posted on BugTraq on
anything connected to the network, dummy." ;)

Anyway, FWIW:

First I noticed that the Mozilla window stopped responding immediately. So
I closed it and breathed deeply. Then I noticed the server hosting the X
font server started thrashing really badly. /home is exported from the
same server via kernel-mode NFS, so file access to /home started freezing
on workstations. Samba access slowed down, but continued to work.

I was able to log on to the server with enough time to SIGKILL the
xfs-daemon process. Unfortunately this wasn't good enough. The server
started running up various processes stuck in "D" state, the OOM killer
went on panic mode and started killing things left and right, mostly from
what I notice apache and apache-ssl processes with messages like "Out of
Memory: Killed process xxxx (apache)". I was also able to do a `free`
after killing xfs-daemon and noticed that there was a lot of free memory
both physically and on swap.

Within less than ten minutes on this relatively lightly-loaded server, I
could not log in to the machine locally, even as root (whose home
directory is not NFS-exported) and load levels shot up to around 25, which
is definitely abnormal. Existing logged-on processes also got stuck in
whatever they were doing (`ps ax`, in particular is what I remember).

Attempted reboots locally via Ctrl-Alt-Delete and Magic SysRq failed
because (1) I had disabled ctrl-alt-delete mapping in inittab "for
security", and (2) I had not compiled Magic SysRq support on this
particular server. More notes to self.

> Description
> ===========
> When loading pages with a specially prepared (or erroneous) stylesheet,
> mozilla and X windows (not restricted to XFree) exhibit any of two
> undesireable behaviours. This seems to depend on the local system
> configuration, especially to the presence of xfs, but bug reports so far
> are inconclusive.
> In one scenario, X simply crashes, taking everything with it. This will result
> in the loss of unsaved work.
> In scenario two, memory useage of the X server explodes until the machine
> reaches the thrashing point, at which point only a hard kill (-9) of the
> X server can save it, provided there are enough system resources left to
> issue the kill.

In my case the workstation was easy to stabilize. I was actually able to
close all windows and exit xfce properly. It's the server running the X
font server that ground to painful halt. I do not know how things would
have been if I had the Magic SysRq enabled.

> The bug is triggered by a huge font setting done through CSS. Depending
> on the end user's system configuration, this will either trigger an
> abort in the XFree86 code ("Beziers this large not supported") or cause
> an explosive use of memory. It is unknown how much memory could get
> consumed, but follow-ups to the mozilla bug verify that machines with 1
> GB of memory still reach the thrashing point.

While I agree with BugTraq posts in response to this that applications
like Mozilla which accept font-sizes from unknown sources should have some
check to prevent such large sizes from crashing X and/or the X Font
Server, I'm alarmed by (1) the way the X font server allows itself to be
crashed like this, and (2) the way the entire Linux kernel seems to have
been unable to handle the situation. While having a central company or
department wide font server may not have been the best choice I made, this
seems like a simple way to drive a stake through a system's heart.

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. :(

 --> Jijo

- -- 
Federico Sevilla III   :  <http://jijo.free.net.ph/>
Network Administrator  :  The Leather Collection, Inc.
GnuPG Key ID           :  0x93B746BE
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Re: Very large font size crashing X Font Server and Grounding Server to

[Alan Cox]

> check to prevent such large sizes from crashing X and/or the X Font
> Server, I'm alarmed by (1) the way the X font server allows itself to be
> crashed like this, and (2) the way the entire Linux kernel seems to have
> been unable to handle the situation. While having a central company or

So turn on the features to conrol it. Set rlimits on the xfs server and 
enable non overcommit (-ac kernel)

Alan

rlimits and non overcommit (was: Very large font size ...)

[Federico Sevilla III]

Hi Alan,
(cc BugTraq and LKML)

On Thu, 13 Jun 2002 at 06:39, Alan Cox wrote:
> > check to prevent such large sizes from crashing X and/or the X Font
> > Server, I'm alarmed by (1) the way the X font server allows itself to
> > be crashed like this, and (2) the way the entire Linux kernel seems to
> > have been unable to handle the situation. While having a central
> > company or
>
> So turn on the features to conrol it. Set rlimits on the xfs server and
> enable non overcommit (-ac kernel)

I am using SGI's XFS, and I think they follow Marcelo's kernels for the
2.4 series, at the moment. Are there any plans of getting non overcommit
from your tree into Marcelo's?

TIA. :)

 --> Jijo

-- 
Federico Sevilla III   :  <http://jijo.free.net.ph/>
Network Administrator  :  The Leather Collection, Inc.
GnuPG Key ID           :  0x93B746BE

Re: rlimits and non overcommit (was: Very large font size ...)

[Keith Owens]

On Thu, 13 Jun 2002 13:57:33 +0800 (PHT), 
Federico Sevilla III <jijo@free.net.ph> wrote:
>On Thu, 13 Jun 2002 at 06:39, Alan Cox wrote:
>> > check to prevent such large sizes from crashing X and/or the X Font
>> > Server, I'm alarmed by (1) the way the X font server allows itself to
>> > be crashed like this, and (2) the way the entire Linux kernel seems to
>> > have been unable to handle the situation. While having a central
>> > company or
>>
>> So turn on the features to conrol it. Set rlimits on the xfs server and
>> enable non overcommit (-ac kernel)
>
>I am using SGI's XFS, and I think they follow Marcelo's kernels for the

SGI's XFS != xfs server.  SGI XFS == journalling filesystem.
xfs server == font server for the X windowing system.

Re: rlimits and non overcommit

[Federico Sevilla III]

On Thu, 13 Jun 2002 at 16:11, Keith Owens wrote:
> On Thu, 13 Jun 2002 13:57:33 +0800 (PHT),
> Federico Sevilla III <jijo@free.net.ph> wrote:
> >On Thu, 13 Jun 2002 at 06:39, Alan Cox wrote:
> >> > check to prevent such large sizes from crashing X and/or the X Font
> >> > Server, I'm alarmed by (1) the way the X font server allows itself to
> >> > be crashed like this, and (2) the way the entire Linux kernel seems to
> >> > have been unable to handle the situation. While having a central
> >> > company or
> >>
> >> So turn on the features to conrol it. Set rlimits on the xfs server and
> >> enable non overcommit (-ac kernel)
> >
> >I am using SGI's XFS, and I think they follow Marcelo's kernels for the
>
> SGI's XFS != xfs server.  SGI XFS == journalling filesystem.
> xfs server == font server for the X windowing system.

Argh. And we get hit by similarly-named projects. :(

To clarify: I wanted to know if there were plans of getting non overcommit
from the AC tree into Marcelo's mainline 2.4 tree, because I use SGI's XFS
(journalling filesystem), and thus use the -xfs kernel tree, which follows
-marcelo and not -ac.

Apologies.

 --> Jijo

-- 
Federico Sevilla III   :  <http://jijo.free.net.ph/>
Network Administrator  :  The Leather Collection, Inc.
GnuPG Key ID           :  0x93B746BE

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

[Matti Aarnio]

On Thu, Jun 13, 2002 at 09:44:33AM +0800, Federico Sevilla III wrote:
> From:	Federico Sevilla III <jijo@free.net.ph>
> To:	BugTraq Mailing List <bugtraq@securityfocus.com>,
> 	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
> 
> (Note: bug originally posted on BugTraq, this response is cross-posted to
> the LKML because of the way the server hosting the X font server ground to
> a halt.)

  It really is nothing new.  (Besides of XFree86's Type1 engine calling 
  abort() when it does not like to do something ..)
...
> 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. :(

  ANY very big program with active working set larger than memory size
  has problems at Linux / Linux has problems handling it.  Indeed the
  problem is _not_ new, nor trivial to solve efficiently.  The ultimate
  situation is called "trashing", where a program to proceed a page of
  code needs to be moved into memory, and to make room for that, some
  other program page must be moved out..  What makes the issue more
  difficult is that the memory is used for lots and lots of different
  kinds of buffers and caches as well, and playing a balancing act on
  them all is quite difficult.

  This is recurring topic at  linux-kernel  list, and has its own
  list called  linux-mm  (not at vger, though.)

  Others may have different views on the issue, but I think that:
     2.0:        fairly tolerable OOM/trashing behaviour
     2.2:        got worse
     2.4.early:  rather terrible
     2.4.late:   improved somewhat, roughly in par with 2.2

>  --> Jijo
> - -- 
> Federico Sevilla III   :  <http://jijo.free.net.ph/>
...
> Please read the FAQ at  http://www.tux.org/lkml/

/Matti Aarnio

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

[rjh]

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

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

[Security Coordinator]

On Thursday 13 June 2002 12:26, rjh@world.std.com 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

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

[Matthew Wakeling]

On Thu, 13 Jun 2002, Federico Sevilla III wrote:

> I was able to log on to the server with enough time to SIGKILL the
> xfs-daemon process. Unfortunately this wasn't good enough. The server
> started running up various processes stuck in "D" state, the OOM killer
> went on panic mode and started killing things left and right, mostly from
> what I notice apache and apache-ssl processes with messages like "Out of
> Memory: Killed process xxxx (apache)". I was also able to do a `free`
> after killing xfs-daemon and noticed that there was a lot of free memory
> both physically and on swap.
> 
> Within less than ten minutes on this relatively lightly-loaded server, I
> could not log in to the machine locally, even as root (whose home
> directory is not NFS-exported) and load levels shot up to around 25, which
> is definitely abnormal. Existing logged-on processes also got stuck in
> whatever they were doing (`ps ax`, in particular is what I remember).

It has always puzzled me that a process using lots of memory can bring 
down an entire (otherwise relatively idle) server to the extent that one 
cannot even get mingetty on a local terminal to respond to keypresses. I 
can confirm that the described situation is not just a one-off.

It is my experience that a single process using large amounts of memory 
causes the system to start swapping. Once it starts swapping, every 
process that does anything (apart from indefinite wait) goes into "I'm 
ready to do some processing, but my memory is swapped out" state, and the 
whole system collapses.

I don't know if Linux prioritises swap-ins as well as CPU time, but 
perhaps this would be a good way of stabilising this circumstance. One 
could arrange the dynamic process priorities so that the bad neighbour 
(read: exploited xfs-daemon) gets less than its fair share of physical 
RAM, therefore letting the rest of the system live. Perhaps one could 
limit the number of processes that are having their swap-in serviced at 
any one time, and make the ones that are being serviced the ones with the 
highest dynamic priority.

I don't know what exactly is causing the problem, but it is possible that 
pages are being paged in, the process runs on them for a quantum, and then 
paged out. In this case, it might help to dictate a minimum time a page 
can be "in", measured in the amount of work the owning process manages to 
do.

My other quibble is that the out-of-memory killer seems to choose 
processes fairly randomly. Ideally, it should kill the process that is 
causing the problem - which in my experience is always the biggest process 
in the system, or the process which has grown the fastest recently. 
However, it should probably be positively discouraged from killing things 
like apache (where most of the process space is shared), or gettys (which 
are firstly small, and secondly likely to be respawned by someone at the 
first opportunity).

> Attempted reboots locally via Ctrl-Alt-Delete and Magic SysRq failed
> because (1) I had disabled ctrl-alt-delete mapping in inittab "for
> security", and (2) I had not compiled Magic SysRq support on this
> particular server. More notes to self.

I think that if you had not disabled ctrl-alt-delete, it wouldn't have 
done much good anyway, since usually that key combination just tells init 
to spawn a shutdown process, which has just as much chance of getting 
resources as any other process in the system.

> While I agree with BugTraq posts in response to this that applications
> like Mozilla which accept font-sizes from unknown sources should have some
> check to prevent such large sizes from crashing X and/or the X Font
> Server, I'm alarmed by (1) the way the X font server allows itself to be
> crashed like this, and (2) the way the entire Linux kernel seems to have
> been unable to handle the situation.

I am in complete agreement with both points, but particularly that the 
kernel should be able to cope with the situation gracefully. I know one 
can set limits on processes, but the kernel should still be able to cope 
if we don't.

> Suggestions on how to work around this on multiple levels would definitely
> be appreciated.

My suggestion would be to set a maximum core size for the xfs-daemon 
process. Given your setup, something like 400MB seems appropriate - maybe 
a little lower. Details for doing this seem to differ from linux to linux. 
Having done that, I would make sure xfs respawns when it dies - that way 
someone can't just DOS your whole network by asking for a large font.
Finally, wait for the xfs developers to put in a font size limit, or patch 
the source yourself.

Apart from that, I wouldn't move xfs to a bigger server unless you have 
already had people complaining about its performance. Moving it to a 
bigger system just changes the constant in the equation - the attacker 
would only need to specify a 100000 point font instead of a 50000 point 
font to bring the system down.

I doubt any of my kernel suggestions have not already been thought about, 
but it was worth a try. Please can this problem be fixed soon?

Matthew

-- 
"If I wanted to kill a battleship, I'd use a s?!tload of Harpoons."
"NT is a lot cheaper."  -- Paul Tomblin & Petro

Re: Very large font size crashing X Font Server and Grounding Server to a Halt

[Bernd Eckenfels]

In article <Pine.LNX.4.44.0206132128570.4999-100000@server3.jumpleads.com> you wrote:
> It has always puzzled me that a process using lots of memory can bring 
> down an entire (otherwise relatively idle) server to the extent that one 
> cannot even get mingetty on a local terminal to respond to keypresses. I 
> can confirm that the described situation is not just a one-off.

Well, with crrent versions you have to have 2 or 3 of them to realy make a
server trtash. Something like:

tail /dev/zero 

recovers with the oom killer

tail /dev/zero & tail /dev/zero & tail /dev/zero

will most likely trash for minutes.

Some kind of penalty could be introduced, because this is better to set up
than ulimit. On a system with large amounts of users you simply cant set the
ulimit of a single user to mainram/concurrent-users or
mainram/concurrent-evil-users.

Greetings
Bernd