linux-kernel - Re: DOS by unprivileged user

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Message-ID: <1524199618.5757.3.camel@gmx.de>
Date:   Fri, 20 Apr 2018 06:46:58 +0200
From:   Mike Galbraith <efault@....de>
To:     Ferry Toth <ftoth@...fort.nl>
Cc:     lkml <linux-kernel@...r.kernel.org>
Subject: Re: DOS by unprivileged user

On Thu, 2018-04-19 at 21:13 +0200, Ferry Toth wrote:
> It appears any ordinary user can easily create a DOS on linux.
> 
> One sure way to reproduce this is to open gitk on the linux kernel repo 
> (SIC) on a machine with 8GB RAM 16 GB swap on a HDD with btrfs and quad core 
> + hyperthreading. But I will be easy enough to get the same effect with more 
> RAM, other fs etc.
> 
> In this case gitk allocates more and more memory (until my system freezes 
> 6.5GB of 7.5GB avaiable), the system starts swapping or writing to tmp files 
> (can't investigate as there is no time until it freezes) and the io wait 
> goes to 100% on all cores. At this point it is impossible to login from 
> remote and local keyboard and mouse are frozen. Hard reset is the only way 
> out at this point.

datapoint: my i4790/ext4 box running master.yesterday booted mem=8G
became highly unpleasant to use, but I retained control, and the all
cores going to 100% thing did not happen at any time.

I didn't try constraining on the gitk user, just turned it loose a few
times to see if it managed to render box effectively dead.  It failed
to kill my box, but (expectedly) did make it suck rocks.

	-Mike