linux-kernel - Re: DOS by unprivileged user

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Message-ID: <20180423071311.GA5768@amd>
Date:   Mon, 23 Apr 2018 09:13:11 +0200
From:   Pavel Machek <pavel@....cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     vcaputo@...garu.com, Ferry Toth <ftoth@...fort.nl>,
        linux-kernel@...r.kernel.org
Subject: Re: DOS by unprivileged user

On Sun 2018-04-22 18:27:38, Michal Hocko wrote:
> On Sun 22-04-18 10:43:00, vcaputo@...garu.com wrote:
> > On Sun, Apr 22, 2018 at 12:16:54PM +0200, Pavel Machek wrote:
> > > On Thu 2018-04-19 21:13:35, 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.
> > > 
> > > You may want to disable swap.
> > > 
> > 
> > I run without swap on my laptops, and still observe long periods of
> > thrashing on the road towards OOM.  What seems to occur is the active
> > file-backed mappings of executables/libraries become a sort of swap
> > area, repeatedly being discarded and faulted back in as the context
> > switches occur.
> > 
> > If there's any good way to prevent this, I'd like to know.
> 
> I am afraid there is none yet. Johannes had some ground work for
> page cache trashing detection https://marc.info/?i=20170727153010.23347-1-hannes%40cmpxchg.org
> but there was no version of the patchseries for quite some time and
> there was no integration into the oom detection which would be
> non-trivial as well.
> 
> I realize this sucks. But the reality is that this is far from trivial
> to resolve without introducing pre-mature OOM killer invocations.

Another problem is that what "unusable machine" in X/web browser
situation may be normal load for build server...

I guess one way would be "hey, this is my X server; if it is waiting
for disk for more than 10 seconds, you probably want to OOM kill
someone. Ouch and same goes for my window manager". 

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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