linux-kernel - Re: Possible deny of service with memfd

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Date:   Thu, 4 Feb 2021 16:32:38 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
cc:     Christian Koenig <christian.koenig@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Possible deny of service with memfd_create()

On Thu, 4 Feb 2021, Michal Hocko wrote:
> On Thu 04-02-21 17:32:20, Christian Koenig wrote:
> > Hi Michal,
> > 
> > as requested in the other mail thread the following sample code gets my test
> > system down within seconds.
> > 
> > The issue is that the memory allocated for the file descriptor is not
> > accounted to the process allocating it, so the OOM killer pics whatever
> > process it things is good but never my small test program.
> > 
> > Since memfd_create() doesn't need any special permission this is a rather
> > nice deny of service and as far as I can see also works with a standard
> > Ubuntu 5.4.0-65-generic kernel.
> 
> Thanks for following up. This is really nasty but now that I am looking
> at it more closely, this is not really different from tmpfs in general.
> You are free to create files and eat the memory without being accounted
> for that memory because that is not seen as your memory from the sysstem
> POV. You would have to map that memory to be part of your rss.
> 
> The only existing protection right now is to use memoery cgroup
> controller because the tmpfs memory is accounted to the process which
> faults the memory in (or write to the file).
> 
> I am not sure there is a good way to handle this in general
> unfortunatelly. Shmem is is just tricky (e.g. how to you deal with left
> overs after the fd is closed?). Maybe memfd_create can be more clever
> and account memory to all owners of the fd but even that sounds far from
> trivial from the accounting POV. It is true that tmpfs can at least
> control who can write to it which is not the case for memfd but then we
> hit the backward compatibility wall.

Yes, no solution satisfactory, and memcg best, but don't forget
echo 2 >/proc/sys/vm/overcommit_memory

Hugh