linux-kernel - Re: [PATCH 1/3] memcg: limit the number of thresholds per-memcg

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Date:	Wed, 7 Aug 2013 15:46:54 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Anton Vorontsov <anton.vorontsov@...aro.org>
Subject: Re: [PATCH 1/3] memcg: limit the number of thresholds per-memcg

On Wed 07-08-13 09:22:10, Tejun Heo wrote:
> Hello,
> 
> On Wed, Aug 07, 2013 at 01:28:25PM +0200, Michal Hocko wrote:
> > There is no limit for the maximum number of threshold events registered
> > per memcg. This might lead to an user triggered memory depletion if a
> > regular user is allowed to register on memory.[memsw.]usage_in_bytes
> > eventfd interface.
> > 
> > Let's be more strict and cap the number of events that might be
> > registered. MAX_THRESHOLD_EVENTS value is more or less random. The
> > expectation is that it should be high enough to cover reasonable
> > usecases while not too high to allow excessive resources consumption.
> > 1024 events consume something like 16KB which shouldn't be a big deal
> > and it should be good enough.
> 
> I don't think the memory consumption per-se is the issue to be handled
> here (as kernel memory consumption is a different generic problem) but
> rather that all listeners, regardless of their priv level, cgroup
> membership and so on, end up contributing to this single shared
> contiguous table,

The table is per-memcg but you are right that everybody who has file
write access to the particular group's usage file can register to it.

> which makes it quite easy to do DoS attack on it if
> the event control is actually delegated to untrusted security domain,

OK, I have obviously misunderstood your concern mentioned in the other
email. Could you be more specific what is the DoS scenario which was
your concern, then?

[...]
> Can you please update the patch description to reflect the actual
> problem?

As soon as I understand what is your concern ;)
-- 
Michal Hocko
SUSE Labs
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