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

From: Michal Hocko
Date: Wed Aug 07 2013 - 09:47:00 EST

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
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