linux-kernel - Re: [PATCH 1/3] epoll: increase default max_user

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Message-ID: <20090128205119.GA8587@brong.net>
Date:	Thu, 29 Jan 2009 07:51:19 +1100
From:	Bron Gondwana <brong@...tmail.fm>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Bron Gondwana <brong@...tmail.fm>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...e.de>
Subject: Re: [PATCH 1/3] epoll: increase default max_user_instances to 1024

On Wed, Jan 28, 2009 at 08:52:51AM -0800, Davide Libenzi wrote:
> On Wed, 28 Jan 2009, Vegard Nossum wrote:
> 
> > On Wed, Jan 28, 2009 at 6:32 AM, Bron Gondwana <brong@...tmail.fm> wrote:
> > > That's clearly not happening here - so it seems that maybe our "happy
> > > medium" is actually in closer inspection of what's going on rather than
> > > a blanket low N to keep N^2 down.
> > 
> > Mh, could another solution to this all be to limit the number times
> > you can add a single epoll descriptor to another descriptor's set?
> 
> In the example that was posted, a single fd was added a single time inside 
> the other 1000+ fds. Epoll already has detection for too long chains and 
> closed loops, but you can't put those in the fast path. And epoll_ctl() is 
> one of those.

Not even if you're adding an epoll watcher inside another epoll watcher?

The problem I have here is that "a single fd was added a single time
inside the other 1000+ fds" is different behaviour to the daemons out
there.  They're pretty much all using flat layouts:

process 1:
 epoll_watcher:
  leaf fd
  leaf fd 2
  leaf fd 3
  leaf fd 4
  ...

process 2:
 epoll_watcher:
  ...

While the attack happens inside a single process.

Indeed, if you had a _per_process_ watcher limit, you would stop the
attack working while not breaking at least postfix and apache.  I'm not
sure what Java's doing under the hood, I have a feeling it's more
thready.

But most of all a way of detecting between a leaf fd and an epoll
watcher fd in epoll_ctl and doing deeper tests if it's an epoll watcher
that's being added would stop the attack.

Bron.

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