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

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Message-ID: <20090128113640.2ea5a9fb@lxorguk.ukuu.org.uk>
Date:	Wed, 28 Jan 2009 11:36:40 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Bron Gondwana <brong@...tmail.fm>
Cc:	Ray Lee <ray-lk@...rabbit.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Bron Gondwana <brong@...tmail.fm>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/3] epoll: increase default max_user_instances to 1024

> > "A kernel upgrade in a -stable series point release fixed a security DoS"
> 
> Alan, that's a complete load of bollocks.  It broke common configurations 
> of java, postfix and apache on real-world machines, causing significant
> actual denials of service in previously reliable configurations.

It fixed a security DoS. I was merely pointing out that the description
provided before was bogus, incomplete and loaded.

> viable within the code.  The DoS works by creating epoll descriptors
> watching other epoll descriptors, which strikes me as a much less
> real-world actual use pattern than a bunch of separate daemons with an
> epoll watcher each.

Deliberate attackers don't have to follow typical usage patterns.

> If it's possible to count watches only if they're added to another epoll
> instance, then we'd have a metric that still catches the N^2 attack, but
> doesn't interact with the common non-attacky use-case.

Agreed entirely.
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