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

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Date:	Tue, 27 Jan 2009 21:30:39 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Bron Gondwana <brong@...tmail.fm>
cc:	Ray Lee <ray-lk@...rabbit.org>,
	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

On Wed, 28 Jan 2009, Bron Gondwana wrote:

> On Tue, Jan 27, 2009 at 08:14:36PM -0800, Davide Libenzi wrote:
> > On Tue, 27 Jan 2009, Ray Lee wrote:
> > > Userspace is not broken here, and the whole idea of a -stable series
> > > is that administrators can upgrade to them without having to worry
> > > about things getting broken or making specific configuration changes
> > > by point release.
> > 
> > The reason Greg took it, was that in a multiuser systems, that's a DoS 
> > EZ-PZ Lemon Squeezie.
> 
> Ok - we're at an impasse here.
> 
> You know the code a whole lot better than me.
> 
> Is there anything you can think of that will allow us to block the DOS
> without breaking every medium to heavily loaded postfix and apache site
> out there.
> 
> Somthing that doesn't require the administrators of every single
> machine in one or the other class to tune their configurations?

Making the initial value of max_instances dependent on the amount of 
memory we can tollerate a user to exploit with the trick showed before.
Allowing up to 1% of lower memory, should roughly result in:

512MB -> ~225
1GB   -> ~310
2GB   -> ~440

We could ssume that heavily loaded mail and web servers to have an amount 
of RAM sufficent to get an high-enough default max_instances.




- Davide


---
 fs/eventpoll.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Index: linux-2.6.mod/fs/eventpoll.c
===================================================================
--- linux-2.6.mod.orig/fs/eventpoll.c	2009-01-27 21:12:29.000000000 -0800
+++ linux-2.6.mod/fs/eventpoll.c	2009-01-27 21:19:06.000000000 -0800
@@ -1419,7 +1419,9 @@
 	struct sysinfo si;
 
 	si_meminfo(&si);
-	max_user_instances = 128;
+	max_user_instances =
+		int_sqrt((((si.totalram - si.totalhigh) / 100) << PAGE_SHIFT) /
+			 EP_ITEM_COST);
 	max_user_watches = (((si.totalram - si.totalhigh) / 32) << PAGE_SHIFT) /
 		EP_ITEM_COST;
 
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