Re: [PATCH 1/3] epoll: increase default max_user_instances to 1024

Davide Libenzi
2009-01-28T05:30:51Z

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;