linux-kernel - Re: [patch] drop epoll max_user_instances and rely only on max_user

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Date:	Thu, 29 Jan 2009 10:46:04 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>, Willy Tarreau <w@....eu>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Bron Gondwana <brong@...tmail.fm>
Subject: Re: [patch] drop epoll max_user_instances and rely only on
 max_user_watches

On Thu, 29 Jan 2009, Andrew Morton wrote:

> 
> > Subject: [patch] drop epoll max_user_instances and rely only on max_user_watches
> 
> nanonit: please prepare titles in the form "subsystem-id:
> what-i-did-to-it", so a suitable name here would be
> 
> 	epoll: drop max_user_instances and rely only on max_user_watches

Sorry, I forgot, again. I sent those by hand, while I have that logic in 
my scripts.



> On Wed, 28 Jan 2009 20:56:07 -0800 (PST) Davide Libenzi <davidel@...ilserver.org> wrote:
> 
> > Linus suggested to put limits where the money is, and max_user_watches 
> > already does that w/out the need of max_user_instances. That has the 
> > advantage to mitigate the potential DoS while allowing pretty generous 
> > default behavior.
> 
> A reader of this changelog would be wondering what this DoS is.

	for i 1..1021
		fd[i] = epoll_create()
	for i 1..1021
		for j 1..1021, j != i
			epoll_ctl(fd[i], ADD, fd[j])

I'm not sure we want to advertise it too much though.


> > Allowing top 4% of low memory (per user) to be allocated in epoll 
> > watches, we have:
> > 
> > LOMEM    MAX_WATCHES (per user)
> > 512MB    ~178000
> > 1GB      ~356000
> > 2GB      ~712000
> > 
> > A box with 512MB of lomem, will meet some challenge in hitting 180K 
> > watches, socket buffers math teaches us.
> > No more max_user_instances limits then.
> 
> So the max consumable memory is
> 
> 	number-of-users * max_user_watches * sizeof(whatever)
> 
> ?
> 
> So if enough users gang up (or if one person has access to a lot of
> UIDs), there's still a DoS?
> 
> I suspect we can live with that.

I think so, given that we've seen that putting too restrictive limits 
creates more problems. A similar thing, even though milder, exist on 
sys_poll too. sys_poll eats (on a 32bit box) 28 bytes of kernel memory for 
each 12 bytes of user memory, worse figures on a 64bit box.



> I assume that because you based all this on all the other patches, you
> view it as 2.6.30 material?
> 
> > @@ -581,10 +570,6 @@
> 
> please use `diff -p'.  It helps.

Need to add to my quilt diff options :)



> The code I have is
> 
> 	if (error < 0)
> 		ep_free(ep);
> 	else
> 		atomic_inc(&ep->user->epoll_devs);
> 
> so I obviously nuked the `else' as well.

That's the right thing.


- Davide


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