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Date: Wed, 29 Aug 2007 12:29:40 +0100
Subject: Re: Fork Bombing Patch
From: "Simon Arlott" <simon@fire.lp0.eu>
To: "Anand Jahagirdar" <anandjigar@gmail.com>
Cc: "Chris Snook" <csnook@redhat.com>, "Krzysztof Halasa" <khc@pm.waw.pl>,
       linux-kernel@vger.kernel.org
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On Wed, August 29, 2007 10:48, Anand Jahagirdar wrote:
> Hi
>                 printk_ratelimit function takes care of flooding the
> syslog. due to printk_ratelimit function syslog will not be flooded
> anymore. as soon as administrator gets this message, he can take
> action against that user (may be block user's access on server). i
> think the my fork patch is very useful and helps administrator lot.
>                 i would also like to mention that in some of the cases
> ulimit solution wont work. in that case fork bombing takes the machine
> and server needs a reboot. i am sure in that situation this printk
> statement helps administrator to know what has happened.

If ulimit "wont work" in some situations, how is it going to trigger this printk?
(When doesn't it work?)

> Anand
>
> On 8/24/07, Chris Snook <csnook@redhat.com> wrote:
>> Krzysztof Halasa wrote:
>> > Hi,
>> >
>> > "Anand Jahagirdar" <anandjigar@gmail.com> writes:
>> >
>> >>    I am forwarding one more improved patch which i have modified as
>> >> per your suggestions. Insted of KERN_INFO i have used KERN_NOTICE and
>> >> i have added one more if block to check hard limit. how good it is?
>> >
>> > Not very, still lacks "#ifdef CONFIG_something" and the required
>> > Kconfig change (or other runtime thing defaulting to "no printk").
>>
>> Wrapping a single printk that's unrelated to debugging in an #ifdef
>> CONFIG_* or a sysctl strikes me as abuse of those configuration
>> facilities.  Where would we draw the line for other patches wanting to
>> do similar things?
>>
>> I realized that even checking the hard limit it insufficient, because
>> that can be lowered (but not raised) by unprivileged processes.  If we
>> can't do this unconditionally (and we can't, because the log pollution
>> would be intolerable for many people) then we shouldn't do it at all.
>>
>> Anand -- I appreciate the effort, but I think you should reconsider
>> precisely what problem you're trying to solve here.  This approach can't
>> tell the difference between legitimate self-regulation of resource
>> utilization and a real attack.  Worse, in the event of a real attack, it
>> could be used to make it more difficult for the administrator to notice
>> something much more serious than a forkbomb.
>>
>> I suspect that userspace might be a better place to solve this problem.
>>  You could run your monitoring app with elevated or even realtime
>> priority to ensure it will still function, and you have much more
>> freedom in making the reporting configurable.  You can also look at much
>> more data than we could ever allow in fork.c, and possibly detect
>> attacks that this patch would miss if a clever attacker stayed just
>> below the limit.
>>
>>        -- Chris
>>
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-- 
Simon Arlott
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