linux-kernel - Re: [local DoS] Re: Linux 2.6.24-rc4

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Date:	Tue, 4 Dec 2007 17:00:05 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Luiz Fernando N. Capitulino" <lcapitulino@...driva.com.br>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	herton@...driva.com.br, dvgevers@...all.nl
Subject: Re: [local DoS] Re: Linux 2.6.24-rc4


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> On Tue, 4 Dec 2007, Luiz Fernando N. Capitulino wrote:
> >
> > 	sched_rr_get_interval(1, NULL);
> 
> Looks like we have a zero "cfs_rq->load.weight".
> 
> Ingo? Both sched_slice() and __sched_slice() do a divide by the 
> runqueue weight, and at least dequeue_task_fair() explicitly checks 
> for that being zero, so clearly zero is a possible value. Hmm?

yeah, i can reproduce this crash too.

The problem is on SMP: if sched_rr_get_interval() gets a task from an 
otherwise idle runqueue, then rq->load.weight is 0. Normally 
sched_slice() is only used on a busy runqueue. So the correct fixup site 
is not in sched_slice() but in sys_sched_rr_get_interval() - i'm working 
on the right fix, i hope to be able to send a pull request in a few 
minutes.

	Ingo
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