linux-kernel - Re: x86_64 system lockup from userspace using setitimer()

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Date:	Tue, 13 Mar 2007 21:33:05 +0100
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Chuck Ebbert <cebbert@...hat.com>
Cc:	Johannes Bauer <JohannesBauer@....de>,
	linux-kernel@...r.kernel.org, schwab@...e.de
Subject: Re: x86_64 system lockup from userspace using setitimer()

On Tue, 2007-03-13 at 16:02 -0400, Chuck Ebbert wrote:
> >     struct itimerval tim = {
> >         .it_interval = {
> >             .tv_sec = 140735669863712,
> >             .tv_usec = 4199521
> >         },
> Could this be fixed by:
> 
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=8bfd9a7a229b5f3d3eda5d7d45c2eebec5b4ba16
> 
> [PATCH] hrtimers: prevent possible itimer DoS

No. The possible DoS is only when high res timers are enabled, which is
not the case in 2.6.20.

Looking at the values 

140735669863712 = 0x7FFF 939C 0520

We convert second to nanoseconds:

140735669863712 * 1e9 =  0x1DCD 4BC3 6B82 914B 4000

The seconds value is limited to LONG_MAX, but on a 64 bit machine, the
140735669863712 is inside LONG_MAX and we have an multiplication
overflow.

I'm not sure, how this results in a DoS, but I will look into this
tomorrow morning, when I'm more awake.

	tglx


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