linux-kernel - Re: [PATCH] x86: check for valid irq_cfg pointer in smp_irq_move_cleanup

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Message-ID: <20120524153607.GA25715@sgi.com>
Date:	Thu, 24 May 2012 10:36:07 -0500
From:	Dimitri Sivanich <sivanich@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Yinghai Lu <yinghai@...nel.org>,
	Naga Chumbalkar <nagananda.chumbalkar@...com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: check for valid irq_cfg pointer in
 smp_irq_move_cleanup_interrupt

On Thu, May 24, 2012 at 04:53:06PM +0200, Thomas Gleixner wrote:
> On Wed, 23 May 2012, Suresh Siddha wrote:
> > On Wed, 2012-05-23 at 15:02 -0500, Dimitri Sivanich wrote:
> > > OK.  Hopefully this covers it.
> > 
> > Sorry No. Now you will understand why Thomas wanted detailed changelog.
> > I found one more issue with the help of your new modification to the
> > changelog.
> > 
> > > A NULL pointer dereference can occur in smp_irq_move_cleanup_interrupt() if
> > > we haven't yet setup the irq_cfg pointer in the irq_desc.irq_data.chip_data.
> > > 
> > > In create_irq_nr() there is a window where we have set vector_irq in
> > > __assign_irq_vector(), but not yet called irq_set_chip_data() to set the
> > > irq_cfg pointer.
> > > 
> > > Should an IRQ_MOVE_CLEANUP_VECTOR hit the cpu in question during this time,
> > > smp_irq_move_cleanup_interrupt() will attempt to process the aforementioned
> > > irq, but panic when accessing irq_cfg.
> > > 
> > > There is also a window in destroy_irq() where we've cleared the irq_cfg
> > > pointer in free_irq_cfg(), but have not yet called irq_free_desc().  Note
> > > that we have cleared vector_irq in __clear_irq_vector() prior to free_irq_cfg(),
> > > but smp_irq_move_cleanup_interrupt() might've already referenced the irq_desc.
> > 
> > So, what happens if the irq_desc gets freed by the destroy_irq() in the
> > sparse irq case? smp_irq_move_cleanup_interrupt() will refer to freed
> > irq desc memory! Right?
> >
> > May we should really do something like the appended (untested patch)?
> > Can you please review and give this a try? Let me review a bit more to
> > see if this really fixes the issue.
> 
> It's fixing the problem.
> 
> But this move_cleanup stuff could be made less stupid.
> 
> The check for irq_desc is superflous. irq_cfg() calls
> irq_get_chip_data() which will return NULL if the irq descriptor is
> not there.
> 
> To avoid the lookup business completely we should really store
> irq_desc instead of the irq number in the per cpu vector array, that
> would also get rid of the lookup in the irq delivery path.

So if the irq_desc gets deallocated you would clear all corresponding
per cpu vector_irq references before deallocation, protecting accesses
by smp_irq_move_cleanup_interrupt?

> 
> Now that still needs to iterate over all vectors, but this could be
> optimized in a second step.
> 
> In complete_move() we send the IPI to all cpus in the old mask. We
> really should set the corresponding vector bit in a per cpu bitfield
> on those cpus in the mask. The cleanup can rely on the bits and avoid
> looking at 200+ vectors to find a single one. 

This part does sound more efficient at first glance.

> 
> Thoughts?
> 
> 	tglx
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