linux-kernel - Re: x86: fix race in create_irq_nr on irq

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Date:	Wed, 3 Feb 2010 09:42:16 -0800
From:	Brandon Philips <bphilips@...e.de>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	YinghaiLu@...e.de, Suresh Siddha <suresh.b.siddha@...el.com>,
	linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: x86: fix race in create_irq_nr on irq_desc

On 02:20 Wed 03 Feb 2010, Yinghai Lu wrote:
> On 02/02/2010 07:31 PM, Brandon Philips wrote:
> > Race in create_irq_nr():
> > 
> > - Thread 1 loops through and calls irq_to_desc_alloc_node with new=0x66.
> > 
> > - Thread 2 has exited the loop with irq=0x66 and calls dynamic_irq_init(0x66)
> >   setting desc->chip_data = NULL
> > 
> > - Thread 1 then dereferences NULL via desc_new->chip_data->vector
> 
> two threads get same irq?

This race happened when two drivers were setting up MSI-X at the same
time via pci_enable_msix(). See this dmesg excerpt:

[   85.170610] ixgbe 0000:02:00.1: irq 97 for MSI/MSI-X
[   85.170611]   alloc irq_desc for 99 on node -1
[   85.170613] igb 0000:08:00.1: irq 98 for MSI/MSI-X
[   85.170614]   alloc kstat_irqs on node -1
[   85.170616] alloc irq_2_iommu on node -1
[   85.170617]   alloc irq_desc for 100 on node -1
[   85.170619]   alloc kstat_irqs on node -1
[   85.170621] alloc irq_2_iommu on node -1
[   85.170625] ixgbe 0000:02:00.1: irq 99 for MSI/MSI-X
[   85.170626]   alloc irq_desc for 101 on node -1
[   85.170628] igb 0000:08:00.1: irq 100 for MSI/MSI-X
[   85.170630]   alloc kstat_irqs on node -1
[   85.170631] alloc irq_2_iommu on node -1
[   85.170635]   alloc irq_desc for 102 on node -1
[   85.170636]   alloc kstat_irqs on node -1
[   85.170639] alloc irq_2_iommu on node -1
[   85.170646] BUG: unable to handle kernel NULL pointer dereference
at 0000000000000088

As you can see igb and ixgbe are both alternating on create_irq_nr()
via pci_enable_msix() in their probe function. So, let me rewrite my
explanation using this example:

ixgbe: While looping through irq_desc_ptrs[] via create_irq_nr() ixgbe
choses irq_desc_ptrs[102] and exits the loop, drops vector_lock and
calls dynamic_irq_init. Then it sets irq_desc_ptrs[102]->chip_data =
NULL via dynamic_irq_init().

igb: Grabs the vector_lock now and starts looping over irq_desc_ptrs[]
via create_irq_nr(). It gets to irq_desc_ptrs[102] and does this:

	cfg_new = irq_desc_ptrs[102]->chip_data;
	if (cfg_new->vector != 0)
		continue;

This hits the NULL deref.

Does that make sense? It is sort of a rare thing to reproduce- took
40+ reboots.

Thanks,

	Brandon
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