linux-kernel - Re: [PATCH] kmmio/mmiotrace: fix double free of kmmio_fault

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Date:	Mon, 7 Jun 2010 15:33:21 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Marcin Slusarz <marcin.slusarz@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Pekka Paalanen <pq@....fi>,
	Stuart Bennett <stuart@...edesktop.org>,
	Christoph Bumiller <e0425955@...dent.tuwien.ac.at>,
	Shinpei KATO <shinpei@...is.s.u-tokyo.ac.jp>,
	nouveau@...ts.freedesktop.org, x86@...nel.org
Subject: Re: [PATCH] kmmio/mmiotrace: fix double free of kmmio_fault_pages


* Marcin Slusarz <marcin.slusarz@...il.com> wrote:

> On Sat, Jun 05, 2010 at 06:49:42PM +0200, Marcin Slusarz wrote:
> > After every iounmap mmiotrace has to free kmmio_fault_pages, but it
> > can't do it directly, so it defers freeing by RCU.
> > 
> > It usually works, but when mmiotraced code calls ioremap-iounmap
> > multiple times without sleeping between (so RCU won't kick in and
> > start freeing) it can be given the same virtual address, so at
> > every iounmap mmiotrace will schedule the same pages for release.
> > Obviously it will explode on second free.
> > 
> > Fix it by marking kmmio_fault_pages which are scheduled for release
> > and not adding them second time.
> > 
> 
> Attached patch for mmiotrace testing module allows to reliably reproduce
> the bug. It can be folded into the main patch.
> 
> ---
> diff --git a/arch/x86/mm/testmmiotrace.c b/arch/x86/mm/testmmiotrace.c
> index 8565d94..5f0937b 100644
> --- a/arch/x86/mm/testmmiotrace.c
> +++ b/arch/x86/mm/testmmiotrace.c
> @@ -90,6 +90,19 @@ static void do_test(unsigned long size)
>  	iounmap(p);
>  }
>  
> +static void do_test2(void)

Please add a comment about what the test function achieves.

> +{
> +	void __iomem *p;
> +	int i;
> +	
> +	for (i = 0; i < 10; ++i) {
> +		p = ioremap_nocache(mmio_address, 4096);

s/4096/PAGE_SIZE

> +		if (p)
> +			iounmap(p);
> +	}
> +	synchronize_rcu(); /* will freeing work? */
> +}
> +
>  static int __init init(void)
>  {
>  	unsigned long size = (read_far) ? (8 << 20) : (16 << 10);
> @@ -104,6 +117,7 @@ static int __init init(void)
>  		   "and writing 16 kB of rubbish in there.\n",
>  		   size >> 10, mmio_address);
>  	do_test(size);
> +	do_test2();

Please name the new function in a bit more meaningful way (such as 
do_test_remap()).

Looks good, please send the full (folded back) patch anew, with Pekka's Ack in 
place as well.

Thanks,

	Ingo
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