linux-kernel - Re: [PATCH] iommu/vt-d: Fix an off-by-one bug in __domain

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Date:	Tue, 2 Dec 2014 13:04:30 +0100
From:	Joerg Roedel <joro@...tes.org>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Jiang Liu <jiang.liu@...ux.intel.com>,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/vt-d: Fix an off-by-one bug in __domain_mapping()

On Tue, Dec 02, 2014 at 10:34:35AM +0000, David Woodhouse wrote:
> __domain_mapping() is an amalgamation of the old domain_pfn_mapping()
> and domain_sg_mapping() functions. When I did that, in commit 9051aa026,
> the 'sg_res' variable was used *only* for tracking how many pages were
> left in the current scatterlist element, before we had to get the next
> one from the sglist.
> 
> For reasons which are lost now, in the case of a simple pfn range I was
> setting 'sg_res = nr_pages + 1' to ensure that we *never* got down to
> sg_res=0 and tried to look for more from the (non-existent, in this
> case) sglist.
> 
> Later in commit 6dd9a7c73 we added large page support, using sg_res in a
> way which actually required it to be accurate. And now we have an
> off-by-one because we'll actually *try* to use a 2GiB large page for a
> mapping of size 0x1ff000, because of that '+1'.
> 
> The BUG_ON is entirely correct here, and correctly highlighted the
> problem.
> 
> However, the +1 is no longer necessary, because the check that needed it
> was also modified to read 'if (sg_res && nr_pages)', which is perfectly
> sufficient and arguably how it should have been done in the first place.
> 
> I had an almost identical patch last week for internal testing, because
> I stupidly hadn't noticed that Jiang had beaten me to it.
> 
> Acked-By: David Woodhouse <David.Woodhouse@...el.com>
> 
> > > 	This issue was introduced in v2.6.31, but intel-iommu.c has
> > > been moved into drivers/iommu in v3.1. So what's the preferred way
> > > to deal with stable kernels between v2.6.31 and v3.1?
> > 
> > Just remove the kernel version marker from the stable tag. The stable
> > kernel maintainers for kernels >3.1 will ask you to backport the patch
> > or just backport it by themselfes.
> 
> I think this is only an issue since commit 6dd9a7c737 added super page
> support in 3.0, isn't it? Before that, the +1 was *needed*.

Okay guys, thanks for the explanations. I applied the patch to the
x86/vt-d branch and changed the stable tag to >= 3.0.


	Joerg

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