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

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Date:	Mon, 1 Dec 2014 17:27:40 +0100
From:	Joerg Roedel <joro@...tes.org>
To:	Jiang Liu <jiang.liu@...ux.intel.com>
Cc:	David Woodhouse <dwmw2@...radead.org>,
	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 Wed, Nov 26, 2014 at 09:42:10AM +0800, Jiang Liu wrote:
> There's an off-by-one bug in function __domain_mapping(), which may
> trigger the BUG_ON(nr_pages < lvl_pages) when
> 	(nr_pages + 1) & superpage_mask == 0

What is the superpage_mask?

> The issue was introduced by commit 9051aa0268dc "intel-iommu: Combine
> domain_pfn_mapping() and domain_sg_mapping()", which sets sg_res to
> "nr_pages + 1" to avoid some of the 'sg_res==0' code paths.
> 
> It's safe to remove extra "+1" because sg_res is only used to calculate
> page size now.

>From your description and the (hard to read) code in __domain_mapping I
don't really understand the issue yet. Can you please elaborate on this
issue can be triggered?

Is the BUG_ON the only issue and, if yes, can that be fixed by just
changing the BUG_ON condition?

> 	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.


	Joerg

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