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

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Date:	Tue, 02 Dec 2014 08:06:09 +0800
From:	Jiang Liu <jiang.liu@...ux.intel.com>
To:	Joerg Roedel <joro@...tes.org>
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 2014/12/2 0:27, Joerg Roedel wrote:
> 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?
Hi Joerg,
	Sorry for the confusion. The really story is:
1) sg_res is set to nr_pages + 1 at the beginning of __domain_mapping()
2) then sg_res is used to choose super page by function
   hardware_largepage_caps(domain, iov_pfn, phys_pfn, sg_res).
The condition to trigger the issue is:
__domain_mapping is called by domain_pfn_mapping() with nr_pages
of 511, so sg_res is 512 and hardware_largepage_caps() will
choose a wrong super page size of 2M, which then trigger
BUG_ON(sg_res < lvl_pages).

So it's not only a BUG_ON() issue, but also causes incorrect super page
selection.
Thanks!
Gerry

> 
>> 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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