linux-kernel - Re: [Xen-devel] [PATCH] xen: grant-table: Check truncation when giving access to a frame

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Date:	Mon, 13 Jun 2016 13:42:09 +0100
From:	Julien Grall <julien.grall@....com>
To:	Paul Durrant <Paul.Durrant@...rix.com>,
	"boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"jgross@...e.com" <jgross@...e.com>,
	"sstabellini@...nel.org" <sstabellini@...nel.org>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
Cc:	Andrew Cooper <Andrew.Cooper3@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"JBeulich@...e.com" <JBeulich@...e.com>,
	"steve.capper@....com" <steve.capper@....com>
Subject: Re: [Xen-devel] [PATCH] xen: grant-table: Check truncation when
 giving access to a frame



On 13/06/16 13:41, Julien Grall wrote:
> Hello Paul,
>
> On 13/06/16 13:12, Paul Durrant wrote:
>>> -----Original Message-----
>>> From: Xen-devel [mailto:xen-devel-bounces@...ts.xen.org] On Behalf Of
>>> Julien Grall
>>> Sent: 13 June 2016 11:51
>>> To: boris.ostrovsky@...cle.com; David Vrabel; jgross@...e.com;
>>> sstabellini@...nel.org; konrad.wilk@...cle.com
>>> Cc: steve.capper@....com; Andrew Cooper; linux-kernel@...r.kernel.org;
>>> xen-devel@...ts.xen.org; Julien Grall; JBeulich@...e.com
>>> Subject: [Xen-devel] [PATCH] xen: grant-table: Check truncation when
>>> giving
>>> access to a frame
>>>
>>> The version 1 of the grant-table protocol only supports frame encoded on
>>> 32-bit.
>>>
>>> When the platform is supporting 48-bit physical address, the frame will
>>> be encoded on 36-bit which will lead a truncation and give access to
>>> the wrong frame.
>>>
>>> On ARM Xen will always allow the guest to use all the physical address,
>>> although today the RAM is always located under 40-bits (see
>>> xen/include/public/arch-arm.h).
>>>
>>> Add a truncation check in gnttab_update_entry_v1 to prevent the guest to
>>> give access to the wrong frame.
>>>
>>> Signed-off-by: Julien Grall <julien.grall@....com>
>>>
>>> ---
>>>      This is limiting us to a 44-bit address space whilst ARM can
>>> support
>>>      up to 48-bit today. This number of bit will increase to 52-bit in
>>>      upcoming processors [1].
>>>
>>>      It might be good to start thinking to extend the version 1 of the
>>>      protocol to use 64-bit frame number.
>>
>> ...or simply use version 2 of the protocol.
>
> On another mail [1], you said that "[v2] didn't scale it became
> bottle-necked on dom0's grant table size,...".
>
> So it looks like to me that version 2 is the wrong way to go.
> The performance should stay the same whether the platform support
> 40-bit, 44-bit, 48-bit, 52-bit address space.

I forgot the link.

[1] 
http://lists.xenproject.org/archives/html/xen-devel/2016-06/msg01606.html

-- 
Julien Grall