linux-kernel - Re: [RFC, PATCH] pagemap: do not leak physical addresses to non-privileged userspace

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Date:	Thu, 19 Mar 2015 13:51:02 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Andy Lutomirski <luto@...capital.net>,
	Mark Seaborn <mseaborn@...omium.org>
CC:	Pavel Machek <pavel@....cz>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>
Subject: Re: [RFC, PATCH] pagemap: do not leak physical addresses to non-privileged
 userspace

On 03/17/2015 02:21 AM, Andy Lutomirski wrote:
> On Mon, Mar 16, 2015 at 5:49 PM, Mark Seaborn <mseaborn@...omium.org> wrote:
>> On 16 March 2015 at 14:11, Pavel Machek <pavel@....cz> wrote:
>>
>>> Can we do anything about that? Disabling cache flushes from userland
>>> should make it no longer exploitable.
>>
>> Unfortunately there's no way to disable userland code's use of
>> CLFLUSH, as far as I know.
>>
>> Maybe Intel or AMD could disable CLFLUSH via a microcode update, but
>> they have not said whether that would be possible.
> 
> The Intel people I asked last week weren't confident.  For one thing,
> I fully expect that rowhammer can be exploited using only reads and
> writes with some clever tricks involving cache associativity.  I don't
> think there are any fully-associative caches, although the cache
> replacement algorithm could make the attacks interesting.

I've been thinking the same. But maybe having to evict e.g. 16-way cache would
mean accessing 16x more lines which could reduce the frequency for a single line
below dangerous levels. Worth trying, though :)

BTW, by using clever access patterns and measurement of access latencies one
could also possibly determine which cache lines alias/colide, without needing to
read pagemap. It would just take longer. Hugepages make that simpler as well.

I just hope we are not going to disable lots of stuff including clflush and e.g.
transparent hugepages just because some part of the currently sold hardware is
vulnerable...

Vlastimil

> --Andy
> 
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