linux-kernel - Re: Minor PKRU bug?

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Date:	Thu, 21 Jul 2016 14:48:51 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Dave Hansen <dave.hansen@...ux.intel.com>
CC:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Minor PKRU bug?

On July 21, 2016 2:45:49 PM PDT, Andy Lutomirski <luto@...capital.net> wrote:
>On Thu, Jul 21, 2016 at 2:35 PM, Dave Hansen
><dave.hansen@...ux.intel.com> wrote:
>> On 07/12/2016 03:59 PM, Andy Lutomirski wrote:
>>> On Tue, Jul 12, 2016 at 3:55 PM, H. Peter Anvin <hpa@...or.com>
>wrote:
>>>> On 07/12/16 08:32, Dave Hansen wrote:
>>>>> On 07/09/2016 02:27 PM, Andy Lutomirski wrote:
>>>>>> is_prefetch in arch/x86/mm/fault.c can be called on a user
>address
>>>>>> that's not readable due to PKRU.  This could break it.  You might
>need
>>>>>> to add a get_user_exec or similar.
>>>>>
>>>>> Thanks for the heads-up.  I think I'll just need a version that
>does
>>>>> something along the lines of stac/clac, but with PKRU.
>>>>>
>>>>> I think I can do it with an "_exec" variant of
>probe_kernel_address(),
>>>>> but it's a bit messy.
>>>>>
>>>> Can this particular codepath even be executed on a PKRU-equipped
>>>> machine?  I thought it was a bug fix for a specific AMD CPU line.
>>>
>>> It can certainly be executed -- do_sigbus will execute it every
>time.
>>> But I guess it doesn't matter if it fails on a PKRU machine, because
>a
>>> failure will just report the signal, and the erratum case can't
>happen
>>> in the first place.
>>
>> Hi Andy,
>>
>> I look at it this way:
>>
>> Systems without prefetch errata always see is_prefetch() return
>false.
>> If is_prefetch() faults when trying to fetch an instruction it
>returns
>> false.  Protection keys will make it do this.
>>
>> Essentially, any pkeys-execute-only code can not have prefetch errata
>> detected inside it.  Any future processor with such an erratum will
>need
>> a different workaround.
>>
>> What do folks think?  Is it worth shoring this up in case of a future
>> erratum?
>>
>> The patch to fix it isn't too invasive (attached).
>
>I like it, except that reading just a single byte is a bit silly.
>OTOH, that's what the current code needs and I see no fundamental
>reason to change it until there's a real user.
>
>--Andy

The thing is that we can't actually test this, since there is no machine on which this code path will ever execute.  That concerns me a bit.
-- 
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