linux-kernel - Re: Minor PKRU bug?

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Date:	Thu, 21 Jul 2016 14:35:41 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Minor PKRU bug?

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).

View attachment "pkeys-900-fix-prefetch.patch" of type "text/x-patch" (5021 bytes)