linux-kernel - Re: Minor PKRU bug?

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Message-ID: <CALCETrViDkmaZCx2rEakqiZmar9cjLjFTmBMDKyapAnxH0euqQ@mail.gmail.com>
Date:	Thu, 21 Jul 2016 14:45:49 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Minor PKRU bug?

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

-- 
Andy Lutomirski
AMA Capital Management, LLC