Re: Minor PKRU bug?

From: Andy Lutomirski
Date: Thu Jul 21 2016 - 17:46:15 EST

On Thu, Jul 21, 2016 at 2:35 PM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
> On 07/12/2016 03:59 PM, Andy Lutomirski wrote:
>> On Tue, Jul 12, 2016 at 3:55 PM, H. Peter Anvin <hpa@xxxxxxxxx> 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 Lutomirski
AMA Capital Management, LLC