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    SubjectRe: Minor PKRU bug?
    On Thu, Jul 21, 2016 at 2:35 PM, Dave Hansen
    <dave.hansen@linux.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@zytor.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

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