On July 21, 2016 2:45:49 PM PDT, Andy Lutomirski <[email protected]> wrote: >On Thu, Jul 21, 2016 at 2:35 PM, Dave Hansen ><[email protected]> wrote: >> On 07/12/2016 03:59 PM, Andy Lutomirski wrote: >>> On Tue, Jul 12, 2016 at 3:55 PM, H. Peter Anvin <[email protected]> >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. -- Sent from my Android device with K-9 Mail. Please excuse brevity and formatting.
