Re: [PATCH] x86/asm/entry/64: better check for canonical address

[Andy Lutomirski]

On Thu, Apr 23, 2015 at 8:49 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Thu, Apr 23, 2015 at 08:41:15AM -0700, Andy Lutomirski wrote:
>> I was rather vague there.  Let me try again:
>>
>> If anyone in the AMD camp really cared, we could add a new bug flag
>> X86_BUG_SYSRET_NEEDS_CANONICAL_RCX and set it on Intel chips only, so
>> we could use alternatives to patch out the check when running on
>> sensible AMD hardware.  This would speed the slow path up by a couple
>> of cycles on AMD chips.
>>
>> Does that make more sense?  We could call it
>> X86_BUG_SYSRET_NEEDS_CANONICAL_RIP if that makes more sense.
>
> Actually "...NEEDS_CANONICAL_RCX" makes more sense as this is what we're
> going to patch out eventually, if it makes sense - the RIP canonicalness
> test is being done as part of SYSRET, just RCX is not being tested.
>
> Tell you what - how about I perf stat this first by commenting out that
> couple of instructions on AMD to see whether it brings anything.
>
> Got an idea for a workload other than a kernel build? :-)
>
> Although a kernel build should do a lot of syscalls too...

Kernel build should be fine.  Or "timing_test_64 10 sys_enosys 1" or
"perf_self_monitor" (warning: WIP).  Make sure you either have context
tracking forced on or something else (audit?) that forces the slow
path, though, or you won't see it at all.

https://git.kernel.org/cgit/linux/kernel/git/luto/misc-tests.git/

--Andy

>
> Thanks.
>
> --
> Regards/Gruss,
>     Boris.
>
> ECO tip #101: Trim your mails when you reply.
> --



-- 
Andy Lutomirski
AMA Capital Management, LLC
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