Re: [RFC PATCH 06/11] x86: make sure _etext includes function sections

Kees Cook
2020-02-10T01:43:46Z

On Fri, Feb 07, 2020 at 10:24:23AM +0100, Peter Zijlstra wrote:
> On Thu, Feb 06, 2020 at 12:02:36PM -0800, Andy Lutomirski wrote:
> > Also, in the shiny new era of
> > Intel-CPUs-can’t-handle-Jcc-spanning-a-cacheline, function alignment
> > may actually matter.
>
> *groan*, indeed. I just went and looked that up. I missed this one in
> all the other fuss :/
>
> So per:
>
> https://www.intel.com/content/dam/support/us/en/documents/processors/mitigations-jump-conditional-code-erratum.pdf
>
> the toolchain mitigations only work if the offset in the ifetch window
> (32 bytes) is preserved. Which seems to suggest we ought to align all
> functions to 32byte before randomizing it, otherwise we're almost
> guaranteed to change this offset by the act of randomizing.

Wheee! This sounds like in needs to be fixed generally, yes? (And I see
"FUNCTION_ALIGN" macro is currently 16 bytes...

--
Kees Cook