linux-kernel - Re: [RFC PATCH 06/11] x86: make sure

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Message-ID: <202002091742.7B1E6BF19@keescook>
Date:   Sun, 9 Feb 2020 17:43:40 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        arjan@...ux.intel.com, rick.p.edgecombe@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [RFC PATCH 06/11] x86: make sure _etext includes function
 sections

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