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

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Message-ID: <20200207092423.GC14914@hirez.programming.kicks-ass.net>
Date:   Fri, 7 Feb 2020 10:24:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>, 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 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.