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Date:   Mon, 29 Mar 2021 11:43:23 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Elena Reshetova <elena.reshetova@...el.com>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Potapenko <glider@...gle.com>,
        Alexander Popov <alex.popov@...ux.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jann Horn <jannh@...gle.com>, Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Randy Dunlap <rdunlap@...radead.org>,
        kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 4/6] x86/entry: Enable random_kstack_offset support

On Sun, Mar 28, 2021 at 04:18:56PM +0200, Thomas Gleixner wrote:
> On Fri, Mar 19 2021 at 14:28, Kees Cook wrote:
> > +
> > +	/*
> > +	 * x86_64 stack alignment means 3 bits are ignored, so keep
> > +	 * the top 5 bits. x86_32 needs only 2 bits of alignment, so
> > +	 * the top 6 bits will be used.
> > +	 */
> > +	choose_random_kstack_offset(rdtsc() & 0xFF);
> 
> Comment mumbles about 5/6 bits and the TSC value is masked with 0xFF and
> then the applied offset is itself limited with 0x3FF.
> 
> Too many moving parts for someone who does not have the details of all
> this memorized.

Each piece is intentional -- I will improve the comments to explain
each level of masking happening (implicit compiler stack alignment mask,
explicit per-arch mask, and the VLA upper-bound protection mask).

-- 
Kees Cook

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