linux-hardening - Re: [PATCH v7 4/6] x86/entry: Enable random_kstack

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Message-ID: <87h7kvcqen.ffs@nanos.tec.linutronix.de>
Date:   Sun, 28 Mar 2021 16:18:56 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Kees Cook <keescook@...omium.org>,
        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 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.

Thanks,

        tglx