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Message-ID: <202103291141.EC2A77731@keescook>
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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