linux-hardening - Re: [PATCH v8 3/6] stack: Optionally randomize kernel stack offset each syscall

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Message-ID: <202103311453.A840B7FC5@keescook>
Date:   Wed, 31 Mar 2021 14:54:57 -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 v8 3/6] stack: Optionally randomize kernel stack offset
 each syscall

On Wed, Mar 31, 2021 at 09:53:26AM +0200, Thomas Gleixner wrote:
> On Tue, Mar 30 2021 at 13:57, Kees Cook wrote:
> > +/*
> > + * Do not use this anywhere else in the kernel. This is used here because
> > + * it provides an arch-agnostic way to grow the stack with correct
> > + * alignment. Also, since this use is being explicitly masked to a max of
> > + * 10 bits, stack-clash style attacks are unlikely. For more details see
> > + * "VLAs" in Documentation/process/deprecated.rst
> > + * The asm statement is designed to convince the compiler to keep the
> > + * allocation around even after "ptr" goes out of scope.
> 
> Nit. That explanation of "ptr" might be better placed right at the
> add_random...() macro.

Ah, yes! Fixed in v9.

> Other than that.
> 
> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>

Thank you for the reviews!

Do you want to take this via -tip (and leave off the arm64 patch until
it is acked), or would you rather it go via arm64? (I've sent v9 now...)

-- 
Kees Cook