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

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Message-ID: <202103312329.394CCA13CF@keescook>
Date:   Wed, 31 Mar 2021 23:31:39 -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 Thu, Apr 01, 2021 at 12:38:31AM +0200, Thomas Gleixner wrote:
> On Wed, Mar 31 2021 at 14:54, Kees Cook wrote:
> > 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.
> 
> Hmm, looking at V9 the "ptr" thing got lost ....

I put the comment inline in the macro directly above the asm().

> > 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...)
> 
> Either way is fine.

Since the arm64 folks have been a bit busy, can you just put this in
-tip and leave off the arm64 patch for now?

Thanks!

-- 
Kees Cook