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Date:   Wed, 25 Mar 2020 13:22:07 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Elena Reshetova <elena.reshetova@intel.com>, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Jann Horn <jannh@google.com>,
        "Perla, Enrico" <enrico.perla@intel.com>,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 5/5] arm64: entry: Enable random_kstack_offset support
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On Wed, Mar 25, 2020 at 01:21:27PM +0000, Mark Rutland wrote:
> On Tue, Mar 24, 2020 at 01:32:31PM -0700, Kees Cook wrote:
> > Allow for a randomized stack offset on a per-syscall basis, with roughly
> > 5 bits of entropy.
> > 
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> 
> Just to check, do you have an idea of the impact on arm64? Patch 3 had
> figures for x86 where it reads the TSC, and it's unclear to me how
> get_random_int() compares to that.

I didn't do a measurement on arm64 since I don't have a good bare-metal
test environment. I know Andy Lutomirki has plans for making
get_random_get() as fast as possible, so that's why I used it here. I
couldn't figure out if there was a comparable instruction like rdtsc in
aarch64 (it seems there's a cycle counter, but I found nothing in the
kernel that seemed to actually use it)?

> Otherwise, this looks sound to me; I'd jsut like to know whether the
> overhead is in the same ballpark.

Thanks!

-Kees

-- 
Kees Cook