Subject: Re: [PATCH v2 5/5] arm64: entry: Enable random_kstack_offset support

From: Kees Cook <[email-masked]>
Date: Wed, 25 Mar 2020 13:22:07 -0700

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 <[email-masked]>
> 
> 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

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