linux-kernel - Re: [PATCH v2 5/5] arm64: entry: Enable random_kstack

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Date:   Mon, 30 Mar 2020 12:26:19 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Alexander Potapenko <glider@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jann Horn <jannh@...gle.com>,
        "Perla, Enrico" <enrico.perla@...el.com>,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/5] arm64: entry: Enable random_kstack_offset support

On Thu, Mar 26, 2020 at 09:31:32AM -0700, Kees Cook wrote:
> On Thu, Mar 26, 2020 at 11:15:21AM +0000, Mark Rutland wrote:
> > On Wed, Mar 25, 2020 at 01:22:07PM -0700, Kees Cook wrote:
> > > 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@...omium.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.
> > 
> > Ok. I suspect I also won't get the chance to test that in the next few
> > days, but if I do I'll try to share the results.
> 
> Okay, thanks! I can try a rough estimate under emulation, but I assume
> that'll be mostly useless. :)
> 
> > My concern here was that, get_random_int() has to grab a spinlock and
> > mess with IRQ masking, so has the potential to block for much longer,
> > but that might not be an issue in practice, and I don't think that
> > should block these patches.
> 
> Gotcha. I was already surprised by how "heavy" the per-cpu access was
> when I looked at the resulting assembly (there looked to be preempt
> stuff, etc). But my hope was that this is configurable so people can
> measure for themselves if they want it, and most people who want this
> feature have a high tolerance for performance trade-offs. ;)
> 
> > > 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)?
> > 
> > AArch64 doesn't have a direct equivalent. The generic counter
> > (CNTxCT_EL0) is the closest thing, but its nominal frequency is
> > typically much lower than the nominal CPU clock frequency (unlike TSC
> > where they're the same). The cycle counter (PMCCNTR_EL0) is part of the
> > PMU, and can't be relied on in the same way (e.g. as perf reprograms it
> > to generate overflow events, and it can stop for things like WFI/WFE).
> 
> Okay, cool; thanks for the details! It's always nice to confirm I didn't
> miss some glaringly obvious solution. ;)
> 
> For a potential v2, should I add your reviewed-by or wait for your
> timing analysis, etc?

I'd rather not give an R-b until I've seen numbers, but please don't
block waiting for that. For the moment, feel free to add:

Acked-by: Mark Rutland <mark.rutland@....com>

... and it's down to Will and Catalin to make the call for arm64.

Thanks,
Mark.