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From: Mark Rutland <mark.rutland@arm.com>
To: Kees Cook <keescook@chromium.org>
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
Date: Mon, 30 Mar 2020 12:26:19 +0100	[thread overview]
Message-ID: <20200330112619.GE1309@C02TD0UTHF1T.local> (raw)
In-Reply-To: <202003260926.83BC44B@keescook>

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@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.
> > 
> > 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@arm.com>

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

Thanks,
Mark.

  reply	other threads:[~2020-03-30 11:26 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-24 20:32 [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall Kees Cook
2020-03-24 20:32 ` [PATCH v2 1/5] jump_label: Provide CONFIG-driven build state defaults Kees Cook
2020-03-24 22:06   ` Peter Zijlstra
2020-03-24 20:32 ` [PATCH v2 2/5] init_on_alloc: Unpessimize default-on builds Kees Cook
2020-03-26 15:48   ` Alexander Potapenko
2020-03-24 20:32 ` [PATCH v2 3/5] stack: Optionally randomize kernel stack offset each syscall Kees Cook
2020-03-30 11:25   ` Mark Rutland
2020-03-30 18:18     ` Kees Cook
2020-03-30 18:27     ` Kees Cook
2020-03-24 20:32 ` [PATCH v2 4/5] x86/entry: Enable random_kstack_offset support Kees Cook
2020-03-28 22:26   ` Kees Cook
2020-03-24 20:32 ` [PATCH v2 5/5] arm64: entry: " Kees Cook
2020-03-25 13:21   ` Mark Rutland
2020-03-25 20:22     ` Kees Cook
2020-03-26 11:15       ` Mark Rutland
2020-03-26 16:31         ` Kees Cook
2020-03-30 11:26           ` Mark Rutland [this message]
2020-04-20 20:54   ` Will Deacon
2020-04-20 22:34     ` Kees Cook
2020-04-21  7:02       ` Will Deacon
2020-03-24 21:28 ` [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall Jann Horn
2020-03-24 23:07   ` Kees Cook
2020-03-25 12:15     ` Reshetova, Elena
2020-03-25 20:27       ` Kees Cook
2020-03-25 23:20         ` Jann Horn
2020-03-26 17:18           ` Kees Cook

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