linux-kernel - Re: [DISCUSSION] kstack offset randomization: bugs and performance

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Message-ID: <aSRtaVjfNc11EtKg@willie-the-truck>
Date: Mon, 24 Nov 2025 14:36:25 +0000
From: Will Deacon <will@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Kees Cook <kees@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Ard Biesheuvel <ardb@...nel.org>,
	Jeremy Linton <jeremy.linton@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Mark Rutland <mark.rutland@....com>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [DISCUSSION] kstack offset randomization: bugs and performance

On Mon, Nov 17, 2025 at 11:31:22AM +0000, Ryan Roberts wrote:
> On 17/11/2025 11:30, Ryan Roberts wrote:
> > Could this give us a middle ground between strong-crng and
> > weak-timestamp-counter? Perhaps the main issue is that we need to store the
> > secret key for a long period?
> > 
> > 
> > Anyway, I plan to work up a series with the bugfixes and performance
> > improvements. I'll add the siphash approach as an experimental addition and get
> > some more detailed numbers for all the options. But wanted to raise it all here
> > first to get any early feedback.

FWIW, I share Mark's concerns about using a counter for this. Given that
the feature currently appears to be both slow _and_ broken I'd vote for
either removing it or switching over to per-thread offsets as a first
step. We already have a per-task stack canary with
CONFIG_STACKPROTECTOR_PER_TASK so I don't understand the reluctance to
do something similar here.

Speeding up the crypto feels like something that could happen separately.

Will