From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 13EFC3587A5; Mon, 19 Jan 2026 10:53:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768819989; cv=none; b=S/HCG1OU3dHsTsr9XBgivlr4eRtXYsZq6K0JpXiTcQMb0FpbJVWLv3d4L0h49Zz3YlK9CaDssdnwMZnx/CQ7cT2CXOYPa7CF9AMQo77dWepj61yhp9w3upJnv6k+tz+Y6fUQnwDYkSZ4r5tePWT5mWOzvlwIJ64op3MpTssIY4U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768819989; c=relaxed/simple; bh=eZcYLrOrZr4lRiCJ3TdUZo06dWdX2VZXenzn28FE97k=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=o31lwCIiha5wQu90dWHZgmidfEPuMi4YhehoDQuOXfDjKUXFZMgMXkMIaOE+LymygMNVAzaJHK8Ji/ftE+gwocbdycAtUUyHALHHnNjuzSqrelXJS9Lu2RlZmBokNGYeTzoVyerWuEReI9+r5Bt+Zz5xmcSkDJziRSaszgStkxo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id CD4E31517; Mon, 19 Jan 2026 02:53:00 -0800 (PST) Received: from J2N7QTR9R3 (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7ABD13F740; Mon, 19 Jan 2026 02:53:02 -0800 (PST) Date: Mon, 19 Jan 2026 10:52:59 +0000 From: Mark Rutland To: Ryan Roberts , Kees Cook Cc: Catalin Marinas , Will Deacon , Huacai Chen , Madhavan Srinivasan , Michael Ellerman , Paul Walmsley , Palmer Dabbelt , Albert Ou , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "Gustavo A. R. Silva" , Arnd Bergmann , "Jason A. Donenfeld" , Ard Biesheuvel , Jeremy Linton , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-hardening@vger.kernel.org Subject: Re: [PATCH v3 0/3] Fix bugs and performance of kstack offset randomisation Message-ID: References: <20260102131156.3265118-1-ryan.roberts@arm.com> Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260102131156.3265118-1-ryan.roberts@arm.com> On Fri, Jan 02, 2026 at 01:11:51PM +0000, Ryan Roberts wrote: > Hi All, Hi Ryan, > As I reported at [1], kstack offset randomisation suffers from a couple of bugs > and, on arm64 at least, the performance is poor. This series attempts to fix > both; patch 1 provides back-portable fixes for the functional bugs. Patches 2-3 > propose a performance improvement approach. > > I've looked at a few different options but ultimately decided that Jeremy's > original prng approach is the fastest. I made the argument that this approach is > secure "enough" in the RFC [2] and the responses indicated agreement. FWIW, the series all looks good to me. I understand you're likely to spin a v4 with a couple of minor tweaks (fixing typos and adding an out-of-line wrapper for a prandom function), but I don't think there's anything material that needs to change. I've given my Ack on all three patches. I've given the series a quick boot test (atop v6.19-rc4) with a bunch of debug options enabled, and all looks well. Kees, do you have any comments? It would be nice if we could queue this up soon. Mark. > More details in the commit logs. > > > Performance > =========== > > Mean and tail performance of 3 "small" syscalls was measured. syscall was made > 10 million times and each individually measured and binned. These results have > low noise so I'm confident that they are trustworthy. > > The baseline is v6.18-rc5 with stack randomization turned *off*. So I'm showing > performance cost of turning it on without any changes to the implementation, > then the reduced performance cost of turning it on with my changes applied. > > **NOTE**: The below results were generated using the RFC patches but there is no > meaningful change, so the numbers are still valid. > > arm64 (AWS Graviton3): > +-----------------+--------------+-------------+---------------+ > | Benchmark | Result Class | v6.18-rc5 | per-task-prng | > | | | rndstack-on | | > | | | | | > +=================+==============+=============+===============+ > | syscall/getpid | mean (ns) | (R) 15.62% | (R) 3.43% | > | | p99 (ns) | (R) 155.01% | (R) 3.20% | > | | p99.9 (ns) | (R) 156.71% | (R) 2.93% | > +-----------------+--------------+-------------+---------------+ > | syscall/getppid | mean (ns) | (R) 14.09% | (R) 2.12% | > | | p99 (ns) | (R) 152.81% | 1.55% | > | | p99.9 (ns) | (R) 153.67% | 1.77% | > +-----------------+--------------+-------------+---------------+ > | syscall/invalid | mean (ns) | (R) 13.89% | (R) 3.32% | > | | p99 (ns) | (R) 165.82% | (R) 3.51% | > | | p99.9 (ns) | (R) 168.83% | (R) 3.77% | > +-----------------+--------------+-------------+---------------+ > > Because arm64 was previously using get_random_u16(), it was expensive when it > didn't have any buffered bits and had to call into the crng. That's what caused > the enormous tail latency. > > > x86 (AWS Sapphire Rapids): > +-----------------+--------------+-------------+---------------+ > | Benchmark | Result Class | v6.18-rc5 | per-task-prng | > | | | rndstack-on | | > | | | | | > +=================+==============+=============+===============+ > | syscall/getpid | mean (ns) | (R) 13.32% | (R) 4.60% | > | | p99 (ns) | (R) 13.38% | (R) 18.08% | > | | p99.9 (ns) | 16.26% | (R) 19.38% | > +-----------------+--------------+-------------+---------------+ > | syscall/getppid | mean (ns) | (R) 11.96% | (R) 5.26% | > | | p99 (ns) | (R) 11.83% | (R) 8.35% | > | | p99.9 (ns) | (R) 11.42% | (R) 22.37% | > +-----------------+--------------+-------------+---------------+ > | syscall/invalid | mean (ns) | (R) 10.58% | (R) 2.91% | > | | p99 (ns) | (R) 10.51% | (R) 4.36% | > | | p99.9 (ns) | (R) 10.35% | (R) 21.97% | > +-----------------+--------------+-------------+---------------+ > > I was surprised to see that the baseline cost on x86 is 10-12% since it is just > using rdtsc. But as I say, I believe the results are accurate. > > > Changes since v2 (RFC) [3] > ========================== > > - Moved late_initcall() to initialize kstack_rnd_state out of > randomize_kstack.h and into main.c. (issue noticed by kernel test robot) > > Changes since v1 (RFC) [2] > ========================== > > - Introduced patch 2 to make prandom_u32_state() __always_inline (needed since > its called from noinstr code) > - In patch 3, prng is now per-cpu instead of per-task (per Ard) > > > [1] https://lore.kernel.org/all/dd8c37bc-795f-4c7a-9086-69e584d8ab24@arm.com/ > [2] https://lore.kernel.org/all/20251127105958.2427758-1-ryan.roberts@arm.com/ > [3] https://lore.kernel.org/all/20251215163520.1144179-1-ryan.roberts@arm.com/ > > Thanks, > Ryan > > > Ryan Roberts (3): > randomize_kstack: Maintain kstack_offset per task > prandom: Convert prandom_u32_state() to __always_inline > randomize_kstack: Unify random source across arches > > arch/Kconfig | 5 ++- > arch/arm64/kernel/syscall.c | 11 ------ > arch/loongarch/kernel/syscall.c | 11 ------ > arch/powerpc/kernel/syscall.c | 12 ------- > arch/riscv/kernel/traps.c | 12 ------- > arch/s390/include/asm/entry-common.h | 8 ----- > arch/x86/include/asm/entry-common.h | 12 ------- > include/linux/prandom.h | 19 +++++++++- > include/linux/randomize_kstack.h | 54 +++++++++++----------------- > init/main.c | 9 ++++- > kernel/fork.c | 1 + > lib/random32.c | 19 ---------- > 12 files changed, 49 insertions(+), 124 deletions(-) > > -- > 2.43.0 >