From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id EDBE1CCF2DF for ; Mon, 19 Jan 2026 10:53:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=SITCyHgDkGbOfobNSbTRHlv37A2ARVPujVGlfz7HAww=; b=h23VcwpLOG+dkb KQrqGG51wbw2sIyiAHLH4P7jRT4BhbzjWAYoG6xkLZ5CjnjNraHhBG5vXntubWrcWvVRp/Q5+GucV /cI9ivVhoPJHZaPEdD8v5gE+mv7a5tcxBDLBGbzKnA15x2dgBVsaN6VUfQ6lcrJ0hpyUfpioZ6Lu0 Qo6gvOsauLbfkxlAbqHz7NKocD3g1u7kLXLGNb7dBJlI9cRUTA+EV23uKULF9ljrD7tvJV6IhGRQn EPFIxraVP3LfNlHC+VQAgkXMGIOyz+yH52uultoKTdbMurWxxRBfGgTLQu6ZtCvTlRrYsb2z+vWKs /C2GSAMqh87aVDid7ODg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vhmsl-00000001p7Y-0Iml; Mon, 19 Jan 2026 10:53:15 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vhmse-00000001p6O-0vr0; Mon, 19 Jan 2026 10:53:14 +0000 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> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20260102131156.3265118-1-ryan.roberts@arm.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260119_025310_138829_8413E82B X-CRM114-Status: GOOD ( 29.87 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org 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 > _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv