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 AB7CAD29C25 for ; Mon, 19 Jan 2026 13:00:28 +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:From:References:Cc:To: Subject:MIME-Version:Date:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=FLKqyoK86x64Nclrwy//lkt4GaYAfkU8ZJ2IlSo9qYQ=; b=MoY8DU42DIfuLa 1a8UpsCqXdi3sbUuhLptmTmAEXd01K1WAIr1ibRrKyyFFkbfXfkRzf13nx564W5HEb8ovoSOORtDc mXZW74f24Sn3DSoo8peifHmifdhFwxddRcYbdi8j5kiGLMI/souOb0Qe6p0YVoJWSOQjOY2z1dtgn h7RnEzET2Krb5gLBv2MJqSuGPbGj4YEgrKJ4A8R0JS6DN3ruV9vxI9iT/b0aZIar0CLfmR4nf3MdU 1QmmTsku25Ko8aJW8sog1P6GxtenjIOeFPGjoVCd1OsBQKo9I2t0thaxyLWyyAzlNrzd8zSU0CTsp PfyonRjSfjTST6zTvfnw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vhorW-000000027CZ-3gQY; Mon, 19 Jan 2026 13:00:06 +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 1vhorU-000000027BD-2TXK; Mon, 19 Jan 2026 13:00:06 +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 AE1A4497; Mon, 19 Jan 2026 04:59:56 -0800 (PST) Received: from [10.57.93.204] (unknown [10.57.93.204]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id F3DC63F740; Mon, 19 Jan 2026 04:59:58 -0800 (PST) Message-ID: Date: Mon, 19 Jan 2026 12:59:57 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 0/3] Fix bugs and performance of kstack offset randomisation Content-Language: en-GB To: Mark Rutland , 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 References: <20260102131156.3265118-1-ryan.roberts@arm.com> From: Ryan Roberts In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260119_050004_716466_D9F0F1F8 X-CRM114-Status: GOOD ( 28.16 ) 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 19/01/2026 10:52, Mark Rutland wrote: > 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. Thanks for the review, Mark! v4 incomming... > > 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