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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (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 618B6FD065A for ; Wed, 11 Mar 2026 08:32:47 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [127.0.0.1]) by lists.ozlabs.org (Postfix) with ESMTP id 4fW3sT5Lcyz3f1p; Wed, 11 Mar 2026 19:32:45 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; arc=none smtp.remote-ip=217.140.110.172 ARC-Seal: i=1; a=rsa-sha256; d=lists.ozlabs.org; s=201707; t=1773217965; cv=none; b=OeTo0pofu271z6Yda6V/+OYgCdRXFPmaNgfztJ5A7ouNKmNe+nctK1sliTe8KJuS2/3/J+rg/cfCdmRCAL66KqURvlpJ6oNLbtgHdC5uvkpSSjzL4MXf5tA+h8m6iw8dlGNfQiFb03F9547kswPS579fuP49QG/rLGjdCiSuFOuHtWqTMxt1PN5ZUY3lxoJtgDNcDczdIEBrAQWrUCD/uPLkyjWADrIUdHW7peg5vIt5h+N9kl3CbAhZJSesKFTkLjPVVHfa323PJUIT9NmMXz1ItLW5Xxz5TJZ6bE7H7o1tqMMAThc7kqubbeAJUt8KtFu4sF0W/TFA7i5ZXXsW7A== ARC-Message-Signature: i=1; a=rsa-sha256; d=lists.ozlabs.org; s=201707; t=1773217965; c=relaxed/relaxed; bh=lWh3I1BJesXJ03ee9rt4h78hZaVgU22THcNWmWBoOOA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=S2U0EJE1XMVLD2fpdLNfJBnN8dhUtCQhJt+6ec7ipJNNNNV41yqt7Bj20Kbm4cJUTuCX50gi4Bukin+mtSGoPkli1/gmnWw6A1pTk+8n8SMIkx9raUsE18g7M7DAdvh1RLkSHi4BKgHJuM2HTta4h0kXsZ3XHX0gT4DnOSuK7WemEjSHYD+CIkpMgV7r6QYQGXut8nyz9ZmSW7KTEDm/7qb0ED108+Ga6pWec827n18beGYnX7obu/8PM7HUJ7hzePsIVpPpZpnBL8oS1z1UnoAnGOyfJfPpsYGDmEtVwscInGnwr6+534BJ7RgCYR+4sAsu6IjXxQ1ugA4Oepo07w== ARC-Authentication-Results: i=1; lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass (client-ip=217.140.110.172; helo=foss.arm.com; envelope-from=ryan.roberts@arm.com; receiver=lists.ozlabs.org) smtp.mailfrom=arm.com Authentication-Results: lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=arm.com (client-ip=217.140.110.172; helo=foss.arm.com; envelope-from=ryan.roberts@arm.com; receiver=lists.ozlabs.org) Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lists.ozlabs.org (Postfix) with ESMTP id 4fW3sS3Jl9z3f1n for ; Wed, 11 Mar 2026 19:32:42 +1100 (AEDT) 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 25D5B20E3; Wed, 11 Mar 2026 01:32:04 -0700 (PDT) Received: from [10.57.83.155] (unknown [10.57.83.155]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D80843F836; Wed, 11 Mar 2026 01:32:05 -0700 (PDT) Message-ID: <7687ded7-ecb1-45fe-bfa6-37d1a04355c8@arm.com> Date: Wed, 11 Mar 2026 08:32:04 +0000 X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 0/2] Fix bugs and performance of kstack offset randomisation Content-Language: en-GB To: 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 , Kees Cook , "Gustavo A. R. Silva" , Arnd Bergmann , Mark Rutland , "Jason A. Donenfeld" , Ard Biesheuvel , Jeremy Linton , David Laight Cc: 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: <20260303150840.3789438-1-ryan.roberts@arm.com> From: Ryan Roberts In-Reply-To: <20260303150840.3789438-1-ryan.roberts@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Kees, I'm keen to get some testing in linux-next and hopefully get this upstream for v7.1 as we previously discussed. Are you willing/able to take this via your tree? Thanks, Ryan On 03/03/2026 15:08, Ryan Roberts wrote: > [Kees; I'm hoping this is now good-to-go via your hardening tree? It would be > good to get some linux-next testing.] > > Hi All, > > 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. Patch 2 > proposes 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. > > 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. I've also rerun the tests > with this version on top of v7.0-rc2 on arm64 and confirmed simialr results. > > arm64 (AWS Graviton3): > +-----------------+--------------+-------------+---------------+ > | Benchmark | Result Class | v6.18-rc5 | per-cpu-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-cpu-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 v4 [5] > ==================== > > - Moved add_random_kstack_offset() later in syscall entry code for powerpc, s390 > and x86. On these platforms it was previously within noinstr sections but for > some exotic Kconfigs, [get|put]_cpu_var() was calling out to instrumentable > code. (reported by kernel test robot) > - Removed what was previously patch 2 (inline version of prandom_u32_state()). > With the above change, there is no longer an issue with calling the > out-of-line version. > > Changes since v3 [4] > ==================== > > - Patch 1: Fixed typo in commit log (per David L) > - Patch 2: Reinstated prandom_u32_state() as out-of-line function, which > forwards to inline version (per David L) > - Patch 3: Added supplementary info about benefits of removing > choose_random_kstack_offset() (per Mark R) > > Changes since v2 [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/ > [4] https://lore.kernel.org/all/20260102131156.3265118-1-ryan.roberts@arm.com/ > [5] https://lore.kernel.org/all/20260119130122.1283821-1-ryan.roberts@arm.com > > Thanks, > Ryan > > > Ryan Roberts (2): > randomize_kstack: Maintain kstack_offset per task > 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 | 16 ++------- > arch/riscv/kernel/traps.c | 12 ------- > arch/s390/include/asm/entry-common.h | 8 ----- > arch/s390/kernel/syscall.c | 2 +- > arch/x86/entry/syscall_32.c | 4 +-- > arch/x86/entry/syscall_64.c | 2 +- > arch/x86/include/asm/entry-common.h | 12 ------- > include/linux/randomize_kstack.h | 54 +++++++++++----------------- > init/main.c | 9 ++++- > kernel/fork.c | 1 + > 13 files changed, 37 insertions(+), 110 deletions(-) > > -- > 2.43.0 >