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 F415A1FA859; Thu, 27 Nov 2025 14:18:22 +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=1764253104; cv=none; b=S/M3GON8cZkarG8y8T0k9n9PwahKx8dEWSKGU5+uLq22fi6CyHeWDl+vUjGjw7E2qFKp0OUIxnMdBHYFHRrzwNVUwGF6tLzTIUg3RaAq5fHR+7/cjqz8o5Llk2c5/izpa4Hh9cTDpgd+O50Gu71XHyQo4GNH0BNRIFuA1acBbDg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764253104; c=relaxed/simple; bh=pPFp+C2cwjO+LvPZIvlRoct3nNUNkGI+QSqqL1g2pvg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=D5PHv0W0FimEiw6kKcAidk+YSO5O/230BZrwJ9Y+aVe/smzcnf8CA7BWxTI4pfF/Su5K+eTZEceYXSEMdgHxO9TfdAZpzmaPUT/IWCTNSufvl4PPQT6TC3WJHjb8fVC5BJgK6h9ybunxL8axTPktZAyEdG5ESgExpvZ3TtZbebE= 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 E0CFB176A; Thu, 27 Nov 2025 06:18:14 -0800 (PST) Received: from [10.57.87.167] (unknown [10.57.87.167]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 70FEE3F73B; Thu, 27 Nov 2025 06:18:20 -0800 (PST) Message-ID: Date: Thu, 27 Nov 2025 14:18:18 +0000 Precedence: bulk X-Mailing-List: linux-hardening@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC/RFT PATCH 0/6] Improve get_random_u8() for use in randomize kstack Content-Language: en-GB To: Ard Biesheuvel Cc: Ard Biesheuvel , linux-hardening@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Kees Cook , Will Deacon , Arnd Bergmann , Jeremy Linton , Catalin Marinas , Mark Rutland , "Jason A. Donenfeld" References: <20251127092226.1439196-8-ardb+git@google.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 27/11/2025 12:28, Ard Biesheuvel wrote: > On Thu, 27 Nov 2025 at 13:12, Ryan Roberts wrote: >> >> On 27/11/2025 09:22, Ard Biesheuvel wrote: >>> From: Ard Biesheuvel >>> >>> Ryan reports that get_random_u16() is dominant in the performance >>> profiling of syscall entry when kstack randomization is enabled [0]. >>> >>> This is the reason many architectures rely on a counter instead, and >>> that, in turn, is the reason for the convoluted way the (pseudo-)entropy >>> is gathered and recorded in a per-CPU variable. >>> >>> Let's try to make the get_random_uXX() fast path faster, and switch to >>> get_random_u8() so that we'll hit the slow path 2x less often. Then, >>> wire it up in the syscall entry path, replacing the per-CPU variable, >>> making the logic at syscall exit redundant. >> >> I ran the same set of syscall benchmarks for this series as I've done for my >> series. >> > > Thanks! > > >> 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, and >> finally cost of turning it on with Ard's changes applied: >> >> arm64 (AWS Graviton3): >> +-----------------+--------------+-------------+---------------+-----------------+ >> | Benchmark | Result Class | v6.18-rc5 | per-task-prng | fast-get-random | >> | | | rndstack-on | | | >> +=================+==============+=============+===============+=================+ >> | syscall/getpid | mean (ns) | (R) 15.62% | (R) 3.43% | (R) 11.93% | >> | | p99 (ns) | (R) 155.01% | (R) 3.20% | (R) 11.00% | >> | | p99.9 (ns) | (R) 156.71% | (R) 2.93% | (R) 11.39% | >> +-----------------+--------------+-------------+---------------+-----------------+ >> | syscall/getppid | mean (ns) | (R) 14.09% | (R) 2.12% | (R) 10.44% | >> | | p99 (ns) | (R) 152.81% | 1.55% | (R) 9.94% | >> | | p99.9 (ns) | (R) 153.67% | 1.77% | (R) 9.83% | >> +-----------------+--------------+-------------+---------------+-----------------+ >> | syscall/invalid | mean (ns) | (R) 13.89% | (R) 3.32% | (R) 10.39% | >> | | p99 (ns) | (R) 165.82% | (R) 3.51% | (R) 10.72% | >> | | p99.9 (ns) | (R) 168.83% | (R) 3.77% | (R) 11.03% | >> +-----------------+--------------+-------------+---------------+-----------------+ >> > > What does the (R) mean? > >> So this fixes the tail problem. I guess get_random_u8() only takes the slow path >> every 768 calls, whereas get_random_u16() took it every 384 calls. I'm not sure >> that fully explains it though. >> >> But it's still a 10% cost on average. >> >> Personally I think 10% syscall cost is too much to pay for 6 bits of stack >> randomisation. 3% is better, but still higher than we would all prefer, I'm sure. >> > > Interesting! > > So the only thing that get_random_u8() does that could explain the > delta is calling into the scheduler on preempt_enable(), given that it > does very little beyond that. > > Would you mind repeating this experiment after changing the > put_cpu_var() to preempt_enable_no_resched(), to test this theory? This has no impact on performance.