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 29EE6D111A8 for ; Thu, 27 Nov 2025 15:57:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type: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=bg1owQV5V7behflaSwXEAFke18eD2nGFCLjM9u9H2Xo=; b=pRezHlXmY6a75VKN94MRMiFo8g baVgx9gXrYW5tpK6VR0nxvz4FuFci1VV7JPD4YyQwWH2f7iDarXDpbY1Y0ZW6D0z5gXxdHlDAf8pX W/FvZEUBcjNf5Zya6KkRfev5v5dBeYfbYvQbhMuwaIObn3Ay0FewBPLFRSfFOUV+d092FYSluMQHV PIeMczVDe4TRVxmd/YVcjLhDUZAZtP7SKzPRtNVgDmsfFVcX9oAiEGNZaXsOZnVRuXQDb8FiK8t3A v52tLWzkiT935eR8CtdAO7KsAL3KAfpa3m37wunCfmdH7ZYQmj5lOzxGUKoHbO3nbHPgqhwIUgBQp rmbDlT+A==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vOeMk-0000000Gt2J-1ZKb; Thu, 27 Nov 2025 15:57:06 +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 1vOeMh-0000000Gt1j-3JZV for linux-arm-kernel@lists.infradead.org; Thu, 27 Nov 2025 15:57:05 +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 87DC6176A; Thu, 27 Nov 2025 07:56:55 -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 1DBFC3F73B; Thu, 27 Nov 2025 07:57:00 -0800 (PST) Message-ID: Date: Thu, 27 Nov 2025 15:56:59 +0000 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 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251127_075703_936538_4FE19360 X-CRM114-Status: GOOD ( 27.39 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 27/11/2025 15:03, Ard Biesheuvel wrote: > On Thu, 27 Nov 2025 at 15:18, Ryan Roberts wrote: >> >> 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. >> > > Thanks. But this is really rather surprising: what else could be > taking up that time, given that on the fast path, there are only some > loads and stores to the buffer, and a cmpxchg64_local(). Could it be > the latter that is causing so much latency? I suppose the local > cmpxchg() semantics don't really exist on arm64, and this uses the > exact same LSE instruction that would be used for an ordinary > cmpxchg(), unlike on x86 where it appears to omit the LOCK prefix. > > In any case, there is no debate that your code is faster on arm64. The results I have for x86 show it's faster than the rdtsc too, although that's also somewhat surprising. I'll run your series on x86 to get the equivalent data. > I > also think that using prandom for this purpose is perfectly fine, even > without reseeding: with a 2^113 period and only 6 observable bits per > 32 bit sample, predicting the next value reliably is maybe not > impossible, but hardly worth the extensive effort, given that we're > not generating cryptographic keys here. > > So the question is really whether we want to dedicate 16 bytes per > task for this. I wouldn't mind personally, but it is something our > internal QA engineers tend to obsess over. Yeah that's a good point. Is this something we could potentially keep at the start of the kstack? Is there any precident for keeping state there at the moment? For arm64, I know there is a general feeling that 16K for the stack more than enough (but we are stuck with it because 8K isn't quite enough). So it would be "for free". I guess it would be tricky to do this in an arch-agnostic way though... Thanks, Ryan