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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D4B62C433FE for ; Sun, 10 Apr 2022 23:03:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233802AbiDJXFl (ORCPT ); Sun, 10 Apr 2022 19:05:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36110 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233642AbiDJXFk (ORCPT ); Sun, 10 Apr 2022 19:05:40 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3EA1F1835A for ; Sun, 10 Apr 2022 16:03:28 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 41A51B80ECA; Sun, 10 Apr 2022 23:03:26 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id DC2BEC385A4; Sun, 10 Apr 2022 23:03:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1649631804; bh=3UW/ugtsAFc3LmqMEdhTjA4sBbB84OlrF236JWrXPJQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=NR9pk0uBM1asFnTpcfZ/n5egUA0L5wn0LwY4g0wU3t6o8+s1sxwXOHR2AYAWEi6pV I2vKpdgHuCEtkbj5jgEl+6Q+z2lm60QHNjVPUndluZnUmfI2C4+KbpzY9A5VH3UpK3 P5r9Evysc6lL+rwlKglGhMFJbnFeXyKAGFovS8I7ute7Bmg1zu32YUyz4TLsfBLCTE fC/iqfphZbrVckNWFSyEQ+mV/51gW4BSq6CcPUm/8Qyydr7JRQY1FMsyTuSVmBdOKr 0g+a0Sxr4+W04a25Z9gGkMyiR61smTiTbf6h82pbMVql8l89m1uCYkKbbielAEJF1K 7i38S8/QIO/7A== Date: Sun, 10 Apr 2022 16:03:22 -0700 From: Eric Biggers To: "Jason A. Donenfeld" Cc: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, arnd@arndb.de, Theodore Ts'o , Dominik Brodowski , Russell King , Catalin Marinas , Will Deacon , Geert Uytterhoeven , Thomas Bogendoerfer , Paul Walmsley , Palmer Dabbelt , Albert Ou , "David S . Miller" , Richard Weinberger , Anton Ivanov , Johannes Berg , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H . Peter Anvin" , Chris Zankel , Max Filippov , John Stultz , Stephen Boyd , linux-arm-kernel@lists.infradead.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org, sparclinux@vger.kernel.org, linux-um@lists.infradead.org, x86@kernel.org, linux-xtensa@linux-xtensa.org Subject: Re: [PATCH RFC v1 00/10] archs/random: fallback to using sched_clock() if no cycle counter Message-ID: References: <20220408182145.142506-1-Jason@zx2c4.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220408182145.142506-1-Jason@zx2c4.com> Precedence: bulk List-ID: X-Mailing-List: linux-m68k@vger.kernel.org On Fri, Apr 08, 2022 at 08:21:35PM +0200, Jason A. Donenfeld wrote: > By my first guess, we have ktime_get_boottime_ns(), jiffies, and > sched_clock(). It seems like sched_clock() has already done a lot of > work in being always available with some incrementing value, falling > back to jiffies as necessary. So this series goes with that as a > fallback, for when the architecture doesn't define random_get_entropy in > its own way and when there's no working cycle counter. Won't this interact badly with how try_to_generate_entropy() (a.k.a. the "Linus Jitter Dance") detects the presence of an appropriate timer currently? stack.cycles = random_get_entropy(); /* Slow counter - or none. Don't even bother */ if (stack.cycles == random_get_entropy()) return; So if random_get_entropy() always returns 0, then try_to_generate_entropy() won't run. However, if random_get_entropy() is even just a low-precision timer, then try_to_generate_entropy() will have a chance of running, since the timer might change between the two calls to random_get_entropy(). And if try_to_generate_entropy() does run, then it credits 1 bit of entropy for every iteration, regardless of the timer's precision. This is an existing problem, but this patchset will make it worse, as it changes a lot of cases from "no timer" to "low precision timer". Perhaps try_to_generate_entropy() should check the timer at least 3 times and verify that it changed each time? - Eric