From patchwork Fri May 6 10:21:28 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Jason A. Donenfeld" X-Patchwork-Id: 571112 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 80481C433EF for ; Fri, 6 May 2022 10:21:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1390933AbiEFKZd (ORCPT ); Fri, 6 May 2022 06:25:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35354 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1388420AbiEFKZa (ORCPT ); Fri, 6 May 2022 06:25:30 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 51D7E5C678; Fri, 6 May 2022 03:21:47 -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 dfw.source.kernel.org (Postfix) with ESMTPS id DCAD762136; Fri, 6 May 2022 10:21:46 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 85689C385AE; Fri, 6 May 2022 10:21:45 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="EyO1temq" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1651832503; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=VpiWB2Vq+d4qOaMoopKF7IjPVOx0k28nRUqidYxVKyc=; b=EyO1temqXZy2n9LGeaXn+pQ7B9ff5vG8jlv22d9nAyzNv8KS+HAEookH/C9hx87WDD4OwI w1S0fffLH7vWyOR6Dw/QBq9tERNJT6/kS3XiiGB3XEEO88KFPjD5UivY0FaKO3EkYthzYV 6FSPsdlrOtYv4y6F39Wm8URptDzddUE= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id dee2ddae (TLSv1.3:AEAD-AES256-GCM-SHA384:256:NO); Fri, 6 May 2022 10:21:43 +0000 (UTC) From: "Jason A. Donenfeld" To: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, nathan@kernel.org, "Jason A. Donenfeld" , Thomas Gleixner , Arnd Bergmann , Theodore Ts'o Subject: [PATCH v8] timekeeping: Add raw clock fallback for random_get_entropy() Date: Fri, 6 May 2022 12:21:28 +0200 Message-Id: <20220506102128.487998-1-Jason@zx2c4.com> In-Reply-To: <87y1zkmg0l.ffs@tglx> References: <87y1zkmg0l.ffs@tglx> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org The addition of random_get_entropy_fallback() provides access to whichever time source has the highest frequency, which is useful for gathering entropy on platforms without available cycle counters. It's not necessarily as good as being able to quickly access a cycle counter that the CPU has, but it's still something, even when it falls back to being jiffies-based. In the event that a given arch does not define get_cycles(), falling back to the get_cycles() default implementation that returns 0 is really not the best we can do. Instead, at least calling random_get_entropy_fallback() would be preferable, because that always needs to return _something_, even falling back to jiffies eventually. It's not as though random_get_entropy_fallback() is super high precision or guaranteed to be entropic, but basically anything that's not zero all the time is better than returning zero all the time. Finally, since random_get_entropy_fallback() is used during extremely early boot when randomizing freelists in mm_init(), it can be called before timekeeping has been initialized. In that case there really is nothing we can do; jiffies hasn't even started ticking yet. So just give up and return 0. Suggested-by: Thomas Gleixner Signed-off-by: Jason A. Donenfeld Reviewed-by: Thomas Gleixner Cc: Arnd Bergmann Cc: Theodore Ts'o --- Changes v7->v8: - Use READ_ONCE around clock and check timekeeping_suspended before reading, suggested by Thomas. Changes v6->v7: - Return 0 if we're called before timekeeping has been initialized, reported by Nathan. include/linux/timex.h | 8 ++++++++ kernel/time/timekeeping.c | 15 +++++++++++++++ 2 files changed, 23 insertions(+) diff --git a/include/linux/timex.h b/include/linux/timex.h index 5745c90c8800..3871b06bd302 100644 --- a/include/linux/timex.h +++ b/include/linux/timex.h @@ -62,6 +62,8 @@ #include #include +unsigned long random_get_entropy_fallback(void); + #include #ifndef random_get_entropy @@ -74,8 +76,14 @@ * * By default we use get_cycles() for this purpose, but individual * architectures may override this in their asm/timex.h header file. + * If a given arch does not have get_cycles(), then we fallback to + * using random_get_entropy_fallback(). */ +#ifdef get_cycles #define random_get_entropy() ((unsigned long)get_cycles()) +#else +#define random_get_entropy() random_get_entropy_fallback() +#endif #endif /* diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index dcdcb85121e4..461d17c0f3d6 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -17,6 +17,7 @@ #include #include #include +#include #include #include #include @@ -2380,6 +2381,20 @@ static int timekeeping_validate_timex(const struct __kernel_timex *txc) return 0; } +/** + * random_get_entropy_fallback - Returns the raw clock source value, + * used by random.c for platforms with no valid random_get_entropy(). + */ +unsigned long random_get_entropy_fallback(void) +{ + struct tk_read_base *tkr = &tk_core.timekeeper.tkr_mono; + struct clocksource *clock = READ_ONCE(tkr->clock); + + if (unlikely(timekeeping_suspended || !clock)) + return 0; + return clock->read(clock); +} +EXPORT_SYMBOL_GPL(random_get_entropy_fallback); /** * do_adjtimex() - Accessor function to NTP __do_adjtimex function