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 X-Spam-Level: X-Spam-Status: No, score=-13.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CECA2C4741F for ; Thu, 5 Nov 2020 15:29:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6DE462151B for ; Thu, 5 Nov 2020 15:29:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1604590195; bh=foAW702CtoshU7IWzq9HQprEHlBZdWORtipJNquMtSg=; h=From:To:Cc:Subject:Date:List-ID:From; b=P76rrylz1CD4Uboxm7bJbSHRBOl7+nMwDBlvsqBuA3bpvFbFecFRuqYOB8vwGH/5e yEw2EnAYOk0tRe4bPg0cpOZjqdSXta8XVQ7736mdZvA2AsPdIghEAy4N85BHy/KG3J J0Jj0aMjpOOrbBMCIdDRqva2gGOf1wLgMLgQCq9E= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731171AbgKEP3x (ORCPT ); Thu, 5 Nov 2020 10:29:53 -0500 Received: from mail.kernel.org ([198.145.29.99]:60264 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730854AbgKEP3w (ORCPT ); Thu, 5 Nov 2020 10:29:52 -0500 Received: from e123331-lin.nice.arm.com (lfbn-nic-1-188-42.w2-15.abo.wanadoo.fr [2.15.37.42]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 8F26C20735; Thu, 5 Nov 2020 15:29:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1604590191; bh=foAW702CtoshU7IWzq9HQprEHlBZdWORtipJNquMtSg=; h=From:To:Cc:Subject:Date:From; b=JmQYjF0wJyxik3FaSbBrfOi0nchfZ72/zrm698mZ2ACRgDx7F8YjPuIOTM6BKKROq HJSC3WY9WXF9Ted7N2lTSD9H25Bs3GbCBD+z5IZ8g59XWSNyjtQ2S0uZafhI7xYIM3 8AJ9zslO5tojU1NEBQhFSAJ5soBSaeeEt4JncNfM= From: Ard Biesheuvel To: tytso@mit.edu Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, maz@kernel.org, mark.rutland@arm.com, broonie@kernel.org, andre.przywara@arm.com, Ard Biesheuvel Subject: [PATCH] random: avoid arch_get_random_seed_long() when collecting IRQ randomness Date: Thu, 5 Nov 2020 16:29:44 +0100 Message-Id: <20201105152944.16953-1-ardb@kernel.org> X-Mailer: git-send-email 2.17.1 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When reseeding the CRNG periodically, arch_get_random_seed_long() is called to obtain entropy from an architecture specific source if one is implemented. In most cases, these are special instructions, but in some cases, such as on ARM, we may want to back this using firmware calls, which are considerably more expensive. Another call to arch_get_random_seed_long() exists in the CRNG driver, in add_interrupt_randomness(), which collects entropy by capturing inter-interrupt timing and relying on interrupt jitter to provide random bits. This is done by keeping a per-CPU state, and mixing in the IRQ number, the cycle counter and the return address every time an interrupt is taken, and mixing this per-CPU state into the entropy pool every 64 invocations, or at least once per second. The entropy that is gathered this way is credited as 1 bit of entropy. Every time this happens, arch_get_random_seed_long() is invoked, and the result is mixed in as well, and also credited with 1 bit of entropy. This means that arch_get_random_seed_long() is called at least once per second on every CPU, which seems excessive, and doesn't really scale, especially in a virtualization scenario where CPUs may be oversubscribed: in cases where arch_get_random_seed_long() is backed by an instruction that actually goes back to a shared hardware entropy source (such as RNDRRS on ARM), we will end up hitting it hundreds of times per second. So let's drop the call to arch_get_random_seed_long() from add_interrupt_randomness(), and instead, rely on crng_reseed() to call the arch hook to get random seed material from the platform. Signed-off-by: Ard Biesheuvel --- drivers/char/random.c | 15 +-------------- 1 file changed, 1 insertion(+), 14 deletions(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 2a41b21623ae..a9c393c1466d 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -1261,8 +1261,6 @@ void add_interrupt_randomness(int irq, int irq_flags) cycles_t cycles = random_get_entropy(); __u32 c_high, j_high; __u64 ip; - unsigned long seed; - int credit = 0; if (cycles == 0) cycles = get_reg(fast_pool, regs); @@ -1298,23 +1296,12 @@ void add_interrupt_randomness(int irq, int irq_flags) fast_pool->last = now; __mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool)); - - /* - * If we have architectural seed generator, produce a seed and - * add it to the pool. For the sake of paranoia don't let the - * architectural seed generator dominate the input from the - * interrupt noise. - */ - if (arch_get_random_seed_long(&seed)) { - __mix_pool_bytes(r, &seed, sizeof(seed)); - credit = 1; - } spin_unlock(&r->lock); fast_pool->count = 0; /* award one bit for the contents of the fast pool */ - credit_entropy_bits(r, credit + 1); + credit_entropy_bits(r, 1); } EXPORT_SYMBOL_GPL(add_interrupt_randomness); -- 2.17.1