From patchwork Wed Feb 9 01:19:11 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: 541224 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 E0D9DC43217 for ; Wed, 9 Feb 2022 02:40:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235563AbiBICkj (ORCPT ); Tue, 8 Feb 2022 21:40:39 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52414 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242676AbiBIBT6 (ORCPT ); Tue, 8 Feb 2022 20:19:58 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8F672C061576; Tue, 8 Feb 2022 17:19:57 -0800 (PST) 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 4F9B4B81E2E; Wed, 9 Feb 2022 01:19:56 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 63807C340EB; Wed, 9 Feb 2022 01:19:54 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="flziGrYH" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1644369593; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=awLAlfu0dEawTGo7j7bialpwVA3+UDTfPgufirjHqnI=; b=flziGrYHxi9FMOZQDeOPRS1Ww+nzdVXJmFKRqJkRKtFe6iGSxTYeGXs7uiw1kmD6d8wn36 E8JAAnH/vQgL3tpLP4r+HXawGrSLn1pjKQFdyI44Ofdh7j2HyBV8AEUlC4P4Y2egGZ/rLc dTokiqRSYonn2nt8otyF/nfBsVhqzZE= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 14f224b6 (TLSv1.3:AEAD-AES256-GCM-SHA384:256:NO); Wed, 9 Feb 2022 01:19:52 +0000 (UTC) From: "Jason A. Donenfeld" To: linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org Cc: tytso@mit.edu, linux@dominikbrodowski.net, ebiggers@kernel.org, "Jason A. Donenfeld" , Eric Biggers Subject: [PATCH v2 1/9] random: use RDSEED instead of RDRAND in entropy extraction Date: Wed, 9 Feb 2022 02:19:11 +0100 Message-Id: <20220209011919.493762-2-Jason@zx2c4.com> In-Reply-To: <20220209011919.493762-1-Jason@zx2c4.com> References: <20220209011919.493762-1-Jason@zx2c4.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org When /dev/random was directly connected with entropy extraction, without any expansion stage, extract_buf() was called for every 10 bytes of data read from /dev/random. For that reason, RDRAND was used rather than RDSEED. At the same time, crng_reseed() was still only called every 5 minutes, so there RDSEED made sense. Those olden days were also a time when the entropy collector did not use a cryptographic hash function, which meant most bets were off in terms of real preimage resistance. For that reason too it didn't matter _that_ much whether RDSEED was mixed in before or after entropy extraction; both choices were sort of bad. But now we have a cryptographic hash function at work, and with that we get real preimage resistance. We also now only call extract_entropy() every 5 minutes, rather than every 10 bytes. This allows us to do two important things. First, we can switch to using RDSEED in extract_entropy(), as Dominik suggested. Second, we can ensure that RDSEED input always goes into the cryptographic hash function with other things before being used directly. This eliminates a category of attacks in which the CPU knows the current state of the crng and knows that we're going to xor RDSEED into it, and so it computes a malicious RDSEED. By going through our hash function, it would require the CPU to compute a preimage on the fly, which isn't going to happen. Cc: Theodore Ts'o Reviewed-by: Eric Biggers Suggested-by: Dominik Brodowski Signed-off-by: Jason A. Donenfeld --- drivers/char/random.c | 22 +++++++++------------- 1 file changed, 9 insertions(+), 13 deletions(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 22d12213d548..ce3c019e5f5f 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -722,13 +722,8 @@ static void crng_reseed(struct crng_state *crng) CHACHA_KEY_SIZE); } spin_lock_irqsave(&crng->lock, flags); - for (i = 0; i < 8; i++) { - unsigned long rv; - if (!arch_get_random_seed_long(&rv) && - !arch_get_random_long(&rv)) - rv = random_get_entropy(); - crng->state[i + 4] ^= buf.key[i] ^ rv; - } + for (i = 0; i < 8; i++) + crng->state[i + 4] ^= buf.key[i]; memzero_explicit(&buf, sizeof(buf)); WRITE_ONCE(crng->init_time, jiffies); spin_unlock_irqrestore(&crng->lock, flags); @@ -1064,16 +1059,17 @@ static void extract_entropy(void *buf, size_t nbytes) unsigned long flags; u8 seed[BLAKE2S_HASH_SIZE], next_key[BLAKE2S_HASH_SIZE]; struct { - unsigned long rdrand[32 / sizeof(long)]; + unsigned long rdseed[32 / sizeof(long)]; size_t counter; } block; size_t i; trace_extract_entropy(nbytes, input_pool.entropy_count); - for (i = 0; i < ARRAY_SIZE(block.rdrand); ++i) { - if (!arch_get_random_long(&block.rdrand[i])) - block.rdrand[i] = random_get_entropy(); + for (i = 0; i < ARRAY_SIZE(block.rdseed); ++i) { + if (!arch_get_random_seed_long(&block.rdseed[i]) && + !arch_get_random_long(&block.rdseed[i])) + block.rdseed[i] = random_get_entropy(); } spin_lock_irqsave(&input_pool.lock, flags); @@ -1081,7 +1077,7 @@ static void extract_entropy(void *buf, size_t nbytes) /* seed = HASHPRF(last_key, entropy_input) */ blake2s_final(&input_pool.hash, seed); - /* next_key = HASHPRF(seed, RDRAND || 0) */ + /* next_key = HASHPRF(seed, RDSEED || 0) */ block.counter = 0; blake2s(next_key, (u8 *)&block, seed, sizeof(next_key), sizeof(block), sizeof(seed)); blake2s_init_key(&input_pool.hash, BLAKE2S_HASH_SIZE, next_key, sizeof(next_key)); @@ -1091,7 +1087,7 @@ static void extract_entropy(void *buf, size_t nbytes) while (nbytes) { i = min_t(size_t, nbytes, BLAKE2S_HASH_SIZE); - /* output = HASHPRF(seed, RDRAND || ++counter) */ + /* output = HASHPRF(seed, RDSEED || ++counter) */ ++block.counter; blake2s(buf, (u8 *)&block, seed, i, sizeof(block), sizeof(seed)); nbytes -= i;