From patchwork Sat Feb 5 16:01:15 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: 12736094 X-Patchwork-Delegate: herbert@gondor.apana.org.au 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 ADFCAC433EF for ; Sat, 5 Feb 2022 16:01:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1380320AbiBEQB4 (ORCPT ); Sat, 5 Feb 2022 11:01:56 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:32910 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1380313AbiBEQBy (ORCPT ); Sat, 5 Feb 2022 11:01:54 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 79A04C06109E; Sat, 5 Feb 2022 08:01:51 -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 330BDB80837; Sat, 5 Feb 2022 16:01:50 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7300EC340E8; Sat, 5 Feb 2022 16:01:48 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="kOqaC9Lm" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1644076907; 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=92AzTeDNszWxPGwVhED34nbJSL0a9CpnN0i00CIDelQ=; b=kOqaC9LmA3NG0oFypyCWnNaz/+O/OXgpv8ygJRn9fvvxC/HBML3W4YDe9rAN29cdMNEa8i rXreKeJhr6FslU40Gl0DbfFJ++rzB8ruxRbHMBv3WKGMBJnsTEKOiEmAG96Li3ZCRVq8QG XICcZurjmpn1oip7gez3nuMjw63WzsE= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id ff19cd4b (TLSv1.3:AEAD-AES256-GCM-SHA384:256:NO); Sat, 5 Feb 2022 16:01:47 +0000 (UTC) From: "Jason A. Donenfeld" To: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org Cc: "Jason A. Donenfeld" , Theodore Ts'o , Greg Kroah-Hartman , Dominik Brodowski Subject: [PATCH v3 2/5] random: simplify entropy debiting Date: Sat, 5 Feb 2022 17:01:15 +0100 Message-Id: <20220205160118.252698-3-Jason@zx2c4.com> In-Reply-To: <20220205160118.252698-1-Jason@zx2c4.com> References: <20220205160118.252698-1-Jason@zx2c4.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org Our pool is 256 bits, and we only ever use all of it or don't use it at all, which is decided by whether or not it has at least 128 bits in it. So we can drastically simplify the accounting and cmpxchg loop to do exactly this. While we're at it, we move the minimum bit size into a constant so it can be shared between the two places where it matters. The reason we want any of this is for the case in which an attacker has compromised the current state, and then bruteforces small amounts of entropy added to it. By demanding a particular minimum amount of entropy be present before reseeding, we make that bruteforcing difficult. Note that this rationale no longer includes anything about /dev/random blocking at the right moment, since /dev/random no longer blocks (except for at ~boot), but rather uses the crng. In a former life, /dev/random was different and therefore required a more nuanced account(), but this is no longer. Behaviorally, nothing changes here. This is just a simplification of the code. Cc: Theodore Ts'o Cc: Greg Kroah-Hartman Reviewed-by: Dominik Brodowski Signed-off-by: Jason A. Donenfeld --- drivers/char/random.c | 91 ++++++++--------------------------- include/trace/events/random.h | 30 +++--------- 2 files changed, 27 insertions(+), 94 deletions(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 3c676897583f..85b0b6241729 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -289,12 +289,14 @@ enum poolinfo { POOL_BITS = BLAKE2S_HASH_SIZE * 8, POOL_BITSHIFT = ilog2(POOL_BITS), + POOL_MIN_BITS = POOL_BITS / 2, /* To allow fractional bits to be tracked, the entropy_count field is * denominated in units of 1/8th bits. */ POOL_ENTROPY_SHIFT = 3, #define POOL_ENTROPY_BITS() (input_pool.entropy_count >> POOL_ENTROPY_SHIFT) - POOL_FRACBITS = POOL_BITS << POOL_ENTROPY_SHIFT + POOL_FRACBITS = POOL_BITS << POOL_ENTROPY_SHIFT, + POOL_MIN_FRACBITS = POOL_MIN_BITS << POOL_ENTROPY_SHIFT }; /* @@ -375,8 +377,7 @@ static struct { .lock = __SPIN_LOCK_UNLOCKED(input_pool.lock), }; -static bool extract_entropy(void *buf, size_t nbytes, int min); -static void _extract_entropy(void *buf, size_t nbytes); +static void extract_entropy(void *buf, size_t nbytes); static void crng_reseed(struct crng_state *crng); @@ -467,7 +468,7 @@ static void process_random_ready_list(void) */ static void credit_entropy_bits(int nbits) { - int entropy_count, entropy_bits, orig; + int entropy_count, orig; int nfrac = nbits << POOL_ENTROPY_SHIFT; /* Ensure that the multiplication can avoid being 64 bits wide. */ @@ -527,8 +528,7 @@ static void credit_entropy_bits(int nbits) trace_credit_entropy_bits(nbits, entropy_count >> POOL_ENTROPY_SHIFT, _RET_IP_); - entropy_bits = entropy_count >> POOL_ENTROPY_SHIFT; - if (crng_init < 2 && entropy_bits >= 128) + if (crng_init < 2 && entropy_count >= POOL_MIN_FRACBITS) crng_reseed(&primary_crng); } @@ -618,7 +618,7 @@ static void crng_initialize_secondary(struct crng_state *crng) static void __init crng_initialize_primary(void) { - _extract_entropy(&primary_crng.state[4], sizeof(u32) * 12); + extract_entropy(&primary_crng.state[4], sizeof(u32) * 12); if (crng_init_try_arch_early() && trust_cpu && crng_init < 2) { invalidate_batched_entropy(); numa_crng_init(); @@ -788,8 +788,17 @@ static void crng_reseed(struct crng_state *crng) } buf; if (crng == &primary_crng) { - if (!extract_entropy(&buf, 32, 16)) - return; + int entropy_count; + do { + entropy_count = READ_ONCE(input_pool.entropy_count); + if (entropy_count < POOL_MIN_FRACBITS) + return; + } while (cmpxchg(&input_pool.entropy_count, entropy_count, 0) != entropy_count); + extract_entropy(buf.key, sizeof(buf.key)); + if (random_write_wakeup_bits) { + wake_up_interruptible(&random_write_wait); + kill_fasync(&fasync, SIGIO, POLL_OUT); + } } else { _extract_crng(&primary_crng, buf.block); _crng_backtrack_protect(&primary_crng, buf.block, @@ -1114,52 +1123,11 @@ EXPORT_SYMBOL_GPL(add_disk_randomness); * *********************************************************************/ -/* - * This function decides how many bytes to actually take from the - * given pool, and also debits the entropy count accordingly. - */ -static size_t account(size_t nbytes, int min) -{ - int entropy_count, orig; - size_t ibytes, nfrac; - - BUG_ON(input_pool.entropy_count > POOL_FRACBITS); - - /* Can we pull enough? */ -retry: - entropy_count = orig = READ_ONCE(input_pool.entropy_count); - if (WARN_ON(entropy_count < 0)) { - pr_warn("negative entropy count: count %d\n", entropy_count); - entropy_count = 0; - } - - /* never pull more than available */ - ibytes = min_t(size_t, nbytes, entropy_count >> (POOL_ENTROPY_SHIFT + 3)); - if (ibytes < min) - ibytes = 0; - nfrac = ibytes << (POOL_ENTROPY_SHIFT + 3); - if ((size_t)entropy_count > nfrac) - entropy_count -= nfrac; - else - entropy_count = 0; - - if (cmpxchg(&input_pool.entropy_count, orig, entropy_count) != orig) - goto retry; - - trace_debit_entropy(8 * ibytes); - if (ibytes && POOL_ENTROPY_BITS() < random_write_wakeup_bits) { - wake_up_interruptible(&random_write_wait); - kill_fasync(&fasync, SIGIO, POLL_OUT); - } - - return ibytes; -} - /* * This is an HKDF-like construction for using the hashed collected entropy * as a PRF key, that's then expanded block-by-block. */ -static void _extract_entropy(void *buf, size_t nbytes) +static void extract_entropy(void *buf, size_t nbytes) { unsigned long flags; u8 seed[BLAKE2S_HASH_SIZE], next_key[BLAKE2S_HASH_SIZE]; @@ -1169,6 +1137,8 @@ static void _extract_entropy(void *buf, size_t nbytes) } block; size_t i; + trace_extract_entropy(nbytes, POOL_ENTROPY_BITS()); + for (i = 0; i < ARRAY_SIZE(block.rdrand); ++i) { if (!arch_get_random_long(&block.rdrand[i])) block.rdrand[i] = random_get_entropy(); @@ -1200,25 +1170,6 @@ static void _extract_entropy(void *buf, size_t nbytes) memzero_explicit(&block, sizeof(block)); } -/* - * This function extracts randomness from the "entropy pool", and - * returns it in a buffer. - * - * The min parameter specifies the minimum amount we can pull before - * failing to avoid races that defeat catastrophic reseeding. If we - * have less than min entropy available, we return false and buf is - * not filled. - */ -static bool extract_entropy(void *buf, size_t nbytes, int min) -{ - trace_extract_entropy(nbytes, POOL_ENTROPY_BITS(), _RET_IP_); - if (account(nbytes, min)) { - _extract_entropy(buf, nbytes); - return true; - } - return false; -} - #define warn_unseeded_randomness(previous) \ _warn_unseeded_randomness(__func__, (void *)_RET_IP_, (previous)) diff --git a/include/trace/events/random.h b/include/trace/events/random.h index a2d9aa16a5d7..ad149aeaf42c 100644 --- a/include/trace/events/random.h +++ b/include/trace/events/random.h @@ -79,22 +79,6 @@ TRACE_EVENT(credit_entropy_bits, __entry->bits, __entry->entropy_count, (void *)__entry->IP) ); -TRACE_EVENT(debit_entropy, - TP_PROTO(int debit_bits), - - TP_ARGS( debit_bits), - - TP_STRUCT__entry( - __field( int, debit_bits ) - ), - - TP_fast_assign( - __entry->debit_bits = debit_bits; - ), - - TP_printk("input pool: debit_bits %d", __entry->debit_bits) -); - TRACE_EVENT(add_input_randomness, TP_PROTO(int input_bits), @@ -161,31 +145,29 @@ DEFINE_EVENT(random__get_random_bytes, get_random_bytes_arch, ); DECLARE_EVENT_CLASS(random__extract_entropy, - TP_PROTO(int nbytes, int entropy_count, unsigned long IP), + TP_PROTO(int nbytes, int entropy_count), - TP_ARGS(nbytes, entropy_count, IP), + TP_ARGS(nbytes, entropy_count), TP_STRUCT__entry( __field( int, nbytes ) __field( int, entropy_count ) - __field(unsigned long, IP ) ), TP_fast_assign( __entry->nbytes = nbytes; __entry->entropy_count = entropy_count; - __entry->IP = IP; ), - TP_printk("input pool: nbytes %d entropy_count %d caller %pS", - __entry->nbytes, __entry->entropy_count, (void *)__entry->IP) + TP_printk("input pool: nbytes %d entropy_count %d", + __entry->nbytes, __entry->entropy_count) ); DEFINE_EVENT(random__extract_entropy, extract_entropy, - TP_PROTO(int nbytes, int entropy_count, unsigned long IP), + TP_PROTO(int nbytes, int entropy_count), - TP_ARGS(nbytes, entropy_count, IP) + TP_ARGS(nbytes, entropy_count) ); TRACE_EVENT(urandom_read,