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 08E24C433EF for ; Thu, 2 Dec 2021 11:35:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1357365AbhLBLjC (ORCPT ); Thu, 2 Dec 2021 06:39:02 -0500 Received: from isilmar-4.linta.de ([136.243.71.142]:39184 "EHLO isilmar-4.linta.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1357348AbhLBLjB (ORCPT ); Thu, 2 Dec 2021 06:39:01 -0500 X-isilmar-external: YES X-isilmar-external: YES X-isilmar-external: YES X-isilmar-external: YES X-isilmar-external: YES X-isilmar-external: YES Received: from light.dominikbrodowski.net (brodo.linta [10.2.0.102]) by isilmar-4.linta.de (Postfix) with ESMTPSA id 71F072013FC; Thu, 2 Dec 2021 11:35:37 +0000 (UTC) Received: by light.dominikbrodowski.net (Postfix, from userid 1000) id E42FF2006D; Thu, 2 Dec 2021 12:35:32 +0100 (CET) Date: Thu, 2 Dec 2021 12:35:32 +0100 From: Dominik Brodowski To: tytso@mit.edu, Jason@zx2c4.com Cc: "Ivan T. Ivanov" , Ard Biesheuvel , linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v3, resend] random: fix crash on multiple early calls to add_bootloader_randomness() Message-ID: References: <20211012082708.121931-1-iivanov@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org If add_bootloader_randomness() or add_hwgenerator_randomness() is called for the first time during early boot, crng_init equals 0. Then, crng_fast_load() gets called -- which is safe to do even if the input pool is not yet properly set up. If the added entropy suffices to increase crng_init to 1, future calls to add_bootloader_randomness() or add_hwgenerator_randomness() used to progress to credit_entropy_bits(). However, if the input pool is not yet properly set up, the cmpxchg call within that function can lead to an infinite recursion. This is not only a hypothetical problem, as qemu on arm64 may provide bootloader entropy via EFI and via devicetree. As crng_global_init_time is set to != 0 once the input pool is properly set up, check (also) for this condition to determine which branch to take. Calls to crng_fast_load() do not modify the input pool; therefore, the entropy_count for the input pool must not be modified at that early stage. Reported-by: Ivan T. Ivanov Fixes: 18b915ac6b0a ("efi/random: Treat EFI_RNG_PROTOCOL output as bootloader randomness") Tested-by: Ivan T. Ivanov Signed-off-by: Dominik Brodowski --- re-send to new co-maintainer of random.c v2->v3: onle one unlikely (Ard Biesheuvel) v1->v2: fix commit message; unmerge Reported-and-tested-by-tag (Ard Biesheuvel) drivers/char/random.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index 605969ed0f96..18fe804c1bf8 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -1763,8 +1763,8 @@ static void __init init_std_data(struct entropy_store *r) } /* - * Note that setup_arch() may call add_device_randomness() - * long before we get here. This allows seeding of the pools + * add_device_randomness() or add_bootloader_randomness() may be + * called long before we get here. This allows seeding of the pools * with some platform dependent data very early in the boot * process. But it limits our options here. We must use * statically allocated structures that already have all @@ -2274,7 +2274,12 @@ void add_hwgenerator_randomness(const char *buffer, size_t count, { struct entropy_store *poolp = &input_pool; - if (unlikely(crng_init == 0)) { + /* We cannot do much with the input pool until it is set up in + * rand_initalize(); therefore just mix into the crng state. + * As this does not affect the input pool, we cannot credit + * entropy for this. + */ + if (unlikely(crng_init == 0 || crng_global_init_time == 0)) { crng_fast_load(buffer, count); return; }