From patchwork Wed Aug 23 09:01:04 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Babis Chalios X-Patchwork-Id: 13361866 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 0C7D6EE49A0 for ; Wed, 23 Aug 2023 09:15:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230503AbjHWJPB (ORCPT ); Wed, 23 Aug 2023 05:15:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58148 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235386AbjHWJJg (ORCPT ); Wed, 23 Aug 2023 05:09:36 -0400 Received: from smtp-fw-9103.amazon.com (smtp-fw-9103.amazon.com [207.171.188.200]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E2827423A; Wed, 23 Aug 2023 02:02:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.es; i=@amazon.es; q=dns/txt; s=amazon201209; t=1692781353; x=1724317353; h=from:to:cc:subject:date:message-id:mime-version: content-transfer-encoding; bh=AwpaYCib8MlS9D8QHLxoHGJ4Ok7wJP8PXV9mzZNz5cU=; b=t2e9wHgKsqWdnnWgWRUYiKc3pw7qzvqeaK6E6mrmgKWGIAbAYSKVa0yv gWbmoJtqjAYyq525f1KTXNaHxTQrQN3DzKnSOEiFXnjJAmmyLgCks5rdK X0eF1Gr9EKGr6ErfPRM/u1j8YE4bisGaNCp6N1/tKT/eHdQZxkVeS2tqv Y=; X-IronPort-AV: E=Sophos;i="6.01,195,1684800000"; d="scan'208";a="1150066203" Received: from pdx4-co-svc-p1-lb2-vlan2.amazon.com (HELO email-inbound-relay-iad-1a-m6i4x-366646a6.us-east-1.amazon.com) ([10.25.36.210]) by smtp-border-fw-9103.sea19.amazon.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 23 Aug 2023 09:01:23 +0000 Received: from EX19D017EUA001.ant.amazon.com (iad12-ws-svc-p26-lb9-vlan2.iad.amazon.com [10.40.163.34]) by email-inbound-relay-iad-1a-m6i4x-366646a6.us-east-1.amazon.com (Postfix) with ESMTPS id E9BB4A3D10; Wed, 23 Aug 2023 09:01:19 +0000 (UTC) Received: from EX19D037EUB003.ant.amazon.com (10.252.61.119) by EX19D017EUA001.ant.amazon.com (10.252.50.71) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1118.37; Wed, 23 Aug 2023 09:01:19 +0000 Received: from f4d4887fdcfb.ant.amazon.com (10.1.212.9) by EX19D037EUB003.ant.amazon.com (10.252.61.119) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1118.37; Wed, 23 Aug 2023 09:01:13 +0000 From: Babis Chalios To: Olivia Mackall , Herbert Xu , Theodore Ts'o , "Jason A. Donenfeld" , "Michael S. Tsirkin" , "Jason Wang" , Xuan Zhuo , , , CC: , , , , , Subject: [RFC PATCH 0/2] Propagating reseed notifications to user space Date: Wed, 23 Aug 2023 11:01:04 +0200 Message-ID: <20230823090107.65749-1-bchalios@amazon.es> X-Mailer: git-send-email 2.37.1 (Apple Git-137.1) MIME-Version: 1.0 X-Originating-IP: [10.1.212.9] X-ClientProxiedBy: EX19D040UWA004.ant.amazon.com (10.13.139.93) To EX19D037EUB003.ant.amazon.com (10.252.61.119) Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org User space often implements PRNGs that use /dev/random as entropy source. We can not expect that this randomness sources stay completely unknown forever. For various reasons, the originating PRNG seed may become known at which point the PRNG becomes insecure for further random number generation. Events that can lead to that are for example fast computers reversing the PRNG function using a number of inputs or Virtual Machine clones which carry seed values into their clones. During LPC 2022 Jason, Alex, Michael and me brainstormed on how to atomically expose a notification to user space that it should reseed. Atomicity is key for the VM clone case. This patchset implements a potential path to do so. This patchset introduces an epoch value as the means of communicating to the guest the need to reseed. The epoch is a 32bit value with the following form: RNG epoch *-------------*---------------------* | notifier id | epoch counter value | *-------------*---------------------* 8 bits 24 bits Changes in this value signal moments in time that PRNGs need to be re-seeded. As a result, the intended use of the epoch from user space PRNGs is to cache the epoch value every time they reseed using kernel entropy, then control that its value hasn't changed before giving out random numbers. If the value has changed the PRNG needs to reseed before producing any more random bits. The API for getting hold of this value is offered through /dev/(u)random. We introduce a new ioctl for these devices, which creates an anonymous file descriptor. User processes can call the ioctl() to get the anon fd and then mmap it to a single page. That page contains the value of the epoch at offset 0. Naturally, random.c is the component that maintains the RNG epoch. During initialization it allocates a single global page which holds the epoch value. Moreover, it exposes an API to kernel subsystems (notifiers) which can report events that require PRNG reseeding. Notifiers register with random.c and receive an 8-bit notifier id (up to 256 subscribers should be enough) and a pointer to the epoch. Notifying, then, is equivalent to writing in the epoch address a new epoch value. Notifiers write epoch values that include the notifier ID on the higher 8 bits and increasing counter values on the 24 remaining bits. This guarantees that two notifiers cannot ever write the same epoch value, since notificator IDs are unique. The first patch of this series implements the epoch mechanism. It adds the logic in the random.c to maintain the epoch page and expose the user space facing API. It also adds the internal API that allows kernel systems to register as notifiers. Based on this API, we can implement notifiers. As example notifier, this patch set only contains a virtio-rng backend. If we agree on this path forward, the natural next notifier would be an in-kernel timer with a user settable frequency that allows the kernel to notify all its user space applications to reseed regularly. Michael sent out a proposal for an extension to virtio-rng [1] which we can use to turn virtio-rng devices to RNG epoch notifiers. Briefly, the proposal defines the concept of "entropy leaks", i.e. events in time that cause entropy to decrease, such as VM snapshots. A virtio-rng driver can program the device to perform a number of operations on guest memory when entropy leaks happen. The proposal defines two possible operations: * "fill-on-leak": Fill a buffer with random bytes * "copy-on-leak": Copy a source buffer in a destination buffer The second patch implements the entropy leak reporting feature of virtio-rng and registers virtio-rng devices as epoch notifiers with random.c. It ensures that one copy-on-leak operation is always in-flight to increase the epoch value when a VM snapshot occurs. The intention is that the VMM will perform these operations before resuming vCPUs, so that user space processes will observe the changes in the epoch value atomically. Additionally, it always keeps a fill-on-leak command in the queue, so that we can get some fresh entropy when VM snapshots occur. This is an RFC, so that we can discuss whether the proposed ABI works. Also, I'd like to hear people's opinion on the internal registration API, 8/24 split etc. If we decide that this approach works, I 'm happy to add documentation for it, with examples on how user space can make use of it. [1] https://lore.kernel.org/virtio-dev/20221121162756.350032-1-mst@redhat.com/ Babis Chalios (2): random: emit reseed notifications for PRNGs virtio-rng: implement entropy leak feature drivers/char/hw_random/virtio-rng.c | 189 +++++++++++++++++++++++++++- drivers/char/random.c | 147 ++++++++++++++++++++++ include/linux/random.h | 28 +++++ include/uapi/linux/random.h | 11 ++ include/uapi/linux/virtio_rng.h | 3 + 5 files changed, 373 insertions(+), 5 deletions(-)