From patchwork Tue Apr 19 16:04:07 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: 564120 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 1CC2FC433EF for ; Tue, 19 Apr 2022 16:04:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1354294AbiDSQHQ (ORCPT ); Tue, 19 Apr 2022 12:07:16 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38696 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1354300AbiDSQHO (ORCPT ); Tue, 19 Apr 2022 12:07:14 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BF69D37A14; Tue, 19 Apr 2022 09:04:27 -0700 (PDT) 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 19F1BB81A6A; Tue, 19 Apr 2022 16:04:26 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D27D2C385A7; Tue, 19 Apr 2022 16:04:23 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="QEtpyqiv" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1650384262; 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; bh=/JmmyyrBwFpcr/dbzrAYgzyoan2bua3z1fb9xEKhqFc=; b=QEtpyqivUtXm2oQVWwgBpVIxdsq8vGLBJGYgQqeRHVelRWfVZLMYx4EVZmthQntVtdRiN/ mkw5fGiSOy7n1h88udLjWVP8rVKJtAKYlasbCwV+T8zQuqzDUe+/ns0k2PWeMKXqkdvICS Q1a/Srbcl+lJ82Px6nS9XCf7OEMK5w0= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 310310a8 (TLSv1.3:AEAD-AES256-GCM-SHA384:256:NO); Tue, 19 Apr 2022 16:04:21 +0000 (UTC) From: "Jason A. Donenfeld" To: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, Alexander Graf Cc: "Jason A. Donenfeld" , Dominik Brodowski , Greg Kroah-Hartman , Theodore Ts'o , Jann Horn , Colm MacCarthaigh Subject: [PATCH] random: add fork_event sysctl for polling VM forks Date: Tue, 19 Apr 2022 18:04:07 +0200 Message-Id: <20220419160407.1740458-1-Jason@zx2c4.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org In order to inform userspace of virtual machine forks, this commit adds a "fork_event" sysctl, which does not return any data, but allows userspace processes to poll() on it for notification of VM forks. It avoids exposing the actual vmgenid from the hypervisor to userspace, in case there is any randomness value in keeping it secret. Rather, userspace is expected to simply use getrandom() if it wants a fresh value. For example, the following snippet can be used to print a message every time a VM forks, after the RNG has been reseeded: struct pollfd fd = { .fd = open("/proc/sys/kernel/random/fork_event", O_RDONLY) }; assert(fd.fd >= 0); for (;;) { assert(poll(&fd, 1, -1) > 0); puts("vm fork detected"); } Various programs and libraries that utilize cryptographic operations depending on fresh randomness can invalidate old keys or take other appropriate actions when receiving that event. While this is racier than allowing userspace to mmap/vDSO the vmgenid itself, it's an incremental step forward that's not as heavyweight. Cc: Dominik Brodowski Cc: Greg Kroah-Hartman Cc: Theodore Ts'o Cc: Alexander Graf Cc: Jann Horn Cc: Colm MacCarthaigh Signed-off-by: Jason A. Donenfeld --- Documentation/admin-guide/sysctl/kernel.rst | 6 ++++-- drivers/char/random.c | 21 +++++++++++++++++++++ 2 files changed, 25 insertions(+), 2 deletions(-) diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst index 1144ea3229a3..ddbd603f0be7 100644 --- a/Documentation/admin-guide/sysctl/kernel.rst +++ b/Documentation/admin-guide/sysctl/kernel.rst @@ -1001,7 +1001,7 @@ This is a directory, with the following entries: * ``urandom_min_reseed_secs``: obsolete (used to determine the minimum number of seconds between urandom pool reseeding). This file is writable for compatibility purposes, but writing to it has no effect - on any RNG behavior. + on any RNG behavior; * ``uuid``: a UUID generated every time this is retrieved (this can thus be used to generate UUIDs at will); @@ -1009,8 +1009,10 @@ This is a directory, with the following entries: * ``write_wakeup_threshold``: when the entropy count drops below this (as a number of bits), processes waiting to write to ``/dev/random`` are woken up. This file is writable for compatibility purposes, but - writing to it has no effect on any RNG behavior. + writing to it has no effect on any RNG behavior; +* ``fork_event``: unreadable, but can be poll()'d on for notifications + delivered after the RNG reseeds following a virtual machine fork. randomize_va_space ================== diff --git a/drivers/char/random.c b/drivers/char/random.c index bf89c6f27a19..63fba6f042f7 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -1187,6 +1187,7 @@ EXPORT_SYMBOL_GPL(add_bootloader_randomness); #if IS_ENABLED(CONFIG_VMGENID) static BLOCKING_NOTIFIER_HEAD(vmfork_chain); +static DEFINE_CTL_TABLE_POLL(sysctl_fork_event_poll); /* * Handle a new unique VM ID, which is unique, not secret, so we @@ -1201,6 +1202,8 @@ void add_vmfork_randomness(const void *unique_vm_id, size_t size) pr_notice("crng reseeded due to virtual machine fork\n"); } blocking_notifier_call_chain(&vmfork_chain, 0, NULL); + if (IS_ENABLED(CONFIG_SYSCTL)) + proc_sys_poll_notify(&sysctl_fork_event_poll); } #if IS_MODULE(CONFIG_VMGENID) EXPORT_SYMBOL_GPL(add_vmfork_randomness); @@ -1655,6 +1658,8 @@ const struct file_operations urandom_fops = { * It is writable to avoid breaking old userspaces, but writing * to it does not change any behavior of the RNG. * + * - fork_event - an unreadable file that can be poll()'d on for VM forks. + * ********************************************************************/ #ifdef CONFIG_SYSCTL @@ -1708,6 +1713,14 @@ static int proc_do_rointvec(struct ctl_table *table, int write, void *buffer, return write ? 0 : proc_dointvec(table, 0, buffer, lenp, ppos); } +#if IS_ENABLED(CONFIG_VMGENID) +static int proc_do_nodata(struct ctl_table *table, int write, void *buffer, + size_t *lenp, loff_t *ppos) +{ + return -ENODATA; +} +#endif + static struct ctl_table random_table[] = { { .procname = "poolsize", @@ -1748,6 +1761,14 @@ static struct ctl_table random_table[] = { .mode = 0444, .proc_handler = proc_do_uuid, }, +#if IS_ENABLED(CONFIG_VMGENID) + { + .procname = "fork_event", + .mode = 0444, + .poll = &sysctl_fork_event_poll, + .proc_handler = proc_do_nodata, + }, +#endif { } };