From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-26.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_GIT,USER_IN_DEF_DKIM_WL autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 68565C63798 for ; Fri, 20 Nov 2020 03:04:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 17BC7222BA for ; Fri, 20 Nov 2020 03:04:42 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="jy+3wxEY" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727122AbgKTDEX (ORCPT ); Thu, 19 Nov 2020 22:04:23 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39218 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726988AbgKTDEW (ORCPT ); Thu, 19 Nov 2020 22:04:22 -0500 Received: from mail-pg1-x549.google.com (mail-pg1-x549.google.com [IPv6:2607:f8b0:4864:20::549]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7D492C061A04 for ; Thu, 19 Nov 2020 19:04:22 -0800 (PST) Received: by mail-pg1-x549.google.com with SMTP id v2so5653749pgv.2 for ; Thu, 19 Nov 2020 19:04:22 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=sender:date:in-reply-to:message-id:mime-version:references:subject :from:to:cc; bh=7bRLqN/4G8Vg/p+SruLvrAF5HknNrILrTJkN47+Yh/M=; b=jy+3wxEYjXh2uJxBT5hnkLUqSwcMvOJq7K7163jGJqJndQn+hV52uzfcxnVzk2k7i2 QxLzVlV42ZSMsgRw5im9AXWR981BWPgW8qyhodIrSmP+aJev97b6YM4FFBOUGTE1GkTB GZnlTAJyizj93ZLIANIIcRqmvuWnRQdXyBDWSuOIbY9l0eqtjfMZDhoZCsdagyRyzffj WS9ayYQfLKTj7PCL0EekOKJy19pub543GwDI6/9EjqPNJGDK7JoTv/CQ01J3lE9agX+I d0ja28FpMJ8VAORlvDNm5PQHGCluoL9XRDafZGPVjnjii9vA5/vU/DURLMI73BbKlz+y ZSzA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=7bRLqN/4G8Vg/p+SruLvrAF5HknNrILrTJkN47+Yh/M=; b=nokHq/sKmKv0baT/Uk2uWs0YadHzU7ZUjn59BRw70DsYvU6FF5Bosqf9a7aaK+vHrP Cedj5YgpC8H7GxPVmPUBCpl0s2CTwH04x0qbjcaFU1Wqp48GGu658ueBwa/voFdZU/CZ VJHmAty7MDKRinM5pbLBa/hTzVPOD7OwV83AqUNEb08XYT5lgODJM5dHWsiZAhZrWSKk nqVNCWM2QMBO1d8h4w9FhvYFtTLekU4FtveCJXNf6GEW9F8SrPzHWpu/7LX6j5ePHQpX xcb77oCNMA8kAv8yRqOztzS6/29FSOUkxg9CUqlO5h4qbQ05/pgoj5dgSRdKbOicMS7Q XjOA== X-Gm-Message-State: AOAM532VMb/K4RrBB8HYVIO4sfqeUEZHRnjQq3BLlH4nfY/JtdLSiWW8 qEDNNiTWGI0rwDAq04Y1dFKS3tX6/iV7HyK25Q== X-Google-Smtp-Source: ABdhPJychvoYkRk5RFftYRsayhMxGVviZlTyCQyVi0Xzz+20ZfzBXFAwD+Ni7rRNIeeqK17MbtDJdSLYdGMf/LyCvQ== Sender: "lokeshgidra via sendgmr" X-Received: from lg.mtv.corp.google.com ([100.89.4.209]) (user=lokeshgidra job=sendgmr) by 2002:a05:6a00:c8:b029:18b:b0e:e51 with SMTP id e8-20020a056a0000c8b029018b0b0e0e51mr11622681pfj.37.1605841461865; Thu, 19 Nov 2020 19:04:21 -0800 (PST) Date: Thu, 19 Nov 2020 19:04:11 -0800 In-Reply-To: <20201120030411.2690816-1-lokeshgidra@google.com> Message-Id: <20201120030411.2690816-3-lokeshgidra@google.com> Mime-Version: 1.0 References: <20201120030411.2690816-1-lokeshgidra@google.com> X-Mailer: git-send-email 2.29.2.454.gaff20da3a2-goog Subject: [PATCH v6 2/2] Add user-mode only option to unprivileged_userfaultfd sysctl knob From: Lokesh Gidra To: Kees Cook , Jonathan Corbet , Peter Xu , Andrea Arcangeli , Sebastian Andrzej Siewior , Andrew Morton Cc: Alexander Viro , Stephen Smalley , Eric Biggers , Lokesh Gidra , Daniel Colascione , "Joel Fernandes (Google)" , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kaleshsingh@google.com, calin@google.com, surenb@google.com, jeffv@google.com, kernel-team@android.com, Mike Rapoport , Shaohua Li , Jerome Glisse , Mauro Carvalho Chehab , Johannes Weiner , Mel Gorman , Nitin Gupta , Vlastimil Babka , Iurii Zaikin , Luis Chamberlain , linux-mm@kvack.kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org With this change, when the knob is set to 0, it allows unprivileged users to call userfaultfd, like when it is set to 1, but with the restriction that page faults from only user-mode can be handled. In this mode, an unprivileged user (without SYS_CAP_PTRACE capability) must pass UFFD_USER_MODE_ONLY to userfaultd or the API will fail with EPERM. This enables administrators to reduce the likelihood that an attacker with access to userfaultfd can delay faulting kernel code to widen timing windows for other exploits. The default value of this knob is changed to 0. This is required for correct functioning of pipe mutex. However, this will fail postcopy live migration, which will be unnoticeable to the VM guests. To avoid this, set 'vm.userfault = 1' in /sys/sysctl.conf. The main reason this change is desirable as in the short term is that the Android userland will behave as with the sysctl set to zero. So without this commit, any Linux binary using userfaultfd to manage its memory would behave differently if run within the Android userland. For more details, refer to Andrea's reply [1]. [1] https://lore.kernel.org/lkml/20200904033438.GI9411@redhat.com/ Signed-off-by: Lokesh Gidra Reviewed-by: Andrea Arcangeli --- Documentation/admin-guide/sysctl/vm.rst | 15 ++++++++++----- fs/userfaultfd.c | 10 ++++++++-- 2 files changed, 18 insertions(+), 7 deletions(-) diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst index f455fa00c00f..d06a98b2a4e7 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -873,12 +873,17 @@ file-backed pages is less than the high watermark in a zone. unprivileged_userfaultfd ======================== -This flag controls whether unprivileged users can use the userfaultfd -system calls. Set this to 1 to allow unprivileged users to use the -userfaultfd system calls, or set this to 0 to restrict userfaultfd to only -privileged users (with SYS_CAP_PTRACE capability). +This flag controls the mode in which unprivileged users can use the +userfaultfd system calls. Set this to 0 to restrict unprivileged users +to handle page faults in user mode only. In this case, users without +SYS_CAP_PTRACE must pass UFFD_USER_MODE_ONLY in order for userfaultfd to +succeed. Prohibiting use of userfaultfd for handling faults from kernel +mode may make certain vulnerabilities more difficult to exploit. -The default value is 1. +Set this to 1 to allow unprivileged users to use the userfaultfd system +calls without any restrictions. + +The default value is 0. user_reserve_kbytes diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 605599fde015..894cc28142e7 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -28,7 +28,7 @@ #include #include -int sysctl_unprivileged_userfaultfd __read_mostly = 1; +int sysctl_unprivileged_userfaultfd __read_mostly; static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly; @@ -1966,8 +1966,14 @@ SYSCALL_DEFINE1(userfaultfd, int, flags) struct userfaultfd_ctx *ctx; int fd; - if (!sysctl_unprivileged_userfaultfd && !capable(CAP_SYS_PTRACE)) + if (!sysctl_unprivileged_userfaultfd && + (flags & UFFD_USER_MODE_ONLY) == 0 && + !capable(CAP_SYS_PTRACE)) { + printk_once(KERN_WARNING "uffd: Set unprivileged_userfaultfd " + "sysctl knob to 1 if kernel faults must be handled " + "without obtaining CAP_SYS_PTRACE capability\n"); return -EPERM; + } BUG_ON(!current->mm); -- 2.29.0.rc1.297.gfa9743e501-goog