From patchwork Mon Oct 26 21:00:50 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Lokesh Gidra X-Patchwork-Id: 11858775 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id D7CF861C for ; Mon, 26 Oct 2020 21:00:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B78E0221F7 for ; Mon, 26 Oct 2020 21:00:58 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="Pxi/eqHu" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729066AbgJZVA5 (ORCPT ); Mon, 26 Oct 2020 17:00:57 -0400 Received: from mail-pj1-f73.google.com ([209.85.216.73]:51800 "EHLO mail-pj1-f73.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729067AbgJZVA5 (ORCPT ); Mon, 26 Oct 2020 17:00:57 -0400 Received: by mail-pj1-f73.google.com with SMTP id 21so3583427pje.1 for ; Mon, 26 Oct 2020 14:00:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=sender:date:message-id:mime-version:subject:from:to:cc; bh=8XFAGaMnZpaYT0ztE4+ED+zm8jGUSJs5c4zzKttTIPI=; b=Pxi/eqHudvziXWScDyXkpvDK2QiIuvHS43rBVL+Xc70/Q5dzLbbxPT40atX3PLqd6o dt9KJrBnICKVwr2NPng1LbI91rutUvaFzl1aozHG4HWDaRt7KhMYDAMmF6aDpxi7XDOk QT2H3OusQXLTq3dTB30XF2gqpXpXBdHCpNyaj577OVJMu9CSNnnppB2U1z1+o6lHcOvZ hWqD7IR8U14Mb86Cp4dMZKfsJ6/lDq6Ysaa7GKnVsTghUKku11fCzFnBpox4cDb6ReLG Q6NUHNqunaSvXXK+IKxly7q+I5MTyAjJHGABlKn21sCz5g4Sn/TxGU86GcXbDF26gmpN eHpQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:message-id:mime-version:subject:from :to:cc; bh=8XFAGaMnZpaYT0ztE4+ED+zm8jGUSJs5c4zzKttTIPI=; b=najpnoLFggkF3Hpi/nGTCaPnjGLi6dIae3rd9jRmmqcOeDBDT4VNv+bEoNTI8xC/8g /6jrg4fCEPPf/Kqb1WWPOO8RGR+SAtIMQMmVydXuIkX0m3lKMHkrM0BxuKxbTjhFvX5R VbNI1yPMUyWdAyiKWvCnAyA++3Kci8r1yWsoWoQ39AUfC2Qod68KsP5v1evyd64EsC++ TU0FQyfx2kDZ25Fai16Kh8B0jwmYLY6DKXUHHhsnabEdQORvhOipqrogJGYqSbEP1qzw c6bwiAuLsv7W09NZ9kjnsIwXlJP9rOMakmb64uf4QWjZGpQ21YWQ9oxIT6aJIs84mSqV FBlA== X-Gm-Message-State: AOAM533wgcaFwC84NLILz9/urzgb89nCEH2ZoFoAkOsa9O3X/DYVY+GW JHGgmYo/qg6zRpk425jmFzA3q+ieAVldYIkpcg== X-Google-Smtp-Source: ABdhPJzoCc1eQDlxMsW2Ajct/JrdoHj+uuvNDeZHvYEyQxfSbIe9/4p6t0jstELt4ujFjAMh69PODMXwUc1F7f+XtA== Sender: "lokeshgidra via sendgmr" X-Received: from lg.mtv.corp.google.com ([2620:15c:211:202:f693:9fff:fef4:29dd]) (user=lokeshgidra job=sendgmr) by 2002:a17:90a:2e05:: with SMTP id q5mr1854369pjd.0.1603746056083; Mon, 26 Oct 2020 14:00:56 -0700 (PDT) Date: Mon, 26 Oct 2020 14:00:50 -0700 Message-Id: <20201026210052.3775167-1-lokeshgidra@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.29.0.rc1.297.gfa9743e501-goog Subject: [PATCH v6 0/2] Control over userfaultfd kernel-fault handling 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, nnk@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 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org This patch series is split from [1]. The other series enables SELinux support for userfaultfd file descriptors so that its creation and movement can be controlled. It has been demonstrated on various occasions that suspending kernel code execution for an arbitrary amount of time at any access to userspace memory (copy_from_user()/copy_to_user()/...) can be exploited to change the intended behavior of the kernel. For instance, handling page faults in kernel-mode using userfaultfd has been exploited in [2, 3]. Likewise, FUSE, which is similar to userfaultfd in this respect, has been exploited in [4, 5] for similar outcome. This small patch series adds a new flag to userfaultfd(2) that allows callers to give up the ability to handle kernel-mode faults with the resulting UFFD file object. It then adds a 'user-mode only' option to the unprivileged_userfaultfd sysctl knob to require unprivileged callers to use this new flag. The purpose of this new interface is to decrease the chance of an unprivileged userfaultfd user taking advantage of userfaultfd to enhance security vulnerabilities by lengthening the race window in kernel code. [1] https://lore.kernel.org/lkml/20200211225547.235083-1-dancol@google.com/ [2] https://duasynt.com/blog/linux-kernel-heap-spray [3] https://duasynt.com/blog/cve-2016-6187-heap-off-by-one-exploit [4] https://googleprojectzero.blogspot.com/2016/06/exploiting-recursion-in-linux-kernel_20.html [5] https://bugs.chromium.org/p/project-zero/issues/detail?id=808 Changes since v5: - Added printk_once when unprivileged_userfaultfd is set to 0 and userfaultfd syscall is called without UFFD_USER_MODE_ONLY in the absence of CAP_SYS_PTRACE capability. Changes since v4: - Added warning when bailing out from handling kernel fault. Changes since v3: - Modified the meaning of value '0' of unprivileged_userfaultfd sysctl knob. Setting this knob to '0' now allows unprivileged users to use userfaultfd, but can handle page faults in user-mode only. - The default value of unprivileged_userfaultfd sysctl knob is changed to '0'. Changes since v2: - Removed 'uffd_flags' and directly used 'UFFD_USER_MODE_ONLY' in userfaultfd(). Changes since v1: - Added external references to the threats from allowing unprivileged users to handle page faults from kernel-mode. - Removed the new sysctl knob restricting handling of page faults from kernel-mode, and added an option for the same in the existing 'unprivileged_userfaultfd' knob. Lokesh Gidra (2): Add UFFD_USER_MODE_ONLY Add user-mode only option to unprivileged_userfaultfd sysctl knob Documentation/admin-guide/sysctl/vm.rst | 15 ++++++++++----- fs/userfaultfd.c | 20 +++++++++++++++++--- include/uapi/linux/userfaultfd.h | 9 +++++++++ 3 files changed, 36 insertions(+), 8 deletions(-)