From patchwork Mon Oct 26 21:00:52 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Lokesh Gidra X-Patchwork-Id: 11858779 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 74CB661C for ; Mon, 26 Oct 2020 21:01:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4A084221F7 for ; Mon, 26 Oct 2020 21:01:08 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="qUQ0TpNV" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729199AbgJZVBG (ORCPT ); Mon, 26 Oct 2020 17:01:06 -0400 Received: from mail-yb1-f201.google.com ([209.85.219.201]:37237 "EHLO mail-yb1-f201.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729165AbgJZVBG (ORCPT ); Mon, 26 Oct 2020 17:01:06 -0400 Received: by mail-yb1-f201.google.com with SMTP id h6so12516049ybk.4 for ; Mon, 26 Oct 2020 14:01:03 -0700 (PDT) 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=qUQ0TpNVF/6695W1cT5dGQhemHPYRPVve802NdyObcUXlPd6c3stDHVd1mIZ0R9eYJ I2UHv9LxFCZhP1FQrl2GaUQ/WqVYVjm78mGGggW8O/ByiGHopRQQsdKs5sTtwtcCkp46 MOk84ZlRjr83ZZ9k87sskBOD2HG942PvsYUVgI7IQeuJWQI2h2KstlF/Pv4h1vYubIcT MQkOFXtVZGdnGk54/ur9rHHGX/e6t3nK7EPjyfMP/jNDHvB5B6/aHlor6ouyhvhDrqRj sRAZM7Z0rqozcZl7nJIydgV4bijKcNwphJY5Y6H4rlZ8nMiuq7P1Os+JIxxLJ370EIdB XBmA== 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=Uek8OXomdohLvbpkNXLvysWqhFwh363+ruV20a0AFKEsXXnnawpM52HIXR8bbjGyr+ 0NBw2iGRkFWfaY+/xrd3lBTlOTelGU8oV4Gk62yXWqY2PUVoZUZAhkjogazNR2ASk1Va d//amyOXMfopPbWmsM5UBr6hZRA3QiAp+ibz/WTiUFUOvADMBKa65K9AfXGX2SnFZwSR yXV0BsemM0WOnWOv8t5CFl/nhVrqW/xzFw9ABSHaJ18dJb5JExZS/fRTTJGadXStnlVU 62HpCenjIk3G0CN5o5/MEmMAQ3OkCHZs6hM0NXiq0E7algpwP8beh7rHFcPOWpOe2ATE FeLw== X-Gm-Message-State: AOAM530SsA1LmZhiJ6xbfJFgeUnYkrIwh2EwH6aJdiYr4ZbbMWCJ3pXl nrtMPAzVlRqBn26VKDF2sc+p2pLFsE0eX00lKQ== X-Google-Smtp-Source: ABdhPJwJFVtsOUaTVRuXfb3Nz4/S8X9LHGpjLsx9VPYaqBfCR0ehkImK/3tlePxr6gX82slucTbmbk948IwHR51PXw== 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:a25:1c1:: with SMTP id 184mr26679062ybb.243.1603746062678; Mon, 26 Oct 2020 14:01:02 -0700 (PDT) Date: Mon, 26 Oct 2020 14:00:52 -0700 In-Reply-To: <20201026210052.3775167-1-lokeshgidra@google.com> Message-Id: <20201026210052.3775167-3-lokeshgidra@google.com> Mime-Version: 1.0 References: <20201026210052.3775167-1-lokeshgidra@google.com> X-Mailer: git-send-email 2.29.0.rc1.297.gfa9743e501-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, 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 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);