From patchwork Tue Jan 21 12:23:42 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: "Dr. David Alan Gilbert" X-Patchwork-Id: 1226507 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org (client-ip=209.51.188.17; helo=lists.gnu.org; envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org; receiver=) Authentication-Results: ozlabs.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: ozlabs.org; dkim=pass (1024-bit key; unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b=izLUGSNd; dkim-atps=neutral Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 4828Jb4QhDz9sNF for ; Wed, 22 Jan 2020 00:19:27 +1100 (AEDT) Received: from localhost ([::1]:53682 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ittRE-0002XI-97 for incoming@patchwork.ozlabs.org; Tue, 21 Jan 2020 08:19:24 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:52082) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1itsf1-0000xS-AW for qemu-devel@nongnu.org; Tue, 21 Jan 2020 07:29:39 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1itsew-0003oW-OL for qemu-devel@nongnu.org; Tue, 21 Jan 2020 07:29:35 -0500 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:30476 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1itsew-0003oO-KO for qemu-devel@nongnu.org; Tue, 21 Jan 2020 07:29:30 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1579609770; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=yMjXJG5M4fPIhAmY/ITGMUqpnE1NsPH9e14qkbQhuHA=; b=izLUGSNdKCT0zkhCUK4ts8T1xa+dFhwa0uckvNf8hX6wRoRCbu6B+FYp6OU5OiYWtVkl2B NFZRGB5JhBy/xr6fabSR5Ke+xb78Kpudpx5/9HfJIgZ/bmAcDeN6YlMp+8ssmVdfINgTK8 3SJPPXfARpbN5POtPLr+IGkfKa73c3E= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-347-G0-100OcMvSfVWMYriUCUA-1; Tue, 21 Jan 2020 07:29:28 -0500 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id C992C800D55; Tue, 21 Jan 2020 12:29:27 +0000 (UTC) Received: from dgilbert-t580.localhost (unknown [10.36.118.32]) by smtp.corp.redhat.com (Postfix) with ESMTP id 62F4F60FC5; Tue, 21 Jan 2020 12:29:08 +0000 (UTC) From: "Dr. David Alan Gilbert (git)" To: qemu-devel@nongnu.org, stefanha@redhat.com, vgoyal@redhat.com, berrange@redhat.com, slp@redhat.com, philmd@redhat.com Subject: [PATCH v2 058/109] virtiofsd: add security guide document Date: Tue, 21 Jan 2020 12:23:42 +0000 Message-Id: <20200121122433.50803-59-dgilbert@redhat.com> In-Reply-To: <20200121122433.50803-1-dgilbert@redhat.com> References: <20200121122433.50803-1-dgilbert@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-MC-Unique: G0-100OcMvSfVWMYriUCUA-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: m.mizuma@jp.fujitsu.com, misono.tomohiro@jp.fujitsu.com Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" From: Stefan Hajnoczi Many people want to know: what's up with virtiofsd and security? This document provides the answers! Signed-off-by: Stefan Hajnoczi Reviewed-by: Daniel P. Berrangé --- docs/tools/index.rst | 2 + docs/tools/virtiofsd-security.rst | 118 ++++++++++++++++++++++++++++++ 2 files changed, 120 insertions(+) create mode 100644 docs/tools/virtiofsd-security.rst diff --git a/docs/tools/index.rst b/docs/tools/index.rst index adbcd3bc72..b9f8b70995 100644 --- a/docs/tools/index.rst +++ b/docs/tools/index.rst @@ -10,3 +10,5 @@ Contents: .. toctree:: :maxdepth: 2 + + virtiofsd-security diff --git a/docs/tools/virtiofsd-security.rst b/docs/tools/virtiofsd-security.rst new file mode 100644 index 0000000000..61ce551344 --- /dev/null +++ b/docs/tools/virtiofsd-security.rst @@ -0,0 +1,118 @@ +======================== +Virtiofsd Security Guide +======================== + +Introduction +============ +This document covers security topics for users of virtiofsd, the daemon that +implements host<->guest file system sharing. Sharing files between one or more +guests and the host raises questions about the trust relationships between +these entities. By understanding these topics users can safely deploy +virtiofsd and control access to their data. + +Architecture +============ +The virtiofsd daemon process acts as a vhost-user device backend, implementing +the virtio-fs device that the corresponding device driver inside the guest +interacts with. + +There is one virtiofsd process per virtio-fs device instance. For example, +when two guests have access to the same shared directory there are still two +virtiofsd processes since there are two virtio-fs device instances. Similarly, +if one guest has access to two shared directories, there are two virtiofsd +processes since there are two virtio-fs device instances. + +Files are created on the host with uid/gid values provided by the guest. +Furthermore, virtiofsd is unable to enforce file permissions since guests have +the ability to access any file within the shared directory. File permissions +are implemented in the guest, just like with traditional local file systems. + +Security Requirements +===================== +Guests have root access to the shared directory. This is necessary for root +file systems on virtio-fs and similar use cases. + +When multiple guests have access to the same shared directory, the guests have +a trust relationship. A broken or malicious guest could delete or corrupt +files. It could exploit symlink or time-of-check-to-time-of-use (TOCTOU) race +conditions against applications in other guests. It could plant device nodes +or setuid executables to gain privileges in other guests. It could perform +denial-of-service (DoS) attacks by consuming available space or making the file +system unavailable to other guests. + +Guests are restricted to the shared directory and cannot access other files on +the host. + +Guests should not be able to gain arbitrary code execution inside the virtiofsd +process. If they do, the process is sandboxed to prevent escaping into other +parts of the host. + +Daemon Sandboxing +================= +The virtiofsd process handles virtio-fs FUSE requests from the untrusted guest. +This attack surface could give the guest access to host resources and must +therefore be protected. Sandboxing mechanisms are integrated into virtiofsd to +reduce the impact in the event that an attacker gains control of the process. + +As a general rule, virtiofsd does not trust inputs from the guest, aside from +uid/gid values. Input validation is performed so that the guest cannot corrupt +memory or otherwise gain arbitrary code execution in the virtiofsd process. + +Sandboxing adds restrictions on the virtiofsd so that even if an attacker is +able to exploit a bug, they will be constrained to the virtiofsd process and +unable to cause damage on the host. + +Seccomp Whitelist +----------------- +Many system calls are not required by virtiofsd to perform its function. For +example, ptrace(2) and execve(2) are not necessary and attackers are likely to +use them to further compromise the system. This is prevented using a seccomp +whitelist in virtiofsd. + +During startup virtiofsd installs a whitelist of allowed system calls. All +other system calls are forbidden for the remaining lifetime of the process. +This list has been built through experience of running virtiofsd on several +flavors of Linux and observing which system calls were encountered. + +It is possible that previously unexplored code paths or newer library versions +will invoke system calls that have not been whitelisted yet. In this case the +process terminates and a seccomp error is captured in the audit log. The log +can typically be viewed using ``journalctl -xe`` and searching for ``SECCOMP``. + +Should it be necessary to extend the whitelist, system call numbers from the +audit log can be translated to names through a CPU architecture-specific +``.tbl`` file in the Linux source tree. They can then be added to the +whitelist in ``seccomp.c`` in the virtiofsd source tree. + +Mount Namespace +--------------- +During startup virtiofsd enters a new mount namespace and releases all mounts +except for the shared directory. This makes the file system root `/` the +shared directory. It is impossible to access files outside the shared +directory since they cannot be looked up by path resolution. + +Several attacks, including `..` traversal and symlink escapes, are prevented by +the mount namespace. + +The current virtiofsd implementation keeps a directory file descriptor to +/proc/self/fd open in order to implement several FUSE requests. This file +descriptor could be used by attackers to access files outside the shared +directory. This limitation will be addressed in a future release of virtiofsd. + +Other Namespaces +---------------- +Virtiofsd enters new pid and network namespaces during startup. The pid +namespace prevents the process from seeing other processes running on the host. +The network namespace removes network connectivity from the process. + +Deployment Best Practices +========================= +The shared directory should be a separate file system so that untrusted guests +cannot cause a denial-of-service by using up all available inodes or exhausting +free space. + +If the shared directory is also accessible from a host mount namespace, it is +recommended to keep a parent directory with rwx------ permissions so that other +users on the host are unable to access any setuid executables or device nodes +in the shared directory. The `nosuid` and `nodev` mount options can also be +used to prevent this issue.