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=-15.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_2 autolearn=ham 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 C527AC433E0 for ; Tue, 26 Jan 2021 17:27:18 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 60139206D4 for ; Tue, 26 Jan 2021 17:27:18 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 60139206D4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kaod.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:50242 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l4S7Z-0005b6-Go for qemu-devel@archiver.kernel.org; Tue, 26 Jan 2021 12:27:17 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:35902) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l4Rww-0005zR-1b for qemu-devel@nongnu.org; Tue, 26 Jan 2021 12:16:18 -0500 Received: from smtpout1.mo804.mail-out.ovh.net ([79.137.123.220]:44149) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l4Rwn-0000Tf-KT for qemu-devel@nongnu.org; Tue, 26 Jan 2021 12:16:17 -0500 Received: from mxplan5.mail.ovh.net (unknown [10.109.138.109]) by mo804.mail-out.ovh.net (Postfix) with ESMTPS id 888C1862A515; Tue, 26 Jan 2021 18:16:06 +0100 (CET) Received: from kaod.org (37.59.142.100) by DAG8EX1.mxp5.local (172.16.2.71) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2106.2; Tue, 26 Jan 2021 18:16:05 +0100 Authentication-Results: garm.ovh; auth=pass (GARM-100R00380ccec42-bc0c-4d1b-8e6d-1eb6481ae219, 0BEDCD57DDE4FFFD0A05CC08AE31DEE925320B36) smtp.auth=groug@kaod.org X-OVh-ClientIp: 78.197.208.248 Date: Tue, 26 Jan 2021 18:16:04 +0100 From: Greg Kurz To: Stefan Hajnoczi Subject: Re: [Virtio-fs] [PATCH v2] virtiofsd: prevent opening of special files (CVE-2020-35517) Message-ID: <20210126181604.1a4c69c6@bahia.lan> In-Reply-To: <20210126103502.260758-1-stefanha@redhat.com> References: <20210126103502.260758-1-stefanha@redhat.com> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.32; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Originating-IP: [37.59.142.100] X-ClientProxiedBy: DAG1EX1.mxp5.local (172.16.2.1) To DAG8EX1.mxp5.local (172.16.2.71) X-Ovh-Tracer-GUID: 8eedb017-f306-469f-b380-66ed1bf4d043 X-Ovh-Tracer-Id: 5644699185187363180 X-VR-SPAMSTATE: OK X-VR-SPAMSCORE: -100 X-VR-SPAMCAUSE: gggruggvucftvghtrhhoucdtuddrgeduledrvdeigdefjecutefuodetggdotefrodftvfcurfhrohhfihhlvgemucfqggfjpdevjffgvefmvefgnecuuegrihhlohhuthemucehtddtnecusecvtfgvtghiphhivghnthhsucdlqddutddtmdenucfjughrpeffhffvuffkjghfofggtgfgihesthejredtredtvdenucfhrhhomhepifhrvghgucfmuhhriicuoehgrhhouhhgsehkrghougdrohhrgheqnecuggftrfgrthhtvghrnhepfedutdeijeejveehkeeileetgfelteekteehtedtieefffevhffflefftdefleejnecukfhppedtrddtrddtrddtpdefjedrheelrddugedvrddutddtnecuvehluhhsthgvrhfuihiivgeptdenucfrrghrrghmpehmohguvgepshhmthhpqdhouhhtpdhhvghlohepmhigphhlrghnhedrmhgrihhlrdhovhhhrdhnvghtpdhinhgvtheptddrtddrtddrtddpmhgrihhlfhhrohhmpehgrhhouhhgsehkrghougdrohhrghdprhgtphhtthhopehvghhohigrlhesrhgvughhrghtrdgtohhm Received-SPF: pass client-ip=79.137.123.220; envelope-from=groug@kaod.org; helo=smtpout1.mo804.mail-out.ovh.net X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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: Daniel Berrange , qemu-devel@nongnu.org, P J P , virtio-fs@redhat.com, Alex Xu , Laszlo Ersek , vgoyal@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, 26 Jan 2021 10:35:02 +0000 Stefan Hajnoczi wrote: > A well-behaved FUSE client does not attempt to open special files with > FUSE_OPEN because they are handled on the client side (e.g. device nodes > are handled by client-side device drivers). > > The check to prevent virtiofsd from opening special files is missing in > a few cases, most notably FUSE_OPEN. A malicious client can cause > virtiofsd to open a device node, potentially allowing the guest to > escape. or pretty much anything nasty you can think of, e.g. DoS if the malicious client repeatedly asks virtiofsd to open FIFOs the other side of which is never opened. > This can be exploited by a modified guest device driver. It is > not exploitable from guest userspace since the guest kernel will handle > special files inside the guest instead of sending FUSE requests. > > This patch adds the missing checks to virtiofsd. This is a short-term > solution because it does not prevent a compromised virtiofsd process > from opening device nodes on the host. > > Reported-by: Alex Xu > Fixes: CVE-2020-35517 > Reviewed-by: Dr. David Alan Gilbert > Reviewed-by: Vivek Goyal > Signed-off-by: Stefan Hajnoczi > --- The patch looks pretty good to me. It just seems to be missing a change in lo_create(): fd = openat(parent_inode->fd, name, (fi->flags | O_CREAT) & ~O_NOFOLLOW, mode); A malicious guest could have created anything called ${name} in this directory before calling FUSE_CREATE and we'll open it blindly, or I'm missing something ? > v2: > * Add doc comment clarifying that symlinks are traversed client-side > [Daniel] > > This issue was diagnosed on public IRC and is therefore already known > and not embargoed. > > A stronger fix, and the long-term solution, is for users to mount the > shared directory and any sub-mounts with nodev, as well as nosuid and > noexec. Unfortunately virtiofsd cannot do this automatically because > bind mounts added by the user after virtiofsd has launched would not be > detected. I suggest the following: > > 1. Modify libvirt and Kata Containers to explicitly set these mount > options. > 2. Then modify virtiofsd to check that the shared directory has the > necessary options at startup. Refuse to start if the options are > missing so that the user is aware of the security requirements. > > As a bonus this also increases the likelihood that other host processes > besides virtiofsd will be protected by nosuid/noexec/nodev so that a > malicious guest cannot drop these files in place and then arrange for a > host process to come across them. > > Additionally, user namespaces have been discussed. They seem like a > worthwhile addition as an unprivileged or privilege-separated mode > although there are limitations with respect to security xattrs and the > actual uid/gid stored on the host file system not corresponding to the > guest uid/gid. > --- > tools/virtiofsd/passthrough_ll.c | 85 +++++++++++++++++++++----------- > 1 file changed, 57 insertions(+), 28 deletions(-) > > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c > index 5fb36d9407..b722f43809 100644 > --- a/tools/virtiofsd/passthrough_ll.c > +++ b/tools/virtiofsd/passthrough_ll.c > @@ -555,6 +555,30 @@ static int lo_fd(fuse_req_t req, fuse_ino_t ino) > return fd; > } > > +/* > + * Open a file descriptor for an inode. Returns -EBADF if the inode is not a > + * regular file or a directory. Use this helper function instead of raw > + * openat(2) to prevent security issues when a malicious client opens special > + * files such as block device nodes. Symlink inodes are also rejected since > + * symlinks must already have been traversed on the client side. > + */ > +static int lo_inode_open(struct lo_data *lo, struct lo_inode *inode, > + int open_flags) > +{ > + g_autofree char *fd_str = g_strdup_printf("%d", inode->fd); > + int fd; > + > + if (!S_ISREG(inode->filetype) && !S_ISDIR(inode->filetype)) { > + return -EBADF; > + } > + > + fd = openat(lo->proc_self_fd, fd_str, open_flags); > + if (fd < 0) { > + return -errno; > + } > + return fd; > +} > + > static void lo_init(void *userdata, struct fuse_conn_info *conn) > { > struct lo_data *lo = (struct lo_data *)userdata; > @@ -684,8 +708,7 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr, > if (fi) { > truncfd = fd; > } else { > - sprintf(procname, "%i", ifd); > - truncfd = openat(lo->proc_self_fd, procname, O_RDWR); > + truncfd = lo_inode_open(lo, inode, O_RDWR); > if (truncfd < 0) { > goto out_err; > } > @@ -1725,7 +1748,6 @@ static struct lo_inode_plock *lookup_create_plock_ctx(struct lo_data *lo, > pid_t pid, int *err) > { > struct lo_inode_plock *plock; > - char procname[64]; > int fd; > > plock = > @@ -1742,12 +1764,10 @@ static struct lo_inode_plock *lookup_create_plock_ctx(struct lo_data *lo, > } > > /* Open another instance of file which can be used for ofd locks. */ > - sprintf(procname, "%i", inode->fd); > - > /* TODO: What if file is not writable? */ > - fd = openat(lo->proc_self_fd, procname, O_RDWR); > - if (fd == -1) { > - *err = errno; > + fd = lo_inode_open(lo, inode, O_RDWR); > + if (fd < 0) { > + *err = -fd; > free(plock); > return NULL; > } > @@ -1894,18 +1914,24 @@ static void lo_open(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) > { > int fd; > ssize_t fh; > - char buf[64]; > struct lo_data *lo = lo_data(req); > + struct lo_inode *inode = lo_inode(req, ino); > > fuse_log(FUSE_LOG_DEBUG, "lo_open(ino=%" PRIu64 ", flags=%d)\n", ino, > fi->flags); > > + if (!inode) { > + fuse_reply_err(req, EBADF); > + return; > + } > + > update_open_flags(lo->writeback, lo->allow_direct_io, fi); > > - sprintf(buf, "%i", lo_fd(req, ino)); > - fd = openat(lo->proc_self_fd, buf, fi->flags & ~O_NOFOLLOW); > - if (fd == -1) { > - return (void)fuse_reply_err(req, errno); > + fd = lo_inode_open(lo, inode, fi->flags & ~O_NOFOLLOW); > + if (fd < 0) { > + lo_inode_put(lo, &inode); > + fuse_reply_err(req, -fd); > + return; > } > > pthread_mutex_lock(&lo->mutex); > @@ -1913,6 +1939,7 @@ static void lo_open(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) > pthread_mutex_unlock(&lo->mutex); > if (fh == -1) { > close(fd); > + lo_inode_put(lo, &inode); > fuse_reply_err(req, ENOMEM); > return; > } > @@ -1923,6 +1950,7 @@ static void lo_open(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) > } else if (lo->cache == CACHE_ALWAYS) { > fi->keep_cache = 1; > } > + lo_inode_put(lo, &inode); > fuse_reply_open(req, fi); > } > > @@ -1982,39 +2010,40 @@ static void lo_flush(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) > static void lo_fsync(fuse_req_t req, fuse_ino_t ino, int datasync, > struct fuse_file_info *fi) > { > + struct lo_inode *inode = lo_inode(req, ino); > + struct lo_data *lo = lo_data(req); > int res; > int fd; > - char *buf; > > fuse_log(FUSE_LOG_DEBUG, "lo_fsync(ino=%" PRIu64 ", fi=0x%p)\n", ino, > (void *)fi); > > + if (!inode) { > + fuse_reply_err(req, EBADF); > + return; > + } > + > if (!fi) { > - struct lo_data *lo = lo_data(req); > - > - res = asprintf(&buf, "%i", lo_fd(req, ino)); > - if (res == -1) { > - return (void)fuse_reply_err(req, errno); > - } > - > - fd = openat(lo->proc_self_fd, buf, O_RDWR); > - free(buf); > - if (fd == -1) { > - return (void)fuse_reply_err(req, errno); > + fd = lo_inode_open(lo, inode, O_RDWR); > + if (fd < 0) { > + res = -fd; > + goto out; > } > } else { > fd = lo_fi_fd(req, fi); > } > > if (datasync) { > - res = fdatasync(fd); > + res = fdatasync(fd) == -1 ? errno : 0; > } else { > - res = fsync(fd); > + res = fsync(fd) == -1 ? errno : 0; > } > if (!fi) { > close(fd); > } > - fuse_reply_err(req, res == -1 ? errno : 0); > +out: > + lo_inode_put(lo, &inode); > + fuse_reply_err(req, res); > } > > static void lo_read(fuse_req_t req, fuse_ino_t ino, size_t size, off_t offset,