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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 2711BC7EE23 for ; Wed, 7 Jun 2023 11:03:24 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1q6qwF-0001h2-Nx; Wed, 07 Jun 2023 07:02:53 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1q6qvq-0001fU-7n for qemu-devel@nongnu.org; Wed, 07 Jun 2023 07:02:28 -0400 Received: from kylie.crudebyte.com ([5.189.157.229]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1q6qvn-0003LO-GG for qemu-devel@nongnu.org; Wed, 07 Jun 2023 07:02:25 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=crudebyte.com; s=kylie; h=Content-Type:Content-Transfer-Encoding: MIME-Version:References:In-Reply-To:Message-ID:Date:Subject:Cc:To:From: Content-ID:Content-Description; bh=4PAJT098JBvy4iaRBItM2y/emNtLvqz+VXCbI5b3HOM=; b=XFQyrF6T6RxVvRoABcsjZhTJxw +OgnmReXprMDXw6Jk9MIVg+9SUeH9Ur1C7mHqMfaOPWGIkL78ss+kzcr1p/7jWWWLCVttqluZz5co xZJG8pEBjVySVFlf1av7eM9c9sfQqqYW/WuzyH0dH9Ad4Uxxl96+qyoKA+Y883A+G7OvI4Br8EsVR qO5n0p1XAbGNqFMAkFKzHZwpjIfsM3Sah/jPNGBNPA55nBrWpcmdwTd9KCw3siN5zipl+Rasy1DFO akxGghnDv8UgosPb7GwsjPIoHMC9mskXpP0nA9I04zyliBjug1/G949RBQXDZQrrnBYLb2xMb9wZE MNdTFwVbzVSqy36hyfM2OJd3CAWPtnuAvGP4yNeNMih9BXGoRoocELCdNZO/BZHdT7LvUtEihublf QbXjEXXqcyr7xZKMfyZ5w2exDQYHns3t5op26W/t9brXITZ73AaD2kgIKGeMy1/LU9clnwcDP8sbI 1Tl50nF3GoVaFiodRq3pLlL6unXqgo6nnHuXEAGnfvMs4EK38rNCW//7fBRMSY1gHasTtMYuQ4Ulj FfrXzgD0r47Buh+9aDR8nWSMBnvCgIXlVpV3oWnQpqS6XrOJJ7nyo5h/y35C9Yv8eqHGb+bu5x/pW 7GMipp/+72k4OXE5rQpPKk+jdGwcpHH8Mnz5Sk4kw=; From: Christian Schoenebeck To: qemu-devel@nongnu.org, Mauro Matteo Cascella Cc: yw s , shawtao1125@gmail.com, jkli@xidian.edu.cn, shenwenbo@zju.edu.cn, Greg Kurz Subject: Re: [PATCH v2] 9pfs: prevent opening special files (CVE-2023-2861) Date: Wed, 07 Jun 2023 13:02:17 +0200 Message-ID: <2567904.5Pe2IrQ8Qc@silver> In-Reply-To: <20230606180028.5305af87@bahia> References: <20230606180028.5305af87@bahia> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Received-SPF: pass client-ip=5.189.157.229; envelope-from=qemu_oss@crudebyte.com; helo=kylie.crudebyte.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tuesday, June 6, 2023 6:00:28 PM CEST Greg Kurz wrote: > Hi Christian, > > On Tue, 06 Jun 2023 15:57:50 +0200 > Christian Schoenebeck wrote: > > > The 9p protocol does not specifically define how server shall behave when > > client tries to open a special file, however from security POV it does > > make sense for 9p server to prohibit opening any special file on host side > > in general. A sane Linux 9p client for instance would never attempt to > > open a special file on host side, it would always handle those exclusively > > on its guest side. A malicious client however could potentially escape > > from the exported 9p tree by creating and opening a device file on host > > side. > > > > With QEMU this could only be exploited in the following unsafe setups: > > > > - Running QEMU binary as root AND 9p 'local' fs driver AND 'passthrough' > > security model. > > > > or > > > > - Using 9p 'proxy' fs driver (which is running its helper daemon as > > root). > > > > These setups were already discouraged for safety reasons before, > > however for obvious reasons we are now tightening behaviour on this. > > > > Fixes: CVE-2023-2861 > > Reported-by: Yanwu Shen > > Reported-by: Jietao Xiao > > Reported-by: Jinku Li > > Reported-by: Wenbo Shen > > Signed-off-by: Christian Schoenebeck > > --- > > v1 -> v2: > > - Add equivalent fix for 'proxy' fs driver. > > - Minor adjustments on commit log. > > > > Note that this might be a bit confusing for reviewers since > v1 was never posted to qemu-devel. Technically, this should > have been posted without the v2 tag. I felt it wouldn't make it any better, as it might otherwise confuse those who already got the previous two patch emails. > > fsdev/virtfs-proxy-helper.c | 48 +++++++++++++++++++++++++++++++++++-- > > hw/9pfs/9p-util.h | 29 ++++++++++++++++++++++ > > 2 files changed, 75 insertions(+), 2 deletions(-) > > > > diff --git a/fsdev/virtfs-proxy-helper.c b/fsdev/virtfs-proxy-helper.c > > index 5cafcd7703..f311519fa3 100644 > > --- a/fsdev/virtfs-proxy-helper.c > > +++ b/fsdev/virtfs-proxy-helper.c > > @@ -26,6 +26,7 @@ > > #include "qemu/xattr.h" > > #include "9p-iov-marshal.h" > > #include "hw/9pfs/9p-proxy.h" > > +#include "hw/9pfs/9p-util.h" > > #include "fsdev/9p-iov-marshal.h" > > > > #define PROGNAME "virtfs-proxy-helper" > > @@ -338,6 +339,49 @@ static void resetugid(int suid, int sgid) > > } > > } > > > > +/* > > + * Open regular file or directory. Attempts to open any special file are > > + * rejected. > > + * > > + * returns file descriptor or -1 on error > > + */ > > +static int open_regular(const char *pathname, int flags, mode_t mode) { > > + int fd; > > + struct stat stbuf; > > + > > + fd = open(pathname, flags, mode); > > + if (fd < 0) { > > + return fd; > > + } > > + > > + /* CVE-2023-2861: Prohibit opening any special file directly on host > > + * (especially device files), as a compromised client could potentially > > + * gain access outside exported tree under certain, unsafe setups. We > > + * expect client to handle I/O on special files exclusively on guest side. > > + */ > > + if (qemu_fstat(fd, &stbuf) < 0) { > > + close_preserve_errno(fd); > > + return -1; > > + } > > + if (!S_ISREG(stbuf.st_mode) && !S_ISDIR(stbuf.st_mode)) { > > + /* Tcreate and Tlcreate 9p messages mandate to immediately open the > > + * created file for I/O. So this is not (necessarily) due to a broken > > + * client, and hence no error message is to be reported in this case. > > + */ > > + if (!(flags & O_CREAT)) { > > Tlcreate is explicitly about creating regular files only (see [1] and > v9fs_lcreate()) and I don't quite see how open() could successfully > create a regular file and the resulting fd is fstat'ed as something > else. > > Tcreate seems to cover more types but again only regular files (with O_CREAT) > or directories (without O_CREAT) are expected here (see v9fs_create()). > > Unless I'm missing something, it seems that the comment and the O_CREAT > check should be removed. > > [1] https://github.com/chaos/diod/blob/master/protocol.md#lcreate----create-regular-file You are right about Tlcreate, but for Tcreate 9p2000.u specifies, quote: "In addition to creating directories with DMDIR, 9P2000.u allows the creation of symlinks (DMSYMLINK), devices (DMDEVICE), named pipes (DMNAMEPIPE), and sockets (DMSOCKET)." http://ericvh.github.io/9p-rfc/rfc9p2000.u.html#anchor17 So I just remove mentioning Tlcreate in the comment? > > > + error_report_once( > > + "9p: broken or compromised client detected; attempt to open " > > + "special file (i.e. neither regular file, nor directory)" > > + ); > > + } > > + close(fd); > > + errno = ENXIO; > > + return -1; > > + } > > + > > + return fd; > > +} > > + > > /* > > * send response in two parts > > * 1) ProxyHeader > > @@ -682,7 +726,7 @@ static int do_create(struct iovec *iovec) > > if (ret < 0) { > > goto unmarshal_err_out; > > } > > - ret = open(path.data, flags, mode); > > + ret = open_regular(path.data, flags, mode); > > if (ret < 0) { > > ret = -errno; > > } > > @@ -707,7 +751,7 @@ static int do_open(struct iovec *iovec) > > if (ret < 0) { > > goto err_out; > > } > > - ret = open(path.data, flags); > > + ret = open_regular(path.data, flags, 0); > > if (ret < 0) { > > ret = -errno; > > } > > diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h > > index c314cf381d..9da1a0538d 100644 > > --- a/hw/9pfs/9p-util.h > > +++ b/hw/9pfs/9p-util.h > > @@ -13,6 +13,8 @@ > > #ifndef QEMU_9P_UTIL_H > > #define QEMU_9P_UTIL_H > > > > +#include "qemu/error-report.h" > > + > > #ifdef O_PATH > > #define O_PATH_9P_UTIL O_PATH > > #else > > @@ -95,6 +97,7 @@ static inline int errno_to_dotl(int err) { > > #endif > > > > #define qemu_openat openat > > +#define qemu_fstat fstat > > #define qemu_fstatat fstatat > > #define qemu_mkdirat mkdirat > > #define qemu_renameat renameat > > @@ -118,6 +121,7 @@ static inline int openat_file(int dirfd, const char *name, int flags, > > mode_t mode) > > { > > int fd, serrno, ret; > > + struct stat stbuf; > > > > #ifndef CONFIG_DARWIN > > again: > > @@ -142,6 +146,31 @@ again: > > return -1; > > } > > > > + /* CVE-2023-2861: Prohibit opening any special file directly on host > > + * (especially device files), as a compromised client could potentially > > + * gain access outside exported tree under certain, unsafe setups. We > > + * expect client to handle I/O on special files exclusively on guest side. > > + */ > > + if (qemu_fstat(fd, &stbuf) < 0) { > > + close_preserve_errno(fd); > > + return -1; > > + } > > + if (!S_ISREG(stbuf.st_mode) && !S_ISDIR(stbuf.st_mode)) { > > + /* Tcreate and Tlcreate 9p messages mandate to immediately open the > > + * created file for I/O. So this is not (necessarily) due to a broken > > + * client, and hence no error message is to be reported in this case. > > + */ > > Same remark as with the proxy helper. > > If you agree with my suggestions, feel free to add my R-b right away. > > Cheers, I'll definitely take the time for another (v3) round in this case. Thanks! > -- > Greg > > > + if (!(flags & O_CREAT)) { > > + error_report_once( > > + "9p: broken or compromised client detected; attempt to open " > > + "special file (i.e. neither regular file, nor directory)" > > + ); > > + } > > + close(fd); > > + errno = ENXIO; > > + return -1; > > + } > > + > > serrno = errno; > > /* O_NONBLOCK was only needed to open the file. Let's drop it. We don't > > * do that with O_PATH since fcntl(F_SETFL) isn't supported, and openat() > >