From nobody Tue Nov 4 15:41:11 2025 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1503969133909501.07426767236166; Mon, 28 Aug 2017 18:12:13 -0700 (PDT) Received: from localhost ([::1]:42205 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dmV4d-0000sF-MD for importer@patchew.org; Mon, 28 Aug 2017 21:12:11 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49241) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dmUD1-00021s-Bm for qemu-devel@nongnu.org; Mon, 28 Aug 2017 20:16:50 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dmUCy-0006OA-51 for qemu-devel@nongnu.org; Mon, 28 Aug 2017 20:16:47 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:50768) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dmUCx-0006ND-QD for qemu-devel@nongnu.org; Mon, 28 Aug 2017 20:16:44 -0400 Received: from pps.filterd (m0098396.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v7T0GIc1010141 for ; Mon, 28 Aug 2017 20:16:42 -0400 Received: from e33.co.us.ibm.com (e33.co.us.ibm.com [32.97.110.151]) by mx0a-001b2d01.pphosted.com with ESMTP id 2cmvjejm5v-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 28 Aug 2017 20:16:42 -0400 Received: from localhost by e33.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! 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Violators will be prosecuted; Mon, 28 Aug 2017 18:16:40 -0600 Received: from b03ledav006.gho.boulder.ibm.com (b03ledav006.gho.boulder.ibm.com [9.17.130.237]) by b03cxnp08028.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id v7T0Gdtk32047306; Mon, 28 Aug 2017 17:16:39 -0700 Received: from b03ledav006.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7BB7CC603E; Mon, 28 Aug 2017 18:16:39 -0600 (MDT) Received: from localhost (unknown [9.80.85.217]) by b03ledav006.gho.boulder.ibm.com (Postfix) with ESMTP id 51764C6043; Mon, 28 Aug 2017 18:16:39 -0600 (MDT) From: Michael Roth To: qemu-devel@nongnu.org Date: Mon, 28 Aug 2017 19:14:54 -0500 X-Mailer: git-send-email 2.7.4 In-Reply-To: <1503965694-10794-1-git-send-email-mdroth@linux.vnet.ibm.com> References: <1503965694-10794-1-git-send-email-mdroth@linux.vnet.ibm.com> MIME-Version: 1.0 X-TM-AS-GCONF: 00 x-cbid: 17082900-0008-0000-0000-0000087D387B X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007630; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000226; SDB=6.00909028; UDB=6.00455849; IPR=6.00689279; BA=6.00005557; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00016909; XFM=3.00000015; UTC=2017-08-29 00:16:41 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17082900-0009-0000-0000-000043C04726 Message-Id: <1503965694-10794-80-git-send-email-mdroth@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:, , definitions=2017-08-28_13:, , signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=60 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1707230000 definitions=main-1708290001 Content-Transfer-Encoding: quoted-printable X-MIME-Autoconverted: from 8bit to quoted-printable by mx0a-001b2d01.pphosted.com id v7T0GIc1010141 X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x [generic] [fuzzy] X-Received-From: 148.163.156.1 Subject: [Qemu-devel] [PATCH 79/79] 9pfs: local: fix fchmodat_nofollow() limitations X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-stable@nongnu.org, Greg Kurz Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Content-Type: text/plain; charset="utf-8" From: Greg Kurz This function has to ensure it doesn't follow a symlink that could be used to escape the virtfs directory. This could be easily achieved if fchmodat() on linux honored the AT_SYMLINK_NOFOLLOW flag as described in POSIX, but it doesn't. There was a tentative to implement a new fchmodat2() syscall with the correct semantics: https://patchwork.kernel.org/patch/9596301/ but it didn't gain much momentum. Also it was suggested to look at an O_PATH based solution in the first place. The current implementation covers most use-cases, but it notably fails if: - the target path has access rights equal to 0000 (openat() returns EPERM), =3D> once you've done chmod(0000) on a file, you can never chmod() again - the target path is UNIX domain socket (openat() returns ENXIO) =3D> bind() of UNIX domain sockets fails if the file is on 9pfs The solution is to use O_PATH: openat() now succeeds in both cases, and we can ensure the path isn't a symlink with fstat(). The associated entry in "/proc/self/fd" can hence be safely passed to the regular chmod() syscall. The previous behavior is kept for older systems that don't have O_PATH. Signed-off-by: Greg Kurz Reviewed-by: Eric Blake Tested-by: Zhi Yong Wu Acked-by: Philippe Mathieu-Daud=C3=A9 (cherry picked from commit 4751fd5328dfcd4fe2f9055728a72a0e3ae56512) Signed-off-by: Michael Roth --- hw/9pfs/9p-local.c | 42 +++++++++++++++++++++++++++++++++++------- hw/9pfs/9p-util.h | 24 +++++++++++++++--------- 2 files changed, 50 insertions(+), 16 deletions(-) diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c index b13ff21..a83dbfb 100644 --- a/hw/9pfs/9p-local.c +++ b/hw/9pfs/9p-local.c @@ -278,17 +278,27 @@ update_map_file: =20 static int fchmodat_nofollow(int dirfd, const char *name, mode_t mode) { + struct stat stbuf; int fd, ret; =20 /* FIXME: this should be handled with fchmodat(AT_SYMLINK_NOFOLLOW). - * Unfortunately, the linux kernel doesn't implement it yet. As an - * alternative, let's open the file and use fchmod() instead. This - * may fail depending on the permissions of the file, but it is the - * best we can do to avoid TOCTTOU. We first try to open read-only - * in case name points to a directory. If that fails, we try write-only - * in case name doesn't point to a directory. + * Unfortunately, the linux kernel doesn't implement it yet. */ - fd =3D openat_file(dirfd, name, O_RDONLY, 0); + + /* First, we clear non-racing symlinks out of the way. */ + if (fstatat(dirfd, name, &stbuf, AT_SYMLINK_NOFOLLOW)) { + return -1; + } + if (S_ISLNK(stbuf.st_mode)) { + errno =3D ELOOP; + return -1; + } + + /* Access modes are ignored when O_PATH is supported. We try O_RDONLY = and + * O_WRONLY for old-systems that don't support O_PATH. + */ + fd =3D openat_file(dirfd, name, O_RDONLY | O_PATH_9P_UTIL, 0); +#if O_PATH_9P_UTIL =3D=3D 0 if (fd =3D=3D -1) { /* In case the file is writable-only and isn't a directory. */ if (errno =3D=3D EACCES) { @@ -302,6 +312,24 @@ static int fchmodat_nofollow(int dirfd, const char *na= me, mode_t mode) return -1; } ret =3D fchmod(fd, mode); +#else + if (fd =3D=3D -1) { + return -1; + } + + /* Now we handle racing symlinks. */ + ret =3D fstat(fd, &stbuf); + if (!ret) { + if (S_ISLNK(stbuf.st_mode)) { + errno =3D ELOOP; + ret =3D -1; + } else { + char *proc_path =3D g_strdup_printf("/proc/self/fd/%d", fd); + ret =3D chmod(proc_path, mode); + g_free(proc_path); + } + } +#endif close_preserve_errno(fd); return ret; } diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h index 517027c..033e7e6 100644 --- a/hw/9pfs/9p-util.h +++ b/hw/9pfs/9p-util.h @@ -13,6 +13,12 @@ #ifndef QEMU_9P_UTIL_H #define QEMU_9P_UTIL_H =20 +#ifdef O_PATH +#define O_PATH_9P_UTIL O_PATH +#else +#define O_PATH_9P_UTIL 0 +#endif + static inline void close_preserve_errno(int fd) { int serrno =3D errno; @@ -22,13 +28,8 @@ static inline void close_preserve_errno(int fd) =20 static inline int openat_dir(int dirfd, const char *name) { -#ifdef O_PATH -#define OPENAT_DIR_O_PATH O_PATH -#else -#define OPENAT_DIR_O_PATH 0 -#endif return openat(dirfd, name, - O_DIRECTORY | O_RDONLY | O_NOFOLLOW | OPENAT_DIR_O_PATH); + O_DIRECTORY | O_RDONLY | O_NOFOLLOW | O_PATH_9P_UTIL); } =20 static inline int openat_file(int dirfd, const char *name, int flags, @@ -43,9 +44,14 @@ static inline int openat_file(int dirfd, const char *nam= e, int flags, } =20 serrno =3D errno; - /* O_NONBLOCK was only needed to open the file. Let's drop it. */ - ret =3D fcntl(fd, F_SETFL, flags); - assert(!ret); + /* 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 opena= t() + * ignored it anyway. + */ + if (!(flags & O_PATH_9P_UTIL)) { + ret =3D fcntl(fd, F_SETFL, flags); + assert(!ret); + } errno =3D serrno; return fd; } --=20 2.7.4