From nobody Wed May 8 20:43:40 2024 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 1502367748107747.0782212564601; Thu, 10 Aug 2017 05:22:28 -0700 (PDT) Received: from localhost ([::1]:52860 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dfmTq-0007sa-9h for importer@patchew.org; Thu, 10 Aug 2017 08:22:26 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47911) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dfmSk-0007QP-2I for qemu-devel@nongnu.org; Thu, 10 Aug 2017 08:21:22 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dfmSg-000450-PV for qemu-devel@nongnu.org; Thu, 10 Aug 2017 08:21:18 -0400 Received: from 17.mo4.mail-out.ovh.net ([46.105.41.16]:40606) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dfmSg-00044I-FR for qemu-devel@nongnu.org; Thu, 10 Aug 2017 08:21:14 -0400 Received: from player694.ha.ovh.net (b6.ovh.net [213.186.33.56]) by mo4.mail-out.ovh.net (Postfix) with ESMTP id C4EE0863A5 for ; Thu, 10 Aug 2017 14:21:10 +0200 (CEST) Received: from [192.168.0.243] (gar31-1-82-66-74-139.fbx.proxad.net [82.66.74.139]) (Authenticated sender: groug@kaod.org) by player694.ha.ovh.net (Postfix) with ESMTPA id F03ED2C008D; Thu, 10 Aug 2017 14:21:04 +0200 (CEST) From: Greg Kurz To: qemu-devel@nongnu.org Date: Thu, 10 Aug 2017 14:21:04 +0200 Message-ID: <150236766413.25933.15038553262682443771.stgit@bahia> User-Agent: StGit/0.17.1-20-gc0b1b-dirty MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" X-Ovh-Tracer-Id: 5743778379035416973 X-VR-SPAMSTATE: OK X-VR-SPAMSCORE: -100 X-VR-SPAMCAUSE: gggruggvucftvghtrhhoucdtuddrfeelkedrkeejgdeglecutefuodetggdotefrodftvfcurfhrohhfihhlvgemucfqggfjpdevjffgvefmvefgnecuuegrihhlohhuthemuceftddtnecusecvtfgvtghiphhivghnthhsucdlqddutddtmd Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 46.105.41.16 Subject: [Qemu-devel] [for-2.10 PATCH v6] 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: zhiyong.wu@ucloud.cn, Michael Tokarev , Philippe =?utf-8?q?Mathieu-Daud=C3=A9?= Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 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 --- v6: - refactored some code as suggested by Philippe v5: - added missing fd =3D=3D -1 check v4: - fixed #if condition - moved out: label above #endif - fixed typo in changelog - added Eric's r-b v3: - O_PATH in a separate block of code - added a reference to the fchmodat2() tentative in the changelog v2: - renamed OPENAT_DIR_O_PATH to O_PATH_9P_UTIL and use it as a replaceme= nt for O_PATH to avoid build breaks on O_PATH-less systems - keep current behavior for O_PATH-less systems - added comments - TODO in 2.11: add _nofollow suffix to openat_dir() and openat_file() --- 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 6e478f4765ef..efb0b79a74bf 100644 --- a/hw/9pfs/9p-local.c +++ b/hw/9pfs/9p-local.c @@ -333,17 +333,27 @@ 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) { @@ -357,6 +367,24 @@ 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 91299a24b8af..dc0d2e29aa3b 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; }