From patchwork Wed Aug 9 16:40:57 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Greg Kurz X-Patchwork-Id: 799888 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Authentication-Results: ozlabs.org; spf=pass (mailfrom) smtp.mailfrom=nongnu.org (client-ip=2001:4830:134:3::11; helo=lists.gnu.org; envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org; receiver=) Received: from lists.gnu.org (lists.gnu.org [IPv6:2001:4830:134:3::11]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 3xSH8t60Ykz9s0Z for ; Thu, 10 Aug 2017 02:41:33 +1000 (AEST) Received: from localhost ([::1]:48952 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dfU2x-0006x8-CM for incoming@patchwork.ozlabs.org; Wed, 09 Aug 2017 12:41:27 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50290) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dfU2f-0006x3-Hr for qemu-devel@nongnu.org; Wed, 09 Aug 2017 12:41:11 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dfU2a-0003Hf-V5 for qemu-devel@nongnu.org; Wed, 09 Aug 2017 12:41:09 -0400 Received: from 19.mo4.mail-out.ovh.net ([87.98.179.66]:41927) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dfU2a-0003HK-Kv for qemu-devel@nongnu.org; Wed, 09 Aug 2017 12:41:04 -0400 Received: from player694.ha.ovh.net (b6.ovh.net [213.186.33.56]) by mo4.mail-out.ovh.net (Postfix) with ESMTP id BDBF8885D2 for ; Wed, 9 Aug 2017 18:41:02 +0200 (CEST) Received: from bahia.lan (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 0F6742C009C; Wed, 9 Aug 2017 18:40:57 +0200 (CEST) From: Greg Kurz To: qemu-devel@nongnu.org Date: Wed, 09 Aug 2017 18:40:57 +0200 Message-ID: <150229685736.21846.2809147507731700887.stgit@bahia.lan> User-Agent: StGit/0.17.1-20-gc0b1b-dirty MIME-Version: 1.0 X-Ovh-Tracer-Id: 4259842301137754509 X-VR-SPAMSTATE: OK X-VR-SPAMSCORE: -100 X-VR-SPAMCAUSE: gggruggvucftvghtrhhoucdtuddrfeelkedrkeehgddutdejucetufdoteggodetrfdotffvucfrrhhofhhilhgvmecuqfggjfdpvefjgfevmfevgfenuceurghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddm X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 87.98.179.66 Subject: [Qemu-devel] [for-2.10 PATCH v4] 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+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" 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), => once you've done chmod(0000) on a file, you can never chmod() again - the target path is UNIX domain socket (openat() returns ENXIO) => 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 --- 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 replacement 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 | 43 ++++++++++++++++++++++++++++++++++++------- hw/9pfs/9p-util.h | 24 +++++++++++++++--------- 2 files changed, 51 insertions(+), 16 deletions(-) diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c index 6e478f4765ef..d9ef57d343c9 100644 --- a/hw/9pfs/9p-local.c +++ b/hw/9pfs/9p-local.c @@ -333,17 +333,27 @@ update_map_file: static int fchmodat_nofollow(int dirfd, const char *name, mode_t mode) { + struct stat stbuf; int fd, ret; /* 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 = 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 = 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 = openat_file(dirfd, name, O_RDONLY | O_PATH_9P_UTIL, 0); +#if O_PATH_9P_UTIL == 0 if (fd == -1) { /* In case the file is writable-only and isn't a directory. */ if (errno == EACCES) { @@ -357,6 +367,25 @@ static int fchmodat_nofollow(int dirfd, const char *name, mode_t mode) return -1; } ret = fchmod(fd, mode); +#else + /* Now we handle racing symlinks. */ + ret = fstat(fd, &stbuf); + if (ret) { + goto out; + } + if (S_ISLNK(stbuf.st_mode)) { + errno = ELOOP; + ret = -1; + goto out; + } + + { + char *proc_path = g_strdup_printf("/proc/self/fd/%d", fd); + ret = chmod(proc_path, mode); + g_free(proc_path); + } +out: +#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 +#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 = errno; @@ -22,13 +28,8 @@ static inline void close_preserve_errno(int fd) 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); } 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 *name, int flags, } serrno = errno; - /* O_NONBLOCK was only needed to open the file. Let's drop it. */ - ret = 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 openat() + * ignored it anyway. + */ + if (!(flags & O_PATH_9P_UTIL)) { + ret = fcntl(fd, F_SETFL, flags); + assert(!ret); + } errno = serrno; return fd; }