From patchwork Tue Aug 8 17:28:36 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Greg Kurz X-Patchwork-Id: 9888855 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 4E64560363 for ; Tue, 8 Aug 2017 17:31:08 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 311B7288C3 for ; Tue, 8 Aug 2017 17:31:08 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 25BD228913; Tue, 8 Aug 2017 17:31:08 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.wl.linuxfoundation.org (Postfix) with ESMTPS id 7A40E288C3 for ; Tue, 8 Aug 2017 17:31:07 +0000 (UTC) Received: from localhost ([::1]:43835 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1df8L1-0003Rc-Bs for patchwork-qemu-devel@patchwork.kernel.org; Tue, 08 Aug 2017 13:30:39 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59699) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1df8JH-000220-Ep for qemu-devel@nongnu.org; Tue, 08 Aug 2017 13:28:52 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1df8JE-0006oD-Ou for qemu-devel@nongnu.org; Tue, 08 Aug 2017 13:28:51 -0400 Received: from 11.mo4.mail-out.ovh.net ([46.105.34.195]:60043) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1df8JE-0006o0-HV for qemu-devel@nongnu.org; Tue, 08 Aug 2017 13:28:48 -0400 Received: from player694.ha.ovh.net (b6.ovh.net [213.186.33.56]) by mo4.mail-out.ovh.net (Postfix) with ESMTP id C9DA08AEEC for ; Tue, 8 Aug 2017 19:28:46 +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 5B2572C0074; Tue, 8 Aug 2017 19:28:43 +0200 (CEST) From: Greg Kurz To: qemu-devel@nongnu.org Date: Tue, 08 Aug 2017 19:28:36 +0200 Message-ID: <150221331695.15915.4904484630739512874.stgit@bahia.lan> User-Agent: StGit/0.17.1-20-gc0b1b-dirty MIME-Version: 1.0 X-Ovh-Tracer-Id: 17640036790665582989 X-VR-SPAMSTATE: OK X-VR-SPAMSCORE: -100 X-VR-SPAMCAUSE: gggruggvucftvghtrhhoucdtuddrfeelkedrkeefgdduudeiucetufdoteggodetrfdotffvucfrrhhofhhilhgvmecuqfggjfdpvefjgfevmfevgfenuceurghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddm X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 46.105.34.195 Subject: [Qemu-devel] [for-2.10 PATCH] 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 Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: "Qemu-devel" X-Virus-Scanned: ClamAV using ClamSMTP 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. 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. Signed-off-by: Greg Kurz --- hw/9pfs/9p-local.c | 44 ++++++++++++++++++++++++++++---------------- hw/9pfs/9p-util.h | 10 +++++++--- 2 files changed, 35 insertions(+), 19 deletions(-) diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c index 6e478f4765ef..b178d627c764 100644 --- a/hw/9pfs/9p-local.c +++ b/hw/9pfs/9p-local.c @@ -333,30 +333,42 @@ update_map_file: static int fchmodat_nofollow(int dirfd, const char *name, mode_t mode) { + struct stat stbuf; int fd, ret; + char *proc_path; /* 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); - if (fd == -1) { - /* In case the file is writable-only and isn't a directory. */ - if (errno == EACCES) { - fd = openat_file(dirfd, name, O_WRONLY, 0); - } - if (fd == -1 && errno == EISDIR) { - errno = EACCES; - } + if (fstatat(dirfd, name, &stbuf, AT_SYMLINK_NOFOLLOW)) { + return -1; } + + if (S_ISLNK(stbuf.st_mode)) { + errno = ELOOP; + return -1; + } + + fd = openat_file(dirfd, name, O_RDONLY | O_PATH, 0); if (fd == -1) { return -1; } - ret = fchmod(fd, mode); + + ret = fstat(fd, &stbuf); + if (ret) { + goto out; + } + + if (S_ISLNK(stbuf.st_mode)) { + errno = ELOOP; + ret = -1; + goto out; + } + + proc_path = g_strdup_printf("/proc/self/fd/%d", fd); + ret = chmod(proc_path, mode); + g_free(proc_path); +out: close_preserve_errno(fd); return ret; } diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h index 91299a24b8af..7c1c8fb1e35c 100644 --- a/hw/9pfs/9p-util.h +++ b/hw/9pfs/9p-util.h @@ -43,9 +43,13 @@ 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 it ignores O_NONBLOCK. + */ + if (!(flags & O_PATH)) { + ret = fcntl(fd, F_SETFL, flags); + assert(!ret); + } errno = serrno; return fd; }