From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932456AbeCKVG0 (ORCPT ); Sun, 11 Mar 2018 17:06:26 -0400 Received: from mail-wr0-f196.google.com ([209.85.128.196]:46302 "EHLO mail-wr0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932370AbeCKVFs (ORCPT ); Sun, 11 Mar 2018 17:05:48 -0400 X-Google-Smtp-Source: AG47ELvaqZ0IkvT1/QFzaFs2dgdZEk0Mkso7rv1cL6MiY8P2UITfZTBjhxwGyDmh+a2Gag8cAyPCBA== From: Christian Brauner To: linux-kernel@vger.kernel.org, ebiederm@xmission.com, torvalds@linux-foundation.org Cc: Christian Brauner Subject: [PATCH 2/3 v2] devpts: resolve devpts bind-mounts Date: Sun, 11 Mar 2018 22:05:36 +0100 Message-Id: <20180311210537.3674-3-christian.brauner@ubuntu.com> X-Mailer: git-send-email 2.15.1 In-Reply-To: <20180311210537.3674-1-christian.brauner@ubuntu.com> References: <20180311210537.3674-1-christian.brauner@ubuntu.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Most libcs will still look at /dev/ptmx when opening the master fd of a pty device. When /dev/ptmx is a bind-mount of /dev/pts/ptmx and the TIOCGPTPEER ioctl() is used to safely retrieve a file descriptor for the slave side of the pty based on the master fd, the /proc/self/fd/{0,1,2} symlinks will point to /. When the kernel tries to look up the root mount of the dentry for the slave file descriptor it will detect that the dentry is escaping its bind-mount since the root mount of the dentry is /dev/pts where the devpts is mounted but the root mount of /dev/ptmx is /dev. Having bind-mounts of /dev/pts/ptmx to /dev/ptmx not working correctly is a regression. In addition, it is also a fairly common scenario in containers employing user namespaces. To handle bind-mounts of /dev/pts/ptmx to /dev/ptmx correctly we need to walk up the bind-mounts for /dev/ptmx in devpts_mntget(). This also means we need to remove the "if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC)" check from devpts_mntget(). However, devpts_acquire() needs to perform that check. The reason is that while the devpts mounted at /dev/pts has devtmpfs mounted at /dev as its parent mount: 21 -- -- / /dev -- 21 -- / /dev/pts they are on different devices -- -- 0:6 / /dev -- -- 0:20 / /dev/pts which has the consequence that the pathname of the directory which forms the root of the /dev/pts mount is /. So if we bind-mount /dev/pts/ptmx to /dev/ptmx we will end up on the same device as the devtmpfs mount on /dev/pts -- -- 0:20 /ptmx /dev/ptmx but given that / is the pathname of the directory which forms the root of the /dev/pts bind-mount, the resulting source will be /ptmx. So in devpts_acquire() we still need to check if "(path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC)" and exit early before we try to call path_pts(). If we call path_pts() in the bind-mount case we will hit path_parent_directory() which will **not** yield /dev as parent directory but rather / which will cause path_pts() to fail since it will not find a "pts" directory underneath /. We should still perform the path_pts() check in devpts_acquire() if we haven't found a suitable devpts filesystem at open time but we should always try to exit early in devpts_acquire() and defer resolving bind-mounts until devpts_mntget(). If we fail in devpts_mntget() we will end up with a wrong /proc//fd/{0,1,2} symlink, if we fail in devpts_acquire() we will end up with a failed open("/dev/ptmx") which is more troublesome. Here's a little reproducer that presupposes a libc that uses TIOCGPTPEER in its openpty() implementation: unshare --mount mount --bind /dev/pts/ptmx /dev/ptmx chmod 666 /dev/ptmx script ls -al /proc/self/fd/0 with output: lrwx------ 1 chb chb 64 Mar 7 16:41 /proc/self/fd/0 -> / Signed-off-by: Christian Brauner Suggested-by: Eric Biederman Suggested-by: Linus Torvalds --- ChangeLog v1->v2: * move removal of if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC) condition to separate patch with non-functional changes ChangeLog v0->v1: * remove /* Has the devpts filesystem already been found? */ if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC) return 0 from devpts_ptmx_path() * check superblock after devpts_ptmx_path() returned --- fs/devpts/inode.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-) diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c index d3ddbb888874..249aba08e9e6 100644 --- a/fs/devpts/inode.c +++ b/fs/devpts/inode.c @@ -154,27 +154,34 @@ static int devpts_ptmx_path(struct path *path) struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi) { + int err; struct path path; path = filp->f_path; path_get(&path); - /* Has the devpts filesystem already been found? */ - if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) { - int err; + if ((DEVPTS_SB(path.mnt->mnt_sb) == fsi) && + (path.mnt->mnt_root == fsi->ptmx_dentry)) { + /* Walk upward while the start point is a bind mount of + * a single file. + */ + while (path.mnt->mnt_root == path.dentry) + if (follow_up(&path) == 0) + break; + } - err = devpts_ptmx_path(&path); - dput(path.dentry); - if (err) { - mntput(path.mnt); - return ERR_PTR(err); - } + err = devpts_ptmx_path(&path); + dput(path.dentry); + if (err) { + mntput(path.mnt); + return ERR_PTR(err); } if (DEVPTS_SB(path.mnt->mnt_sb) != fsi) { mntput(path.mnt); return ERR_PTR(-ENODEV); } + return path.mnt; } -- 2.15.1