From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Brauner Subject: [PATCH 2/4 v5 RESEND] devpts: resolve devpts bind-mounts Date: Tue, 13 Mar 2018 17:55:25 +0100 Message-ID: <20180313165527.24038-3-christian.brauner__18261.5500795238$1520960100$gmane$org@ubuntu.com> References: <20180313165527.24038-1-christian.brauner@ubuntu.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180313165527.24038-1-christian.brauner-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org, torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org Cc: containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Christian Brauner List-Id: containers.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 /. A very simply reproducer for this issue presupposing a libc that uses TIOCGPTPEER in its openpty() implementation is: unshare --mount mount --bind /dev/pts/ptmx /dev/ptmx chmod 666 /dev/ptmx script ls -al /proc/self/fd/0 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. The reason for the current failure is that the kernel tries to verify the useability of the devpts filesystem without resolving the /dev/ptmx bind-mount first. This will lead it to detect that the dentry is escaping its bind-mount. The reason is that while the devpts filesystem mounted at /dev/pts has the devtmpfs mounted at /dev as its parent mount: 21 -- -- / /dev -- 21 -- / /dev/pts devtmpfs and devpts are on different devices -- -- 0:6 / /dev -- -- 0:20 / /dev/pts This has the consequence that the pathname of the parent directory of the devpts filesystem mount at /dev/pts is /. So if /dev/ptmx is a bind-mount of /dev/pts/ptmx then the /dev/ptmx bind-mount and the devpts mount at /dev/pts will end up being located on the same device which is recorded in the superblock of their vfsmount. This means the parent directory of the /dev/ptmx bind-mount will be /ptmx: -- -- ---- /ptmx /dev/ptmx Without the bind-mount resolution patch the kernel will now perform the bind-mount escape check directly on /dev/ptmx. The function responsible for this is devpts_ptmx_path() which calls pts_path() which in turn calls path_parent_directory(). Based on the above explanation, path_parent_directory() will yield / as the parent directory for the /dev/ptmx bind-mount and not the expected /dev. Thus, the kernel detects that /dev/ptmx is escaping its bind-mount and will set /proc//fd/ to /. This patch changes the logic to first resolve any bind-mounts. After the bind-mounts have been resolved (i.e. we have traced it back to the associated devpts mount) devpts_ptmx_path() can be called. In order to guarantee correct path generation for the slave file descriptor the kernel now requires that a pts directory is found in the parent directory of the ptmx bind-mount. This implies that when doing bind-mounts the ptmx bind-mount and the devpts mount should have a common parent directory. A valid example is: mount -t devpts devpts /dev/pts mount --bind /dev/pts/ptmx /dev/ptmx an invalid example is: mount -t devpts devpts /dev/pts mount --bind /dev/pts/ptmx /ptmx This allows us to support: - calling open on ptmx devices located inside non-standard devpts mounts: mount -t devpts devpts /mnt master = open("/mnt/ptmx", ...); slave = ioctl(master, TIOCGPTPEER, ...); - calling open on ptmx devices located outside the devpts mount with a common ancestor directory: mount -t devpts devpts /dev/pts mount --bind /dev/pts/ptmx /dev/ptmx master = open("/dev/ptmx", ...); slave = ioctl(master, TIOCGPTPEER, ...); while failing on ptmx devices located outside the devpts mount without a common ancestor directory: mount -t devpts devpts /dev/pts mount --bind /dev/pts/ptmx /ptmx master = open("/ptmx", ...); slave = ioctl(master, TIOCGPTPEER, ...); in which case save path generation cannot be guaranteed. Signed-off-by: Christian Brauner Suggested-by: Eric Biederman Suggested-by: Linus Torvalds --- ChangeLog v4->v5: * reverse error handling logic to further simplify (Linus) ChangeLog v3->v4: * simplify if condition (Eric) ChangeLog v2->v3: * rework logic to account for non-standard devpts mounts such as mount -t devpts devpts /mnt (Christian) ChangeLog v1->v2: * move removal of if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC) condition to separate patch with non-functional changes (Linus) 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() (Eric) * check superblock after devpts_ptmx_path() returned (Christian) --- fs/devpts/inode.c | 26 ++++++++++++++++---------- 1 file changed, 16 insertions(+), 10 deletions(-) diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c index 71b901936113..542364bf923e 100644 --- a/fs/devpts/inode.c +++ b/fs/devpts/inode.c @@ -160,21 +160,27 @@ struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi) path = filp->f_path; path_get(&path); - /* Has the devpts filesystem already been found? */ - if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) + /* 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; + + /* devpts_ptmx_path() finds a devpts fs or returns an error. */ + if ((path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) || + (DEVPTS_SB(path.mnt->mnt_sb) != fsi)) err = devpts_ptmx_path(&path); dput(path.dentry); - if (err) { - mntput(path.mnt); - return ERR_PTR(err); - } + if (!err) { + if (DEVPTS_SB(path.mnt->mnt_sb) == fsi) + return path.mnt; - if (DEVPTS_SB(path.mnt->mnt_sb) != fsi) { - mntput(path.mnt); - return ERR_PTR(-ENODEV); + err = -ENODEV; } - return path.mnt; + mntput(path.mnt); + return ERR_PTR(err); } struct pts_fs_info *devpts_acquire(struct file *filp) -- 2.15.1