[PATCH 2/3 v3] devpts: resolve devpts bind-mounts

[Christian Brauner]

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(). Since the
contents of /proc/<pid>/fd/<nr> symlinks attached to the slave side of a
file descriptor will always point to a path under the devpts mount we need
to try and ensure that the kernel doesn't falsely get the impression that a
pty slave file descriptor retrieved via TIOCGPTPEER based on a pty master
file descriptor opened via a bind-mount of the ptmx device escapes its
bind-mount. To clarify in pseudo code:

* bind-mount /dev/pts/ptmx to /dev/ptmx
* master = open("/dev/ptmx", O_RDWR | O_NOCTTY | O_CLOEXEC);
* slave = ioctl(master, TIOCGPTPEER, O_RDWR | O_NOCTTY | O_CLOEXEC);

would cause the kernel to think that slave is escaping its bind-mount. 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 at
/dev/pts

-- -- 0:20 /ptmx /dev/ptmx

Without the bind-mount resolution patch here the kernel will now perform
the bind-mount escape check directly on /dev/ptmx. When it hits
devpts_ptmx_path()  calls pts_path() which in turn calls
path_parent_directory(). While one would expect that
path_parent_directory() *should* yield /dev it will yield / since
/dev and /dev/pts are on different devices. This will cause path_pts() to
fail finding a "pts" directory since there is none under /. Thus, the
kernel detects that /dev/ptmx is escaping its bind-mount and will set
/proc/<pid>/fd/<nr> to /.

This patch changes the logic to do bind-mount resolution and after the
bind-mount has been resolved (i.e. we have traced it back to the devpts
mount) we can safely perform devpts_ptmx_path() and check whether we find a
"pts" directory in the parent directory of the devpts mount. Since
path_parent_directory() will now correctly yield /dev as parent directory
for the devpts mount at /dev/pts.

However, we can only perform devpts_ptmx_path() devpts_mntget() if we
either did resolve a bind-mount or did not find a suitable devpts
filesystem. The reason is that we want and need to support non-standard
mountpoints for the devpts filesystem. If we call devpts_ptmx_path()
although we did already find a devpts filesystem and did not resolve
bind-mounts we will fail on devpts mounts such as:

mount -t devpts devpts /mnt

where no "pts" directory will be under /. So change the logic to account
for this.

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 <christian.brauner@xxxxxxxxxx>
Suggested-by: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
ChangeLog v2->v3:
* rework logic to account for non-standard devpts mounts such as
  mount -t devpts devpts /mnt
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 | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index d3ddbb888874..b31362c6c548 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -154,27 +154,35 @@ static int devpts_ptmx_path(struct path *path)
 
 struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
 {
+	bool unwind;
 	struct path path;
+	int err = 0;
 
 	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;
+	unwind = (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 && unwind)
+		if (follow_up(&path) == 0)
+			break;
 
+	if ((path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) || unwind)
 		err = devpts_ptmx_path(&path);
-		dput(path.dentry);
-		if (err) {
-			mntput(path.mnt);
-			return ERR_PTR(err);
-		}
+	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