[PATCH 0/3 v3] devpts: handle bind-mounts

[Christian Brauner]

Hey everyone,

This is the third iteration of this patch. Major changes are:
- rewritten commit message to thoroughly analyse the problem for
  posterity in Subject: [PATCH 2/3 v3] devpts: resolve devpts bind-mounts
  and in this cover letter.
- extended selftests to test for correct handling of /dev/pts/ptmx
  bind-mounts to /dev/ptmx and non-standard devpts mounts such as
  mount -t devpts devpts /mnt

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 -> /

Christian Brauner (3):
  devpts: hoist out check for DEVPTS_SUPER_MAGIC
  devpts: resolve devpts bind-mounts
  selftests: add devpts selftests

 fs/devpts/inode.c                                |  35 ++-
 tools/testing/selftests/Makefile                 |   1 +
 tools/testing/selftests/filesystems/.gitignore   |   1 +
 tools/testing/selftests/filesystems/Makefile     |   2 +-
 tools/testing/selftests/filesystems/devpts_pts.c | 281 +++++++++++++++++++++++
 5 files changed, 308 insertions(+), 12 deletions(-)
 create mode 100644 tools/testing/selftests/filesystems/devpts_pts.c

-- 
2.15.1