Most libcs will still look at /dev/ptmx when opening the master fd of 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 it's 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 this situation correctly we walk up the bind-mounts for the /dev/ptmx
file.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 <[email protected]> Suggested-by: Eric Biederman <[email protected]> Suggested-by: Linus Torvalds <[email protected]> --- fs/devpts/inode.c | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c index e31d6ed3ec32..4059e3e69d57 100644 --- a/fs/devpts/inode.c +++ b/fs/devpts/inode.c @@ -163,6 +163,26 @@ struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi) path = filp->f_path; path_get(&path); + 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; + + /* Is this path a valid devpts filesystem? */ + err = devpts_ptmx_path(&path); + if (err == 0) { + dput(path.dentry); + return path.mnt; + } + + path_put(&path); + path = filp->f_path; + path_get(&path); + } err = devpts_ptmx_path(&path); dput(path.dentry); -- 2.15.1
