userns: Allow the userns root to mount of devpts

- The context in which devpts is mounted has no effect on the creation
  of ptys as the /dev/ptmx interface has been used by unprivileged
  users for many years.

- Only support unprivileged mounts in combination with the newinstance
  option to ensure that mounting of /dev/pts in a user namespace will
  not allow the options of an existing mount of devpts to be modified.

- Create /dev/pts/ptmx as the root user in the user namespace that
  mounts devpts so that it's permissions to be changed.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index 472e6befc54d..073d30b9d1ac 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -243,6 +243,13 @@ static int mknod_ptmx(struct super_block *sb)
         struct dentry *root = sb->s_root;
         struct pts_fs_info *fsi = DEVPTS_SB(sb);
         struct pts_mount_opts *opts = &fsi->mount_opts;
+        kuid_t root_uid;
+        kgid_t root_gid;
+
+        root_uid = make_kuid(current_user_ns(), 0);
+        root_gid = make_kgid(current_user_ns(), 0);
+        if (!uid_valid(root_uid) || !gid_valid(root_gid))
+                return -EINVAL;
 
         mutex_lock(&root->d_inode->i_mutex);
 
@@ -273,6 +280,8 @@ static int mknod_ptmx(struct super_block *sb)
 
         mode = S_IFCHR|opts->ptmxmode;
         init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
+        inode->i_uid = root_uid;
+        inode->i_gid = root_gid;
 
         d_add(dentry, inode);
 
@@ -438,6 +447,12 @@ static struct dentry *devpts_mount(struct file_system_type *fs_type,
         if (error)
                 return ERR_PTR(error);
 
+        /* Require newinstance for all user namespace mounts to ensure
+         * the mount options are not changed.
+         */
+        if ((current_user_ns() != &init_user_ns) && !opts.newinstance)
+                return ERR_PTR(-EINVAL);
+
         if (opts.newinstance)
                 s = sget(fs_type, NULL, set_anon_super, flags, NULL);
         else
@@ -491,6 +506,9 @@ static struct file_system_type devpts_fs_type = {
         .name           = "devpts",
         .mount          = devpts_mount,
         .kill_sb        = devpts_kill_sb,
+#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
+        .fs_flags       = FS_USERNS_MOUNT | FS_USERNS_DEV_MOUNT,
+#endif
 };
 
 /*