Quoting Eric W. Biederman (ebied...@xmission.com):
>
> - 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.
>
> Signed-off-by: "Eric W. Biederman" <ebied...@xmission.com>
Acked-by: Serge Hallyn <serge.hal...@canonical.com>
> ---
> fs/devpts/inode.c | 18 ++++++++++++++++++
> 1 files changed, 18 insertions(+), 0 deletions(-)
>
> diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
> index 472e6be..073d30b 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
> };
>
> /*
> --
> 1.7.5.4
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