[PATCH 03/10] teach move_mount(2) to work with OPEN_TREE_CLONE

[Alan Jenkins]

On 19/02/2019 17:08, David Howells wrote:
> Allow a detached tree created by open_tree(..., OPEN_TREE_CLONE) to be
> attached by move_mount(2).
>
> If by the time of final fput() of OPEN_TREE_CLONE-opened file its tree is
> not detached anymore, it won't be dissolved.  move_mount(2) is adjusted
> to handle detached source.
>
> That gives us equivalents of mount --bind and mount --rbind.
> Thanks also to Alan Jenkins<alan.christopher.jenkins at gmail.com>  for
> providing a whole bunch of ways to break things using this interface.
>
> Signed-off-by: Al Viro<viro at zeniv.linux.org.uk>
> Signed-off-by: David Howells<dhowells at redhat.com>
> Signed-off-by: Al Viro<viro at zeniv.linux.org.uk>
> ---
>
>   fs/namespace.c |   62 ++++++++++++++++++++++++++++++++++++++++++++++++++------
>   1 file changed, 55 insertions(+), 7 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index f10122028a11..56423c60ac7e 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -1840,10 +1840,16 @@ void dissolve_on_fput(struct vfsmount *mnt)
>   	namespace_lock();
>   	lock_mount_hash();
>   	ns = real_mount(mnt)->mnt_ns;
> -	umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
> +	if (ns) {
> +		if (is_anon_ns(ns))
> +			umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
> +		else
> +			ns = NULL;
> +	}
>   	unlock_mount_hash();
>   	namespace_unlock();
> -	free_mnt_ns(ns);
> +	if (ns)
> +		free_mnt_ns(ns);
>   }
>   
>   void drop_collected_mounts(struct vfsmount *mnt)
> @@ -2079,6 +2085,10 @@ static int attach_recursive_mnt(struct mount *source_mnt,
>   		attach_mnt(source_mnt, dest_mnt, dest_mp);
>   		touch_mnt_namespace(source_mnt->mnt_ns);
>   	} else {
> +		if (source_mnt->mnt_ns) {
> +			/* move from anon - the caller will destroy */
> +			list_del_init(&source_mnt->mnt_ns->list);
> +		}
>   		mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
>   		commit_tree(source_mnt);
>   	}
> @@ -2537,13 +2547,37 @@ static inline int tree_contains_unbindable(struct mount *mnt)
>   	return 0;
>   }
>   
> +/*
> + * Check that there aren't references to earlier/same mount namespaces in the
> + * specified subtree.  Such references can act as pins for mount namespaces
> + * that aren't checked by the mount-cycle checking code, thereby allowing
> + * cycles to be made.
> + */
> +static bool check_for_nsfs_mounts(struct mount *subtree)
> +{
> +	struct mount *p;
> +	bool ret = false;
> +
> +	lock_mount_hash();
> +	for (p = subtree; p; p = next_mnt(p, subtree))
> +		if (mnt_ns_loop(p->mnt.mnt_root))
> +			goto out;
> +
> +	ret = true;
> +out:
> +	unlock_mount_hash();
> +	return ret;
> +}
> +
>   static int do_move_mount(struct path *old_path, struct path *new_path)
>   {
>   	struct path parent_path = {.mnt = NULL, .dentry = NULL};
> +	struct mnt_namespace *ns;
>   	struct mount *p;
>   	struct mount *old;
>   	struct mountpoint *mp;
>   	int err;
> +	bool attached;
>   
>   	mp = lock_mount(new_path);
>   	if (IS_ERR(mp))
> @@ -2551,12 +2585,19 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
>   
>   	old = real_mount(old_path->mnt);
>   	p = real_mount(new_path->mnt);
> +	attached = mnt_has_parent(old);
> +	ns = old->mnt_ns;
>   
>   	err = -EINVAL;
> -	if (!check_mnt(p) || !check_mnt(old))
> +	/* The mountpoint must be in our namespace. */
> +	if (!check_mnt(p))
>   		goto out;
>   
> -	if (!mnt_has_parent(old))
> +	/* The thing moved should be either ours or completely unattached. */
> +	if (attached && !check_mnt(old))
> +		goto out;
> +
> +	if (!attached && !is_anon_ns(ns))

I think this is missing a check for ns != NULL, before passing it to 
is_anon_ns().

E.g. in case I called umount2(old_path, MNT_DETACH) beforehand.

>   		goto out;
>   
>   	if (old->mnt.mnt_flags & MNT_LOCKED)
> @@ -2571,7 +2612,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
>   	/*
>   	 * Don't move a mount residing in a shared parent.
>   	 */
> -	if (IS_MNT_SHARED(old->mnt_parent))
> +	if (attached && IS_MNT_SHARED(old->mnt_parent))
>   		goto out;
>   	/*
>   	 * Don't move a mount tree containing unbindable mounts to a destination
> @@ -2580,12 +2621,14 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
>   	if (IS_MNT_SHARED(p) && tree_contains_unbindable(old))
>   		goto out;
>   	err = -ELOOP;
> +	if (!check_for_nsfs_mounts(old))
> +		goto out;
>   	for (; mnt_has_parent(p); p = p->mnt_parent)
>   		if (p == old)
>   			goto out;
>   
>   	err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp,
> -				   &parent_path);
> +				   attached ? &parent_path : NULL);
>   	if (err)
>   		goto out;
>   
> @@ -2594,8 +2637,11 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
>   	list_del_init(&old->mnt_expire);
>   out:
>   	unlock_mount(mp);
> -	if (!err)
> +	if (!err) {
>   		path_put(&parent_path);
> +		if (!attached)
> +			free_mnt_ns(ns);
> +	}
>   	return err;
>   }
>   
> @@ -3289,6 +3335,8 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
>   
>   /*
>    * Move a mount from one place to another.
> + * In combination with open_tree(OPEN_TREE_CLONE [| AT_RECURSIVE]) it can be
> + * used to copy a mount subtree.
>    *
>    * Note the flags value is a combination of MOVE_MOUNT_* flags.
>    */
>
>