teach move_mount(2) to work with OPEN_TREE_CLONE

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@gmail.com> for
providing a whole bunch of ways to break things using this interface.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 55 insertions, 7 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index dc600f53de9d..1e72d19fa4f8 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1842,10 +1842,16 @@ 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)
@@ -2081,6 +2087,10 @@ 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);
         }
@@ -2539,13 +2549,37 @@ 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))
@@ -2553,12 +2587,19 @@ 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))
                 goto out;
 
         if (old->mnt.mnt_flags & MNT_LOCKED)
@@ -2573,7 +2614,7 @@ 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
@@ -2582,12 +2623,14 @@ 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;
 
@@ -2596,8 +2639,11 @@ 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.
  */