vfs: Only support slave subtrees across different user namespaces

Sharing mount subtress with mount namespaces created by unprivileged
users allows unprivileged mounts created by unprivileged users to
propagate to mount namespaces controlled by privileged users.

Prevent nasty consequences by changing shared subtrees to slave
subtress when an unprivileged users creates a new mount namespace.

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

fs/namespace.c

@@ -786,7 +786,7 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
 	if (!mnt)
 		return ERR_PTR(-ENOMEM);
 
-	if (flag & (CL_SLAVE | CL_PRIVATE))
+	if (flag & (CL_SLAVE | CL_PRIVATE | CL_SHARED_TO_SLAVE))
 		mnt->mnt_group_id = 0; /* not a peer of original */
 	else
 		mnt->mnt_group_id = old->mnt_group_id;
@@ -807,7 +807,8 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
 	list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
 	br_write_unlock(&vfsmount_lock);
 
-	if (flag & CL_SLAVE) {
+	if ((flag & CL_SLAVE) ||
+	    ((flag & CL_SHARED_TO_SLAVE) && IS_MNT_SHARED(old))) {
 		list_add(&mnt->mnt_slave, &old->mnt_slave_list);
 		mnt->mnt_master = old;
 		CLEAR_MNT_SHARED(mnt);
@@ -2331,6 +2332,7 @@ static struct mnt_namespace *dup_mnt_ns(struct mnt_namespace *mnt_ns,
 	struct mount *p, *q;
 	struct mount *old = mnt_ns->root;
 	struct mount *new;
+	int copy_flags;
 
 	new_ns = alloc_mnt_ns(user_ns);
 	if (IS_ERR(new_ns))
@@ -2338,7 +2340,10 @@ static struct mnt_namespace *dup_mnt_ns(struct mnt_namespace *mnt_ns,
 
 	down_write(&namespace_sem);
 	/* First pass: copy the tree topology */
-	new = copy_tree(old, old->mnt.mnt_root, CL_COPY_ALL | CL_EXPIRE);
+	copy_flags = CL_COPY_ALL | CL_EXPIRE;
+	if (user_ns != mnt_ns->user_ns)
+		copy_flags |= CL_SHARED_TO_SLAVE;
+	new = copy_tree(old, old->mnt.mnt_root, copy_flags);
 	if (IS_ERR(new)) {
 		up_write(&namespace_sem);
 		free_mnt_ns(new_ns);

fs/pnode.h

@@ -22,6 +22,7 @@
 #define CL_COPY_ALL 		0x04
 #define CL_MAKE_SHARED 		0x08
 #define CL_PRIVATE 		0x10
+#define CL_SHARED_TO_SLAVE	0x20
 
 static inline void set_mnt_shared(struct mount *mnt)
 {