[PATCH 06/17] namespace: keep covered mounts covered in OPEN_TREE_NAMESPACE

[Christian Brauner <brauner@kernel.org>]

open_tree(OPEN_TREE_NAMESPACE) creates a new mount namespace from a
copy of the tree at the given path. The caller needs to be privileged
over its current user namespace because the new mount namespace will be
owned by it. It doesn't need to be privileged over the mount namespace
the tree is copied from. An unprivileged user just needs to create a
user namespace first.

The copy follows the rules of a detached bind mount. Without
AT_RECURSIVE only the mount itself is cloned and its children are left
out. With AT_RECURSIVE unbindable mounts are skipped. Both is fine when
the caller has privileges over the source mount namespace because it
could unmount those mounts anyway. Not so for an unprivileged user in
that mount namespace. For them a mount namespace copy is what unshare()
does. copy_mnt_ns() copies everything including unbindable mounts and
lock_mnt_tree() makes sure nothing can be unmounted in the copy. Nothing
that was covered gets revealed.

create_new_namespace() only does the locking and so the covered content
is right there in the new mount namespace:

  # mount -t tmpfs none /tmp/otn; mkdir /tmp/otn/covered
  # echo hidden > /tmp/otn/covered/under.txt
  # mount -t tmpfs none /tmp/otn/covered

  uid 1000, after unshare(CLONE_NEWUSER):
  fd = open_tree(AT_FDCWD, "/tmp/otn", OPEN_TREE_NAMESPACE);
  setns(fd, CLONE_NEWNS);
  open("/covered/under.txt", O_RDONLY)  -> "hidden"

Covering paths with mounts is how container runtimes mask parts of
/proc and /sys and how admins hide things.

When the caller's user namespace doesn't own the source mount namespace
copy the way unshare() does and refuse a non-recursive copy of a mount
that has anything mounted below the requested directory and copy
unbindable mounts in a recursive copy. A caller that is privileged over
the source mount namespace sees no change.

Fixes: 9b8a0ba68246 ("mount: add OPEN_TREE_NAMESPACE")
Cc: stable@vger.kernel.org # v7.0+
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/namespace.c | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index c2f54636ec9d..de3900dd02f6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2369,6 +2369,18 @@ bool has_locked_children(struct mount *mnt, struct dentry *dentry)
 	return __has_locked_children(mnt, dentry);
 }
 
+/* locks: namespace_shared && pinned(mnt) || mount_locked_reader */
+static bool __has_children(struct mount *mnt, struct dentry *dentry)
+{
+	struct mount *child;
+
+	list_for_each_entry(child, &mnt->mnt_mounts, mnt_child) {
+		if (is_subdir(child->mnt_mountpoint, dentry))
+			return true;
+	}
+	return false;
+}
+
 /*
  * Check that there aren't references to earlier/same mount namespaces in the
  * specified subtree.  Such references can act as pins for mount namespaces
@@ -3141,12 +3153,18 @@ static struct mnt_namespace *create_new_namespace(struct path *path,
 	struct mount *mnt;
 	unsigned int copy_flags = 0;
 	bool locked = false, recurse = flags & MOUNT_COPY_RECURSIVE;
+	bool foreign = user_ns != ns->user_ns;
 
 	if (unlikely(!d_can_lookup(path->dentry)))
 		return ERR_PTR(-ENOTDIR);
 
-	if (user_ns != ns->user_ns)
-		copy_flags |= CL_SLAVE;
+	/*
+	 * Without privileges over the mount namespace the copy is made from
+	 * nothing mounted below @path may be left out. It would reveal what
+	 * it covers. That's what unshare() gives such a caller as well.
+	 */
+	if (foreign)
+		copy_flags |= CL_SLAVE | CL_COPY_UNBINDABLE;
 
 	new_ns = alloc_mnt_ns(user_ns, false);
 	if (IS_ERR(new_ns))
@@ -3180,10 +3198,14 @@ static struct mnt_namespace *create_new_namespace(struct path *path,
 	/*
 	 * We don't emulate unshare()ing a mount namespace. We stick to
 	 * the restrictions of creating detached bind-mounts. It has a
-	 * lot saner and simpler semantics.
+	 * lot saner and simpler semantics. A caller without privileges
+	 * over the mount namespace can't leave out any child though.
 	 */
 	if (flags & MOUNT_COPY_NEW)
 		mnt = clone_mnt(real_mount(path->mnt), path->dentry, copy_flags);
+	else if (foreign && !recurse &&
+		 __has_children(real_mount(path->mnt), path->dentry))
+		mnt = ERR_PTR(-EINVAL);
 	else
 		mnt = __do_loopback(path, recurse, copy_flags);
 	scoped_guard(mount_writer) {
@@ -3200,7 +3222,7 @@ static struct mnt_namespace *create_new_namespace(struct path *path,
 		 * of the real rootfs we created.
 		 */
 		attach_mnt(mnt, new_ns_root, mp.mp);
-		if (user_ns != ns->user_ns)
+		if (foreign)
 			lock_mnt_tree(new_ns_root);
 	}
 

-- 
2.53.0