fs: allow clone_private_mount() for a path on real rootfs

Mounting overlayfs with a directory on real rootfs (initramfs)
as upperdir has failed with following message since commit
db04662e2f4f ("fs: allow detached mounts in clone_private_mount()").

  [    4.080134] overlayfs: failed to clone upperpath

Overlayfs mount uses clone_private_mount() to create internal mount
for the underlying layers.

The commit made clone_private_mount() reject real rootfs because
it does not have a parent mount and is in the initial mount namespace,
that is not an anonymous mount namespace.

This issue can be fixed by modifying the permission check
of clone_private_mount() following [1].

Reviewed-by: Christian Brauner <brauner@kernel.org>
Fixes: db04662e2f4f ("fs: allow detached mounts in clone_private_mount()")
Link: https://lore.kernel.org/all/20250514190252.GQ2023217@ZenIV/
Link: https://lore.kernel.org/all/20250506194849.GT2023217@ZenIV/
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Kazuma Kondo <kazuma-kondo@nec.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namespace.c b/fs/namespace.c
index 6c94ecbe2c2c5e18a2a98ad56a4025e1a6d3016e..854099aafed517bb61ac3016a8e8f6cd3f5bb562 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2493,18 +2493,19 @@ struct vfsmount *clone_private_mount(const struct path *path)
        if (IS_MNT_UNBINDABLE(old_mnt))
                return ERR_PTR(-EINVAL);
 
-       if (mnt_has_parent(old_mnt)) {
-               if (!check_mnt(old_mnt))
-                       return ERR_PTR(-EINVAL);
-       } else {
-               if (!is_mounted(&old_mnt->mnt))
-                       return ERR_PTR(-EINVAL);
-
-               /* Make sure this isn't something purely kernel internal. */
-               if (!is_anon_ns(old_mnt->mnt_ns))
+       /*
+        * Make sure the source mount is acceptable.
+        * Anything mounted in our mount namespace is allowed.
+        * Otherwise, it must be the root of an anonymous mount
+        * namespace, and we need to make sure no namespace
+        * loops get created.
+        */
+       if (!check_mnt(old_mnt)) {
+               if (!is_mounted(&old_mnt->mnt) ||
+                       !is_anon_ns(old_mnt->mnt_ns) ||
+                       mnt_has_parent(old_mnt))
                        return ERR_PTR(-EINVAL);
 
-               /* Make sure we don't create mount namespace loops. */
                if (!check_for_nsfs_mounts(old_mnt))
                        return ERR_PTR(-EINVAL);
        }