clone_private_mnt(): make sure that caller has CAP_SYS_ADMIN in the right userns

JIRA: https://issues.redhat.com/browse/RHEL-129257
CVE: CVE-2025-38499

commit c28f922
Author: Al Viro <viro@zeniv.linux.org.uk>
Date:   Sun Jun 1 20:11:06 2025 -0400

    clone_private_mnt(): make sure that caller has CAP_SYS_ADMIN in the right userns

    What we want is to verify there is that clone won't expose something
    hidden by a mount we wouldn't be able to undo.  "Wouldn't be able to undo"
    may be a result of MNT_LOCKED on a child, but it may also come from
    lacking admin rights in the userns of the namespace mount belongs to.

    clone_private_mnt() checks the former, but not the latter.

    There's a number of rather confusing CAP_SYS_ADMIN checks in various
    userns during the mount, especially with the new mount API; they serve
    different purposes and in case of clone_private_mnt() they usually,
    but not always end up covering the missing check mentioned above.

    Reviewed-by: Christian Brauner <brauner@kernel.org>
    Reported-by: "Orlando, Noah" <Noah.Orlando@deshaw.com>
    Fixes: 427215d ("ovl: prevent private clone if bind mount is not allowed")
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>
Signed-off-by: Abhi Das <adas@redhat.com>

Author: CKI Backport Bot

fs/namespace.c

@@ -2004,6 +2004,11 @@ struct vfsmount *clone_private_mount(const struct path *path)
 	if (!check_mnt(old_mnt))
 		goto invalid;
 
+        if (!ns_capable(old_mnt->mnt_ns->user_ns, CAP_SYS_ADMIN)) {
+		up_read(&namespace_sem);
+		return ERR_PTR(-EPERM);
+	}
+
 	if (__has_locked_children(old_mnt, path->dentry))
 		goto invalid;