do_change_type(): refuse to operate on unmounted/not ours mounts

jira VULN-98611
cve CVE-2025-38498
commit-author Al Viro <[email protected]>
commit 12f147d

Ensure that propagation settings can only be changed for mounts located
in the caller's mount namespace. This change aligns permission checking
with the rest of mount(2).

	Reviewed-by: Christian Brauner <[email protected]>
Fixes: 07b2088 ("beginning of the shared-subtree proper")
	Reported-by: "Orlando, Noah" <[email protected]>
	Signed-off-by: Al Viro <[email protected]>
(cherry picked from commit 12f147d)
	Signed-off-by: Shreeya Patel <[email protected]>

Author: shreeya-patel98

fs/namespace.c

@@ -2323,6 +2323,10 @@ static int do_change_type(struct path *path, int ms_flags)
 		return -EINVAL;
 
 	namespace_lock();
+	if (!check_mnt(mnt)) {
+		err = -EINVAL;
+		goto out_unlock;
+	}
 	if (type == MS_SHARED) {
 		err = invent_group_ids(mnt, recurse);
 		if (err)