nsfs: tighten permission checks for handle opening

Even privileged services should not necessarily be able to see other
privileged service's namespaces so they can't leak information to each
other. Use may_see_all_namespaces() helper that centralizes this policy
until the nstree adapts.

Link: https://patch.msgid.link/20260226-work-visibility-fixes-v1-2-d2c2853313bd@kernel.org
Fixes: 5222470 ("nsfs: support file handles")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Cc: stable@kernel.org # v6.18+
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/nsfs.c

@@ -627,7 +627,7 @@ static struct dentry *nsfs_fh_to_dentry(struct super_block *sb, struct fid *fh,
 		return ERR_PTR(-EOPNOTSUPP);
 	}
 
-	if (owning_ns && !ns_capable(owning_ns, CAP_SYS_ADMIN)) {
+	if (owning_ns && !may_see_all_namespaces()) {
 		ns->ops->put(ns);
 		return ERR_PTR(-EPERM);
 	}