sshd: Return error when proxy policy is misconfigured

MustStrictWhiteListPolicy panics when configured incorrectly. So here we
use the error returning version instead.

internal/sshd/sshd.go

@@ -95,9 +95,14 @@ func (s *Server) listen(ctx context.Context) error {
 	}
 
 	if s.Config.Server.ProxyProtocol {
+		policy, err := s.proxyPolicy()
+		if err != nil {
+			return fmt.Errorf("invalid policy configuration: %w", err)
+		}
+
 		sshListener = &proxyproto.Listener{
 			Listener:          sshListener,
-			Policy:            s.requirePolicy(),
+			Policy:            policy,
 			ReadHeaderTimeout: time.Duration(s.Config.Server.ProxyHeaderTimeout),
 		}
 
@@ -200,22 +205,22 @@ func (s *Server) handleConn(ctx context.Context, nconn net.Conn) {
 	})
 }
 
-func (s *Server) requirePolicy() proxyproto.PolicyFunc {
+func (s *Server) proxyPolicy() (proxyproto.PolicyFunc, error) {
 	if len(s.Config.Server.ProxyAllowed) > 0 {
-		return proxyproto.MustStrictWhiteListPolicy(s.Config.Server.ProxyAllowed)
+		return proxyproto.StrictWhiteListPolicy(s.Config.Server.ProxyAllowed)
 	}
 
 	// Set the Policy value based on config
 	// Values are taken from https://github.com/pires/go-proxyproto/blob/195fedcfbfc1be163f3a0d507fac1709e9d81fed/policy.go#L20
 	switch strings.ToLower(s.Config.Server.ProxyPolicy) {
 	case "require":
-		return staticProxyPolicy(proxyproto.REQUIRE)
+		return staticProxyPolicy(proxyproto.REQUIRE), nil
 	case "ignore":
-		return staticProxyPolicy(proxyproto.IGNORE)
+		return staticProxyPolicy(proxyproto.IGNORE), nil
 	case "reject":
-		return staticProxyPolicy(proxyproto.REJECT)
+		return staticProxyPolicy(proxyproto.REJECT), nil
 	default:
-		return staticProxyPolicy(proxyproto.USE)
+		return staticProxyPolicy(proxyproto.USE), nil
 	}
 }