tcp: mark ISN and timestamp-offset secrets nosave for checkpoint

pkg/tcpip/transport/tcp/BUILD

@@ -162,6 +162,7 @@ go_library(
         "pending_processing_mutex.go",
         "protocol.go",
         "protocol_mutex.go",
+        "protocol_state.go",
         "rack.go",
         "rcv.go",
         "rcv_queue_mutex.go",

pkg/tcpip/transport/tcp/protocol.go

@@ -114,9 +114,11 @@ type protocol struct {
 	// This is immutable after creation.
 	probe TCPProbeFunc `state:"nosave"`
 
-	// The following secrets are initialized once and stay unchanged after.
-	seqnumSecret   [16]byte
-	tsOffsetSecret [16]byte
+	// The following secrets are used for ISN and timestamp-offset
+	// generation. They are not serialized into checkpoint state and are
+	// freshly drawn from the secure RNG on restore (see afterLoad).
+	seqnumSecret   [16]byte `state:"nosave"`
+	tsOffsetSecret [16]byte `state:"nosave"`
 }
 
 // Number returns the tcp protocol number.

pkg/tcpip/transport/tcp/protocol_state.go

@@ -0,0 +1,31 @@
+// Copyright 2026 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tcp
+
+import (
+	"context"
+	"fmt"
+)
+
+// afterLoad is invoked by stateify.
+func (p *protocol) afterLoad(ctx context.Context) {
+	rng := p.stack.SecureRNG()
+	if n, err := rng.Reader.Read(p.seqnumSecret[:]); err != nil || n != len(p.seqnumSecret) {
+		panic(fmt.Sprintf("rng.Reader.Read(seqnumSecret) failed: n=%d err=%v", n, err))
+	}
+	if n, err := rng.Reader.Read(p.tsOffsetSecret[:]); err != nil || n != len(p.tsOffsetSecret) {
+		panic(fmt.Sprintf("rng.Reader.Read(tsOffsetSecret) failed: n=%d err=%v", n, err))
+	}
+}