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

[ibondarenko1]

Summary

pkg/tcpip/transport/tcp/protocol.go is annotated +stateify savable. Two of its fields are 16-byte CSPRNG secrets used for ISN and timestamp-offset generation (RFC 6528):

// protocol.go:88-119 (before this PR)
// +stateify savable
type protocol struct {
    ...
    // The following secrets are initialized once and stay unchanged after.
    seqnumSecret   [16]byte
    tsOffsetSecret [16]byte
}

Neither field carries state:"nosave". The stateify generator therefore writes both 16-byte values verbatim into the checkpoint state stream. A reader of a checkpoint file recovers the secrets and predicts ISNs and timestamp offsets on the restored sandbox.

Background

Sibling fields in the same struct already carry the tag:

• protocol.go:92: mu protocolRWMutex with state:"nosave"
• protocol.go:115: probe TCPProbeFunc with state:"nosave"

The source of randomness itself, secureRNG, is also state:"nosave" at pkg/tcpip/stack/stack.go:159. The absence of the tag on seqnumSecret / tsOffsetSecret is an omission against the established codebase convention, not a deliberate choice.

Linux / BSD reference

Linux stores its ISN secret (net_secret in net/core/secure_seq.c) in kernel memory per netns; FreeBSD and OpenBSD store theirs in tcp_subr.c kernel memory. None of those stacks have a checkpoint/restore equivalent. CRIU does not dump kernel memory, so the Linux ISN secret is not exposed through the closest analog. The checkpoint surface is unique to gVisor save-restore.

CVE-2024-10026 and the NDSS 2025 paper "You Can Rand but You Can't Hide" (Kaplan, Even, Klein; commits 83f75082e5, e54bfde792, f956b5ac17, fixed in v20231204.0.0) improved the cryptographic quality of these same secrets. Persistence into checkpoint state was not addressed by that work.

Change

After this PR, protocol.go:114-120 reads:

probe TCPProbeFunc `state:"nosave"`

// 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"`

New file pkg/tcpip/transport/tcp/protocol_state.go adds an afterLoad hook that redraws both secrets from p.stack.SecureRNG() on restore.

Save: stateify omits both fields from the checkpoint blob, which now contains neither the bytes nor the field names.
Load: stateify restores the protocol with both fields zeroed; afterLoad reseeds from stack.SecureRNG().
Existing live behavior at newProtocol() (initial seeding at construction) is unchanged.

Tests

New file pkg/tcpip/transport/tcp/protocol_state_test.go adds two regression tests:

• TestProtocolSecretsHaveNosaveTag is a reflection-based structural check that both secret fields carry state:"nosave". Removing the tag in a future change would fail this test.
• TestProtocolAfterLoadRegeneratesSecrets zeroes the two secrets (simulating the post-restore state for nosave fields), invokes afterLoad, and asserts both fields become non-zero and differ from the initial values.

$ bazel test //pkg/tcpip/transport/tcp:tcp_test --test_filter='TestProtocolSecretsHaveNosaveTag|TestProtocolAfterLoadRegeneratesSecrets'
--- PASS: TestProtocolSecretsHaveNosaveTag (0.00s)
--- PASS: TestProtocolAfterLoadRegeneratesSecrets (0.00s)
PASS

bazel build //pkg/tcpip/transport/tcp:tcp builds clean against master.

Framing

This is hardening, not an advisory. Disclosure requires read access to a checkpoint file, which is host-side and orchestrator-controlled (g3doc/user_guide/checkpoint_restore.md). The fix is minimal, the convention already exists in the same file, and the patch closes a remaining persistence surface adjacent to the CVE-2024-10026 / NDSS 2025 secret-quality work.

References

• RFC 6528 (Defending against Sequence Number Attacks, Gont/Bellovin, 2012)
• CVE-2024-10026 (NVD)
• NDSS 2025: Kaplan, Even, Klein. "You Can Rand but You Can't Hide."
• gVisor g3doc/user_guide/checkpoint_restore.md
• Sibling state:"nosave": pkg/tcpip/stack/stack.go:159, pkg/tcpip/transport/tcp/protocol.go:92,115.