tls: Add distributed_stek module

This migrates a feature that was previously reserved for enterprise
users, according to https://github.com/caddyserver/caddy/issues/2786.

TLS session ticket keys are sensitive, so they should be rotated on a
regular basis. Only Caddy does this by default. However, a cluster of
servers that rotate keys without synchronization will lose the benefits
of having sessions in the first place if the client is routed to a
different backend. This module coordinates STEK rotation in a fleet so
the same keys are used, and rotated, across the whole cluster. No other
server does this, but Twitter wrote about how they hacked together a
solution a few years ago:
https://blog.twitter.com/engineering/en_us/a/2013/forward-secrecy-at-twitter.html

modules/caddytls/distributedstek/distributedstek.go

@@ -0,0 +1,228 @@
+// Copyright 2015 Matthew Holt and The Caddy 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 distributedstek provides TLS session ticket ephemeral
+// keys (STEKs) in a distributed fashion by utilizing configured
+// storage for locking and key sharing. This allows a cluster of
+// machines to optimally resume TLS sessions in a load-balanced
+// environment without any hassle. This is similar to what
+// Twitter does, but without needing to rely on SSH, as it is
+// built into the web server this way:
+// https://blog.twitter.com/engineering/en_us/a/2013/forward-secrecy-at-twitter.html
+package distributedstek
+
+import (
+	"bytes"
+	"encoding/gob"
+	"encoding/json"
+	"fmt"
+	"log"
+	"time"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/modules/caddytls"
+	"github.com/mholt/certmagic"
+)
+
+func init() {
+	caddy.RegisterModule(Provider{})
+}
+
+// Provider implements a distributed STEK provider.
+type Provider struct {
+	Storage json.RawMessage `json:"storage,omitempty"`
+
+	storage    certmagic.Storage
+	stekConfig *caddytls.SessionTicketService
+	timer      *time.Timer
+}
+
+// CaddyModule returns the Caddy module information.
+func (Provider) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		Name: "tls.stek.distributed",
+		New:  func() caddy.Module { return new(Provider) },
+	}
+}
+
+// Provision provisions s.
+func (s *Provider) Provision(ctx caddy.Context) error {
+	// unpack the storage module to use, if different from the default
+	if s.Storage != nil {
+		val, err := ctx.LoadModuleInline("module", "caddy.storage", s.Storage)
+		if err != nil {
+			return fmt.Errorf("loading TLS storage module: %s", err)
+		}
+		cmStorage, err := val.(caddy.StorageConverter).CertMagicStorage()
+		if err != nil {
+			return fmt.Errorf("creating TLS storage configuration: %v", err)
+		}
+		s.storage = cmStorage
+		s.Storage = nil // allow GC to deallocate
+	}
+
+	// otherwise, use default storage
+	if s.storage == nil {
+		s.storage = ctx.Storage()
+	}
+
+	return nil
+}
+
+// Initialize sets the configuration for s and returns the starting keys.
+func (s *Provider) Initialize(config *caddytls.SessionTicketService) ([][32]byte, error) {
+	// keep a reference to the config; we'll need it when rotating keys
+	s.stekConfig = config
+
+	dstek, err := s.getSTEK()
+	if err != nil {
+		return nil, err
+	}
+
+	// create timer for the remaining time on the interval;
+	// this timer is cleaned up only when rotate() returns
+	s.timer = time.NewTimer(time.Until(dstek.NextRotation))
+
+	return dstek.Keys, nil
+}
+
+// Next returns a channel which transmits the latest session ticket keys.
+func (s *Provider) Next(doneChan <-chan struct{}) <-chan [][32]byte {
+	keysChan := make(chan [][32]byte)
+	go s.rotate(doneChan, keysChan)
+	return keysChan
+}
+
+func (s *Provider) loadSTEK() (distributedSTEK, error) {
+	var sg distributedSTEK
+	gobBytes, err := s.storage.Load(stekFileName)
+	if err != nil {
+		return sg, err // don't wrap, in case error is certmagic.ErrNotExist
+	}
+	dec := gob.NewDecoder(bytes.NewReader(gobBytes))
+	err = dec.Decode(&sg)
+	if err != nil {
+		return sg, fmt.Errorf("STEK gob corrupted: %v", err)
+	}
+	return sg, nil
+}
+
+func (s *Provider) storeSTEK(dstek distributedSTEK) error {
+	var buf bytes.Buffer
+	err := gob.NewEncoder(&buf).Encode(dstek)
+	if err != nil {
+		return fmt.Errorf("encoding STEK gob: %v", err)
+	}
+	err = s.storage.Store(stekFileName, buf.Bytes())
+	if err != nil {
+		return fmt.Errorf("storing STEK gob: %v", err)
+	}
+	return nil
+}
+
+// getSTEK locks and loads the current STEK from storage. If none
+// currently exists, a new STEK is created and persisted. If the
+// current STEK is outdated (NextRotation time is in the past),
+// then it is rotated and persisted. The resulting STEK is returned.
+func (s *Provider) getSTEK() (distributedSTEK, error) {
+	s.storage.Lock(stekLockName)
+	defer s.storage.Unlock(stekLockName)
+
+	// load the current STEKs from storage
+	dstek, err := s.loadSTEK()
+	if _, isNotExist := err.(certmagic.ErrNotExist); isNotExist {
+		// if there is none, then make some right away
+		dstek, err = s.rotateKeys(dstek)
+		if err != nil {
+			return dstek, fmt.Errorf("creating new STEK: %v", err)
+		}
+	} else if err != nil {
+		// some other error, that's a problem
+		return dstek, fmt.Errorf("loading STEK: %v", err)
+	} else if time.Now().After(dstek.NextRotation) {
+		// if current STEKs are outdated, rotate them
+		dstek, err = s.rotateKeys(dstek)
+		if err != nil {
+			return dstek, fmt.Errorf("rotating keys: %v", err)
+		}
+	}
+
+	return dstek, nil
+}
+
+// rotateKeys rotates the keys of oldSTEK and returns the new distributedSTEK
+// with updated keys and timestamps. It stores the returned STEK in storage,
+// so this function must only be called in a storage-provided lock.
+func (s *Provider) rotateKeys(oldSTEK distributedSTEK) (distributedSTEK, error) {
+	var newSTEK distributedSTEK
+	var err error
+
+	newSTEK.Keys, err = s.stekConfig.RotateSTEKs(oldSTEK.Keys)
+	if err != nil {
+		return newSTEK, err
+	}
+
+	now := time.Now()
+	newSTEK.LastRotation = now
+	newSTEK.NextRotation = now.Add(time.Duration(s.stekConfig.RotationInterval))
+
+	err = s.storeSTEK(newSTEK)
+	if err != nil {
+		return newSTEK, err
+	}
+
+	return newSTEK, nil
+}
+
+// rotate rotates keys on a regular basis, sending each updated set of
+// keys down keysChan, until doneChan is closed.
+func (s *Provider) rotate(doneChan <-chan struct{}, keysChan chan<- [][32]byte) {
+	for {
+		select {
+		case <-s.timer.C:
+			dstek, err := s.getSTEK()
+			if err != nil {
+				// TODO: improve this handling
+				log.Printf("[ERROR] Loading STEK: %v", err)
+				continue
+			}
+
+			// send the updated keys to the service
+			keysChan <- dstek.Keys
+
+			// timer channel is already drained, so reset directly (see godoc)
+			s.timer.Reset(time.Until(dstek.NextRotation))
+
+		case <-doneChan:
+			// again, see godocs for why timer is stopped this way
+			if !s.timer.Stop() {
+				<-s.timer.C
+			}
+			return
+		}
+	}
+}
+
+type distributedSTEK struct {
+	Keys                       [][32]byte
+	LastRotation, NextRotation time.Time
+}
+
+const (
+	stekLockName = "stek_check"
+	stekFileName = "stek/stek.bin"
+)
+
+// Interface guard
+var _ caddytls.STEKProvider = (*Provider)(nil)