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// Copyright 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "net/socket/ssl_session_cache_openssl.h"
#include <list>
#include <map>
#include <openssl/rand.h>
#include <openssl/ssl.h>
#include "base/containers/hash_tables.h"
#include "base/lazy_instance.h"
#include "base/logging.h"
#include "base/synchronization/lock.h"
namespace net {
namespace {
// A helper class to lazily create a new EX_DATA index to map SSL_CTX handles
// to their corresponding SSLSessionCacheOpenSSLImpl object.
class SSLContextExIndex {
SSLContextExIndex() {
context_index_ = SSL_CTX_get_ex_new_index(0, NULL, NULL, NULL, NULL);
DCHECK_NE(-1, context_index_);
session_index_ = SSL_SESSION_get_ex_new_index(0, NULL, NULL, NULL, NULL);
DCHECK_NE(-1, session_index_);
int context_index() const { return context_index_; }
int session_index() const { return session_index_; }
int context_index_;
int session_index_;
// static
base::LazyInstance<SSLContextExIndex>::Leaky s_ssl_context_ex_instance =
// Retrieve the global EX_DATA index, created lazily on first call, to
// be used with SSL_CTX_set_ex_data() and SSL_CTX_get_ex_data().
static int GetSSLContextExIndex() {
return s_ssl_context_ex_instance.Get().context_index();
// Retrieve the global EX_DATA index, created lazily on first call, to
// be used with SSL_SESSION_set_ex_data() and SSL_SESSION_get_ex_data().
static int GetSSLSessionExIndex() {
return s_ssl_context_ex_instance.Get().session_index();
// Helper struct used to store session IDs in a SessionIdIndex container
// (see definition below). To save memory each entry only holds a pointer
// to the session ID buffer, which must outlive the entry itself. On the
// other hand, a hash is included to minimize the number of hashing
// computations during cache operations.
struct SessionId {
SessionId(const unsigned char* a_id, unsigned a_id_len)
: id(a_id), id_len(a_id_len), hash(ComputeHash(a_id, a_id_len)) {}
explicit SessionId(const SessionId& other)
: id(, id_len(other.id_len), hash(other.hash) {}
explicit SessionId(SSL_SESSION* session)
: id(session->session_id),
hash(ComputeHash(session->session_id, session->session_id_length)) {}
bool operator==(const SessionId& other) const {
return hash == other.hash && id_len == other.id_len &&
!memcmp(id,, id_len);
const unsigned char* id;
unsigned id_len;
size_t hash;
// Session ID are random strings of bytes. This happens to compute the same
// value as std::hash<std::string> without the extra string copy. See
// base/containers/hash_tables.h. Other hashing computations are possible,
// this one is just simple enough to do the job.
size_t ComputeHash(const unsigned char* id, unsigned id_len) {
size_t result = 0;
for (unsigned n = 0; n < id_len; ++n)
result += 131 * id[n];
return result;
} // namespace
} // namespace net
template <>
struct hash<net::SessionId> {
std::size_t operator()(const net::SessionId& entry) const {
return entry.hash;
} // namespace BASE_HASH_NAMESPACE
namespace net {
// Implementation of the real SSLSessionCache.
// The implementation is inspired by base::MRUCache, except that the deletor
// also needs to remove the entry from other containers. In a nutshell, this
// uses several basic containers:
// |ordering_| is a doubly-linked list of SSL_SESSION handles, ordered in
// MRU order.
// |key_index_| is a hash table mapping unique cache keys (e.g. host/port
// values) to a single iterator of |ordering_|. It is used to efficiently
// find the cached session associated with a given key.
// |id_index_| is a hash table mapping SessionId values to iterators
// of |key_index_|. If is used to efficiently remove sessions from the cache,
// as well as check for the existence of a session ID value in the cache.
// SSL_SESSION objects are reference-counted, and owned by the cache. This
// means that their reference count is incremented when they are added, and
// decremented when they are removed.
// Assuming an average key size of 100 characters, each node requires the
// following memory usage on 32-bit Android, when linked against STLport:
// 12 (ordering_ node, including SSL_SESSION handle)
// 100 (key characters)
// + 24 (std::string header/minimum size)
// + 8 (key_index_ node, excluding the 2 lines above for the key).
// + 20 (id_index_ node)
// --------
// 164 bytes/node
// Hence, 41 KiB for a full cache with a maximum of 1024 entries, excluding
// the size of SSL_SESSION objects and heap fragmentation.
class SSLSessionCacheOpenSSLImpl {
// Construct new instance. This registers various hooks into the SSL_CTX
// context |ctx|. OpenSSL will call back during SSL connection
// operations. |key_func| is used to map a SSL handle to a unique cache
// string, according to the client's preferences.
SSLSessionCacheOpenSSLImpl(SSL_CTX* ctx,
const SSLSessionCacheOpenSSL::Config& config)
: ctx_(ctx), config_(config), expiration_check_(0) {
// NO_INTERNAL_STORE disables OpenSSL's builtin cache, and
// NO_AUTO_CLEAR disables the call to SSL_CTX_flush_sessions
// every 256 connections (this number is hard-coded in the library
// and can't be changed).
SSL_CTX_sess_set_new_cb(ctx_, NewSessionCallbackStatic);
SSL_CTX_sess_set_remove_cb(ctx_, RemoveSessionCallbackStatic);
SSL_CTX_set_generate_session_id(ctx_, GenerateSessionIdStatic);
SSL_CTX_set_timeout(ctx_, config_.timeout_seconds);
SSL_CTX_set_ex_data(ctx_, GetSSLContextExIndex(), this);
// Destroy this instance. Must happen before |ctx_| is destroyed.
~SSLSessionCacheOpenSSLImpl() {
SSL_CTX_set_ex_data(ctx_, GetSSLContextExIndex(), NULL);
SSL_CTX_sess_set_new_cb(ctx_, NULL);
SSL_CTX_sess_set_remove_cb(ctx_, NULL);
SSL_CTX_set_generate_session_id(ctx_, NULL);
// Return the number of items in this cache.
size_t size() const { return key_index_.size(); }
// Retrieve the cache key from |ssl| and look for a corresponding
// cached session ID. If one is found, call SSL_set_session() to associate
// it with the |ssl| connection.
// Will also check for expired sessions every |expiration_check_count|
// calls.
// Return true if a cached session ID was found, false otherwise.
bool SetSSLSession(SSL* ssl) {
std::string cache_key = config_.key_func(ssl);
if (cache_key.empty())
return false;
return SetSSLSessionWithKey(ssl, cache_key);
// Variant of SetSSLSession to be used when the client already has computed
// the cache key. Avoid a call to the configuration's |key_func| function.
bool SetSSLSessionWithKey(SSL* ssl, const std::string& cache_key) {
base::AutoLock locked(lock_);
DCHECK_EQ(config_.key_func(ssl), cache_key);
if (++expiration_check_ >= config_.expiration_check_count) {
expiration_check_ = 0;
KeyIndex::iterator it = key_index_.find(cache_key);
if (it == key_index_.end())
return false;
SSL_SESSION* session = *it->second;
DVLOG(2) << "Lookup session: " << session << " for " << cache_key;
void* session_is_good =
SSL_SESSION_get_ex_data(session, GetSSLSessionExIndex());
if (!session_is_good)
return false; // Session has not yet been marked good. Treat as a miss.
// Move to front of MRU list.
it->second = ordering_.begin();
return SSL_set_session(ssl, session) == 1;
// Return true iff a cached session was associated with the given |cache_key|.
bool SSLSessionIsInCache(const std::string& cache_key) const {
base::AutoLock locked(lock_);
KeyIndex::const_iterator it = key_index_.find(cache_key);
if (it == key_index_.end())
return false;
SSL_SESSION* session = *it->second;
void* session_is_good =
SSL_SESSION_get_ex_data(session, GetSSLSessionExIndex());
return session_is_good;
void MarkSSLSessionAsGood(SSL* ssl) {
SSL_SESSION* session = SSL_get_session(ssl);
// Mark the session as good, allowing it to be used for future connections.
session, GetSSLSessionExIndex(), reinterpret_cast<void*>(1));
// Flush all entries from the cache.
void Flush() {
base::AutoLock lock(lock_);
while (!ordering_.empty()) {
SSL_SESSION* session = ordering_.front();
// Type for list of SSL_SESSION handles, ordered in MRU order.
typedef std::list<SSL_SESSION*> MRUSessionList;
// Type for a dictionary from unique cache keys to session list nodes.
typedef base::hash_map<std::string, MRUSessionList::iterator> KeyIndex;
// Type for a dictionary from SessionId values to key index nodes.
typedef base::hash_map<SessionId, KeyIndex::iterator> SessionIdIndex;
// Return the key associated with a given session, or the empty string if
// none exist. This shall only be used for debugging.
std::string SessionKey(SSL_SESSION* session) {
if (!session)
return std::string("<null-session>");
if (session->session_id_length == 0)
return std::string("<empty-session-id>");
SessionIdIndex::iterator it = id_index_.find(SessionId(session));
if (it == id_index_.end())
return std::string("<unknown-session>");
return it->second->first;
// Remove a given |session| from the cache. Lock must be held.
void RemoveSessionLocked(SSL_SESSION* session) {
DCHECK_GT(session->session_id_length, 0U);
SessionId session_id(session);
SessionIdIndex::iterator id_it = id_index_.find(session_id);
if (id_it == id_index_.end()) {
LOG(ERROR) << "Trying to remove unknown session from cache: " << session;
KeyIndex::iterator key_it = id_it->second;
DCHECK(key_it != key_index_.end());
DCHECK_EQ(session, *key_it->second);
DCHECK_EQ(key_index_.size(), id_index_.size());
// Used internally to flush expired sessions. Lock must be held.
void FlushExpiredSessionsLocked() {
// Unfortunately, OpenSSL initializes |session->time| with a time()
// timestamps, which makes mocking / unit testing difficult.
long timeout_secs = static_cast<long>(::time(NULL));
MRUSessionList::iterator it = ordering_.begin();
while (it != ordering_.end()) {
SSL_SESSION* session = *it++;
// Important, use <= instead of < here to allow unit testing to
// work properly. That's because unit tests that check the expiration
// behaviour will use a session timeout of 0 seconds.
if (session->time + session->timeout <= timeout_secs) {
DVLOG(2) << "Expiring session " << session << " for "
<< SessionKey(session);
// Retrieve the cache associated with a given SSL context |ctx|.
static SSLSessionCacheOpenSSLImpl* GetCache(SSL_CTX* ctx) {
void* result = SSL_CTX_get_ex_data(ctx, GetSSLContextExIndex());
return reinterpret_cast<SSLSessionCacheOpenSSLImpl*>(result);
// Called by OpenSSL when a new |session| was created and added to a given
// |ssl| connection. Note that the session's reference count was already
// incremented before the function is entered. The function must return 1
// to indicate that it took ownership of the session, i.e. that the caller
// should not decrement its reference count after completion.
static int NewSessionCallbackStatic(SSL* ssl, SSL_SESSION* session) {
SSLSessionCacheOpenSSLImpl* cache = GetCache(ssl->ctx);
cache->OnSessionAdded(ssl, session);
return 1;
// Called by OpenSSL to indicate that a session must be removed from the
// cache. This happens when SSL_CTX is destroyed.
static void RemoveSessionCallbackStatic(SSL_CTX* ctx, SSL_SESSION* session) {
// Called by OpenSSL to generate a new session ID. This happens during a
// SSL connection operation, when the SSL object doesn't have a session yet.
// A session ID is a random string of bytes used to uniquely identify the
// session between a client and a server.
// |ssl| is a SSL connection handle. Ignored here.
// |id| is the target buffer where the ID must be generated.
// |*id_len| is, on input, the size of the desired ID. It will be 16 for
// SSLv2, and 32 for anything else. OpenSSL allows an implementation
// to change it on output, but this will not happen here.
// The function must ensure the generated ID is really unique, i.e. that
// another session in the cache doesn't already use the same value. It must
// return 1 to indicate success, or 0 for failure.
static int GenerateSessionIdStatic(const SSL* ssl,
unsigned char* id,
unsigned* id_len) {
if (!GetCache(ssl->ctx)->OnGenerateSessionId(id, *id_len))
return 0;
return 1;
// Add |session| to the cache in association with |cache_key|. If a session
// already exists, it is replaced with the new one. This assumes that the
// caller already incremented the session's reference count.
void OnSessionAdded(SSL* ssl, SSL_SESSION* session) {
base::AutoLock locked(lock_);
DCHECK_GT(session->session_id_length, 0U);
std::string cache_key = config_.key_func(ssl);
KeyIndex::iterator it = key_index_.find(cache_key);
if (it == key_index_.end()) {
DVLOG(2) << "Add session " << session << " for " << cache_key;
// This is a new session. Add it to the cache.
std::pair<KeyIndex::iterator, bool> ret =
key_index_.insert(std::make_pair(cache_key, ordering_.begin()));
it = ret.first;
DCHECK(it != key_index_.end());
} else {
// An existing session exists for this key, so replace it if needed.
DVLOG(2) << "Replace session " << *it->second << " with " << session
<< " for " << cache_key;
SSL_SESSION* old_session = *it->second;
if (old_session != session) {
it->second = ordering_.begin();
id_index_[SessionId(session)] = it;
if (key_index_.size() > config_.max_entries)
DCHECK_EQ(key_index_.size(), id_index_.size());
DCHECK_LE(key_index_.size(), config_.max_entries);
// Shrink the cache to ensure no more than config_.max_entries entries,
// starting with older entries first. Lock must be acquired.
void ShrinkCacheLocked() {
DCHECK_EQ(key_index_.size(), ordering_.size());
DCHECK_EQ(key_index_.size(), id_index_.size());
while (key_index_.size() > config_.max_entries) {
MRUSessionList::reverse_iterator it = ordering_.rbegin();
DCHECK(it != ordering_.rend());
SSL_SESSION* session = *it;
DVLOG(2) << "Evicting session " << session << " for "
<< SessionKey(session);
// Remove |session| from the cache.
void OnSessionRemoved(SSL_SESSION* session) {
base::AutoLock locked(lock_);
DVLOG(2) << "Remove session " << session << " for " << SessionKey(session);
// See GenerateSessionIdStatic for a description of what this function does.
bool OnGenerateSessionId(unsigned char* id, unsigned id_len) {
base::AutoLock locked(lock_);
// This mimics def_generate_session_id() in openssl/ssl/,
// I.e. try to generate a pseudo-random bit string, and check that no
// other entry in the cache has the same value.
const size_t kMaxTries = 10;
for (size_t tries = 0; tries < kMaxTries; ++tries) {
if (RAND_pseudo_bytes(id, id_len) <= 0) {
DLOG(ERROR) << "Couldn't generate " << id_len
<< " pseudo random bytes?";
return false;
if (id_index_.find(SessionId(id, id_len)) == id_index_.end())
return true;
DLOG(ERROR) << "Couldn't generate unique session ID of " << id_len
<< "bytes after " << kMaxTries << " tries.";
return false;
SSL_CTX* ctx_;
SSLSessionCacheOpenSSL::Config config_;
// method to get the index which can later be used with SSL_CTX_get_ex_data()
// or SSL_CTX_set_ex_data().
mutable base::Lock lock_; // Protects access to containers below.
MRUSessionList ordering_;
KeyIndex key_index_;
SessionIdIndex id_index_;
size_t expiration_check_;
SSLSessionCacheOpenSSL::~SSLSessionCacheOpenSSL() { delete impl_; }
size_t SSLSessionCacheOpenSSL::size() const { return impl_->size(); }
void SSLSessionCacheOpenSSL::Reset(SSL_CTX* ctx, const Config& config) {
if (impl_)
delete impl_;
impl_ = new SSLSessionCacheOpenSSLImpl(ctx, config);
bool SSLSessionCacheOpenSSL::SetSSLSession(SSL* ssl) {
return impl_->SetSSLSession(ssl);
bool SSLSessionCacheOpenSSL::SetSSLSessionWithKey(
SSL* ssl,
const std::string& cache_key) {
return impl_->SetSSLSessionWithKey(ssl, cache_key);
bool SSLSessionCacheOpenSSL::SSLSessionIsInCache(
const std::string& cache_key) const {
return impl_->SSLSessionIsInCache(cache_key);
void SSLSessionCacheOpenSSL::MarkSSLSessionAsGood(SSL* ssl) {
return impl_->MarkSSLSessionAsGood(ssl);
void SSLSessionCacheOpenSSL::Flush() { impl_->Flush(); }
} // namespace net