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/* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
/*
* Anti-replay measures for TLS 1.3.
*
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#include "nss.h" /* for NSS_RegisterShutdown */
#include "nssilock.h" /* for PZMonitor */
#include "pk11pub.h"
#include "prmon.h"
#include "prtime.h"
#include "secerr.h"
#include "ssl.h"
#include "sslbloom.h"
#include "sslimpl.h"
#include "tls13hkdf.h"
#include "tls13psk.h"
struct SSLAntiReplayContextStr {
/* The number of outstanding references to this context. */
PRInt32 refCount;
/* Used to serialize access. */
PZMonitor *lock;
/* The filters, use of which alternates. */
sslBloomFilter filters[2];
/* Which of the two filters is active (0 or 1). */
PRUint8 current;
/* The time that we will next update. */
PRTime nextUpdate;
/* The width of the window; i.e., the period of updates. */
PRTime window;
/* This key ensures that the bloom filter index is unpredictable. */
PK11SymKey *key;
};
void
tls13_ReleaseAntiReplayContext(SSLAntiReplayContext *ctx)
{
if (!ctx) {
return;
}
if (PR_ATOMIC_DECREMENT(&ctx->refCount) >= 1) {
return;
}
if (ctx->lock) {
PZ_DestroyMonitor(ctx->lock);
ctx->lock = NULL;
}
PK11_FreeSymKey(ctx->key);
ctx->key = NULL;
sslBloom_Destroy(&ctx->filters[0]);
sslBloom_Destroy(&ctx->filters[1]);
PORT_Free(ctx);
}
/* Clear the current state and free any resources we allocated. */
SECStatus
SSLExp_ReleaseAntiReplayContext(SSLAntiReplayContext *ctx)
{
tls13_ReleaseAntiReplayContext(ctx);
return SECSuccess;
}
SSLAntiReplayContext *
tls13_RefAntiReplayContext(SSLAntiReplayContext *ctx)
{
PORT_Assert(ctx);
PR_ATOMIC_INCREMENT(&ctx->refCount);
return ctx;
}
static SECStatus
tls13_AntiReplayKeyGen(SSLAntiReplayContext *ctx)
{
PK11SlotInfo *slot;
PORT_Assert(ctx);
slot = PK11_GetBestSlot(CKM_HKDF_DERIVE, NULL);
if (!slot) {
PORT_SetError(SEC_ERROR_LIBRARY_FAILURE);
return SECFailure;
}
ctx->key = PK11_KeyGen(slot, CKM_HKDF_KEY_GEN, NULL, 32, NULL);
if (!ctx->key) {
goto loser;
}
PK11_FreeSlot(slot);
return SECSuccess;
loser:
PK11_FreeSlot(slot);
return SECFailure;
}
/* Set a limit on the combination of number of hashes and bits in each hash. */
#define SSL_MAX_BLOOM_FILTER_SIZE 64
/*
* The context created by this function can be called concurrently on multiple
* threads if the server is multi-threaded. A monitor is used to ensure that
* only one thread can access the structures that change over time, but no such
* guarantee is provided for configuration data.
*/
SECStatus
SSLExp_CreateAntiReplayContext(PRTime now, PRTime window, unsigned int k,
unsigned int bits, SSLAntiReplayContext **pctx)
{
SECStatus rv;
if (window <= 0 || k == 0 || bits == 0 || pctx == NULL) {
PORT_SetError(SEC_ERROR_INVALID_ARGS);
return SECFailure;
}
if ((k * (bits + 7) / 8) > SSL_MAX_BLOOM_FILTER_SIZE) {
PORT_SetError(SEC_ERROR_INVALID_ARGS);
return SECFailure;
}
SSLAntiReplayContext *ctx = PORT_ZNew(SSLAntiReplayContext);
if (!ctx) {
return SECFailure; /* Code already set. */
}
ctx->refCount = 1;
ctx->lock = PZ_NewMonitor(nssILockSSL);
if (!ctx->lock) {
goto loser; /* Code already set. */
}
rv = tls13_AntiReplayKeyGen(ctx);
if (rv != SECSuccess) {
goto loser; /* Code already set. */
}
rv = sslBloom_Init(&ctx->filters[0], k, bits);
if (rv != SECSuccess) {
goto loser; /* Code already set. */
}
rv = sslBloom_Init(&ctx->filters[1], k, bits);
if (rv != SECSuccess) {
goto loser; /* Code already set. */
}
/* When starting out, ensure that 0-RTT is not accepted until the window is
* updated. A ClientHello might have been accepted prior to a restart. */
sslBloom_Fill(&ctx->filters[1]);
ctx->current = 0;
ctx->nextUpdate = now + window;
ctx->window = window;
*pctx = ctx;
return SECSuccess;
loser:
tls13_ReleaseAntiReplayContext(ctx);
return SECFailure;
}
SECStatus
SSLExp_SetAntiReplayContext(PRFileDesc *fd, SSLAntiReplayContext *ctx)
{
sslSocket *ss = ssl_FindSocket(fd);
if (!ss) {
return SECFailure; /* Code already set. */
}
tls13_ReleaseAntiReplayContext(ss->antiReplay);
if (ctx != NULL) {
ss->antiReplay = tls13_RefAntiReplayContext(ctx);
} else {
ss->antiReplay = NULL;
}
return SECSuccess;
}
static void
tls13_AntiReplayUpdate(SSLAntiReplayContext *ctx, PRTime now)
{
PR_ASSERT_CURRENT_THREAD_IN_MONITOR(ctx->lock);
if (now >= ctx->nextUpdate) {
ctx->current ^= 1;
ctx->nextUpdate = now + ctx->window;
sslBloom_Zero(ctx->filters + ctx->current);
}
}
PRBool
tls13_InWindow(const sslSocket *ss, const sslSessionID *sid)
{
PRInt32 timeDelta;
/* Calculate the difference between the client's view of the age of the
* ticket (in |ss->xtnData.ticketAge|) and the server's view, which we now
* calculate. The result should be close to zero. timeDelta is signed to
* make the comparisons below easier. */
timeDelta = ss->xtnData.ticketAge -
((ssl_Time(ss) - sid->creationTime) / PR_USEC_PER_MSEC);
/* Only allow the time delta to be at most half of our window. This is
* symmetrical, though it doesn't need to be; this assumes that clock errors
* on server and client will tend to cancel each other out.
*
* There are two anti-replay filters that roll over each window. In the
* worst case, immediately after a rollover of the filters, we only have a
* single window worth of recorded 0-RTT attempts. Thus, the period in
* which we can accept 0-RTT is at most one window wide. This uses PR_ABS()
* and half the window so that the first attempt can be up to half a window
* early and then replays will be caught until the attempts are half a
* window late.
*
* For example, a 0-RTT attempt arrives early, but near the end of window 1.
* The attempt is then recorded in window 1. Rollover to window 2 could
* occur immediately afterwards. Window 1 is still checked for new 0-RTT
* attempts for the remainder of window 2. Therefore, attempts to replay
* are detected because the value is recorded in window 1. When rollover
* occurs again, window 1 is erased and window 3 instated. If we allowed an
* attempt to be late by more than half a window, then this check would not
* prevent the same 0-RTT attempt from being accepted during window 1 and
* later window 3.
*/
PRInt32 allowance = ss->antiReplay->window / (PR_USEC_PER_MSEC * 2);
SSL_TRC(10, ("%d: TLS13[%d]: replay check time delta=%d, allow=%d",
SSL_GETPID(), ss->fd, timeDelta, allowance));
return PR_ABS(timeDelta) < allowance;
}
/* Checks for a duplicate in the two filters we have. Performs maintenance on
* the filters as a side-effect. This only detects a probable replay, it's
* possible that this will return true when the 0-RTT attempt is not genuinely a
* replay. In that case, we reject 0-RTT unnecessarily, but that's OK because
* no client expects 0-RTT to work every time. */
PRBool
tls13_IsReplay(const sslSocket *ss, const sslSessionID *sid)
{
PRBool replay;
unsigned int size;
PRUint8 index;
SECStatus rv;
static const char *label = "anti-replay";
PRUint8 buf[SSL_MAX_BLOOM_FILTER_SIZE];
SSLAntiReplayContext *ctx = ss->antiReplay;
/* If SSL_SetAntiReplayContext hasn't been called with a valid context, then
* treat all attempts at 0-RTT as a replay. */
if (ctx == NULL) {
return PR_TRUE;
}
if (!sid) {
PORT_Assert(ss->xtnData.selectedPsk->type == ssl_psk_external);
} else if (!tls13_InWindow(ss, sid)) {
return PR_TRUE;
}
size = ctx->filters[0].k * (ctx->filters[0].bits + 7) / 8;
PORT_Assert(size <= SSL_MAX_BLOOM_FILTER_SIZE);
rv = tls13_HkdfExpandLabelRaw(ctx->key, ssl_hash_sha256,
ss->xtnData.pskBinder.data,
ss->xtnData.pskBinder.len,
label, strlen(label),
ss->protocolVariant, buf, size);
if (rv != SECSuccess) {
return PR_TRUE;
}
PZ_EnterMonitor(ctx->lock);
tls13_AntiReplayUpdate(ctx, ssl_Time(ss));
index = ctx->current;
replay = sslBloom_Add(&ctx->filters[index], buf);
SSL_TRC(10, ("%d: TLS13[%d]: replay check current window: %s",
SSL_GETPID(), ss->fd, replay ? "replay" : "ok"));
if (!replay) {
replay = sslBloom_Check(&ctx->filters[index ^ 1], buf);
SSL_TRC(10, ("%d: TLS13[%d]: replay check previous window: %s",
SSL_GETPID(), ss->fd, replay ? "replay" : "ok"));
}
PZ_ExitMonitor(ctx->lock);
return replay;
}