Bug 1396487 - Record size limiting extension, r=ekr

Summary: See draft-ietf-tls-record-limit for details.

Reviewers: ekr

Bug #: 1396487

Differential Revision: https://phabricator.services.mozilla.com/D23

21 files changed, 638 insertions, 27 deletions

diff --git a/gtests/ssl_gtest/manifest.mn b/gtests/ssl_gtest/manifest.mn
index 5d893bab3..8547e56d1 100644
--- a/gtests/ssl_gtest/manifest.mn
+++ b/gtests/ssl_gtest/manifest.mn
@@ -36,6 +36,7 @@ CPPSRCS = \
       ssl_loopback_unittest.cc \
       ssl_misc_unittest.cc \
       ssl_record_unittest.cc \
+      ssl_recordsize_unittest.cc \
       ssl_resumption_unittest.cc \
       ssl_renegotiation_unittest.cc \
       ssl_skip_unittest.cc \
diff --git a/gtests/ssl_gtest/ssl_custext_unittest.cc b/gtests/ssl_gtest/ssl_custext_unittest.cc
index c2f582a93..5be62e506 100644
--- a/gtests/ssl_gtest/ssl_custext_unittest.cc
+++ b/gtests/ssl_gtest/ssl_custext_unittest.cc
@@ -68,6 +68,7 @@ static const uint16_t kManyExtensions[] = {
     ssl_next_proto_nego_xtn,
     ssl_renegotiation_info_xtn,
     ssl_tls13_short_header_xtn,
+    ssl_record_size_limit_xtn,
     1,
     0xffff};
 // The list here includes all extensions we expect to use (SSL_MAX_EXTENSIONS),
diff --git a/gtests/ssl_gtest/ssl_gtest.gyp b/gtests/ssl_gtest/ssl_gtest.gyp
index e2a8d830a..17677713d 100644
--- a/gtests/ssl_gtest/ssl_gtest.gyp
+++ b/gtests/ssl_gtest/ssl_gtest.gyp
@@ -37,6 +37,7 @@
         'ssl_loopback_unittest.cc',
         'ssl_misc_unittest.cc',
         'ssl_record_unittest.cc',
+        'ssl_recordsize_unittest.cc',
         'ssl_resumption_unittest.cc',
         'ssl_renegotiation_unittest.cc',
         'ssl_skip_unittest.cc',
diff --git a/gtests/ssl_gtest/ssl_record_unittest.cc b/gtests/ssl_gtest/ssl_record_unittest.cc
index 97bbbba01..4c33c1936 100644
--- a/gtests/ssl_gtest/ssl_record_unittest.cc
+++ b/gtests/ssl_gtest/ssl_record_unittest.cc
@@ -104,15 +104,13 @@ TEST_P(TlsPaddingTest, LastByteOfPadWrong) {
 class RecordReplacer : public TlsRecordFilter {
  public:
   RecordReplacer(const std::shared_ptr<TlsAgent>& a, size_t size)
-      : TlsRecordFilter(a), enabled_(false), size_(size) {}
+      : TlsRecordFilter(a), size_(size) {
+    Disable();
+  }
 
   PacketFilter::Action FilterRecord(const TlsRecordHeader& header,
                                     const DataBuffer& data,
                                     DataBuffer* changed) override {
-    if (!enabled_) {
-      return KEEP;
-    }
-
     EXPECT_EQ(kTlsApplicationDataType, header.content_type());
     changed->Allocate(size_);
 
@@ -120,14 +118,11 @@ class RecordReplacer : public TlsRecordFilter {
       changed->data()[i] = i & 0xff;
     }
 
-    enabled_ = false;
+    Disable();
     return CHANGE;
   }
 
-  void Enable() { enabled_ = true; }
-
  private:
-  bool enabled_;
   size_t size_;
 };
 
diff --git a/gtests/ssl_gtest/ssl_recordsize_unittest.cc b/gtests/ssl_gtest/ssl_recordsize_unittest.cc
new file mode 100644
index 000000000..8b9e28aff
--- /dev/null
+++ b/gtests/ssl_gtest/ssl_recordsize_unittest.cc
@@ -0,0 +1,431 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* 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 "secerr.h"
+#include "ssl.h"
+#include "sslerr.h"
+#include "sslproto.h"
+
+#include "gtest_utils.h"
+#include "scoped_ptrs.h"
+#include "tls_connect.h"
+#include "tls_filter.h"
+#include "tls_parser.h"
+
+namespace nss_test {
+
+// This class tracks the maximum size of record that was sent, both cleartext
+// and plain.  It only tracks records that have an outer type of
+// application_data.  In TLS 1.3, this includes handshake messages.
+class TlsRecordMaximum : public TlsRecordFilter {
+ public:
+  TlsRecordMaximum(const std::shared_ptr<TlsAgent>& a)
+      : TlsRecordFilter(a), max_ciphertext_(0), max_plaintext_(0) {}
+
+  size_t max_ciphertext() const { return max_ciphertext_; }
+  size_t max_plaintext() const { return max_plaintext_; }
+
+ protected:
+  PacketFilter::Action FilterRecord(const TlsRecordHeader& header,
+                                    const DataBuffer& record, size_t* offset,
+                                    DataBuffer* output) override {
+    std::cerr << "max: " << record << std::endl;
+    // Ignore unprotected packets.
+    if (header.content_type() != kTlsApplicationDataType) {
+      return KEEP;
+    }
+
+    max_ciphertext_ = (std::max)(max_ciphertext_, record.len());
+    return TlsRecordFilter::FilterRecord(header, record, offset, output);
+  }
+
+  PacketFilter::Action FilterRecord(const TlsRecordHeader& header,
+                                    const DataBuffer& data,
+                                    DataBuffer* changed) override {
+    max_plaintext_ = (std::max)(max_plaintext_, data.len());
+    return KEEP;
+  }
+
+ private:
+  size_t max_ciphertext_;
+  size_t max_plaintext_;
+};
+
+void CheckRecordSizes(const std::shared_ptr<TlsAgent>& agent,
+                      const std::shared_ptr<TlsRecordMaximum>& record_max,
+                      size_t config) {
+  uint16_t cipher_suite;
+  ASSERT_TRUE(agent->cipher_suite(&cipher_suite));
+
+  size_t expansion;
+  size_t iv;
+  switch (cipher_suite) {
+    case TLS_AES_128_GCM_SHA256:
+    case TLS_AES_256_GCM_SHA384:
+    case TLS_CHACHA20_POLY1305_SHA256:
+      expansion = 16;
+      iv = 0;
+      break;
+
+    case TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256:
+      expansion = 16;
+      iv = 8;
+      break;
+
+    case TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA:
+      // Expansion is 20 for the MAC.  Maximum block padding is 16.  Maximum
+      // padding is added when the input plus the MAC is an exact multiple of
+      // the block size.
+      expansion = 20 + 16 - ((config + 20) % 16);
+      iv = 16;
+      break;
+
+    default:
+      ADD_FAILURE() << "No expansion set for ciphersuite "
+                    << agent->cipher_suite_name();
+      return;
+  }
+
+  switch (agent->version()) {
+    case SSL_LIBRARY_VERSION_TLS_1_3:
+      EXPECT_EQ(0U, iv) << "No IV for TLS 1.3";
+      // We only have decryption in TLS 1.3.
+      EXPECT_EQ(config - 1, record_max->max_plaintext())
+          << "bad plaintext length for " << agent->role_str();
+      break;
+
+    case SSL_LIBRARY_VERSION_TLS_1_2:
+    case SSL_LIBRARY_VERSION_TLS_1_1:
+      expansion += iv;
+      break;
+
+    case SSL_LIBRARY_VERSION_TLS_1_0:
+      break;
+
+    default:
+      ADD_FAILURE() << "Unexpected version " << agent->version();
+      return;
+  }
+
+  EXPECT_EQ(config + expansion, record_max->max_ciphertext())
+      << "bad ciphertext length for " << agent->role_str();
+}
+
+TEST_P(TlsConnectGeneric, RecordSizeMaximum) {
+  uint16_t max_record_size =
+      (version_ >= SSL_LIBRARY_VERSION_TLS_1_3) ? 16385 : 16384;
+  size_t send_size = (version_ >= SSL_LIBRARY_VERSION_TLS_1_3)
+                         ? max_record_size
+                         : max_record_size + 1;
+
+  EnsureTlsSetup();
+  auto client_max = MakeTlsFilter<TlsRecordMaximum>(client_);
+  client_max->EnableDecryption();
+  auto server_max = MakeTlsFilter<TlsRecordMaximum>(server_);
+  server_max->EnableDecryption();
+
+  Connect();
+  client_->SendData(send_size, send_size);
+  server_->SendData(send_size, send_size);
+  server_->ReadBytes(send_size);
+  client_->ReadBytes(send_size);
+
+  CheckRecordSizes(client_, client_max, max_record_size);
+  CheckRecordSizes(server_, server_max, max_record_size);
+}
+
+TEST_P(TlsConnectGeneric, RecordSizeMinimumClient) {
+  EnsureTlsSetup();
+  auto server_max = MakeTlsFilter<TlsRecordMaximum>(server_);
+  server_max->EnableDecryption();
+
+  client_->SetOption(SSL_RECORD_SIZE_LIMIT, 64);
+  Connect();
+  SendReceive(127);  // Big enough for one record, allowing for 1+N splitting.
+
+  CheckRecordSizes(server_, server_max, 64);
+}
+
+TEST_P(TlsConnectGeneric, RecordSizeMinimumServer) {
+  EnsureTlsSetup();
+  auto client_max = MakeTlsFilter<TlsRecordMaximum>(client_);
+  client_max->EnableDecryption();
+
+  server_->SetOption(SSL_RECORD_SIZE_LIMIT, 64);
+  Connect();
+  SendReceive(127);
+
+  CheckRecordSizes(client_, client_max, 64);
+}
+
+TEST_P(TlsConnectGeneric, RecordSizeAsymmetric) {
+  EnsureTlsSetup();
+  auto client_max = MakeTlsFilter<TlsRecordMaximum>(client_);
+  client_max->EnableDecryption();
+  auto server_max = MakeTlsFilter<TlsRecordMaximum>(server_);
+  server_max->EnableDecryption();
+
+  client_->SetOption(SSL_RECORD_SIZE_LIMIT, 64);
+  server_->SetOption(SSL_RECORD_SIZE_LIMIT, 100);
+  Connect();
+  SendReceive(127);
+
+  CheckRecordSizes(client_, client_max, 100);
+  CheckRecordSizes(server_, server_max, 64);
+}
+
+// This just modifies the encrypted payload so to include a few extra zeros.
+class TlsRecordExpander : public TlsRecordFilter {
+ public:
+  TlsRecordExpander(const std::shared_ptr<TlsAgent>& a, size_t expansion)
+      : TlsRecordFilter(a), expansion_(expansion) {}
+
+ protected:
+  virtual PacketFilter::Action FilterRecord(const TlsRecordHeader& header,
+                                            const DataBuffer& data,
+                                            DataBuffer* changed) {
+    if (header.content_type() != kTlsApplicationDataType) {
+      return KEEP;
+    }
+    changed->Allocate(data.len() + expansion_);
+    changed->Write(0, data.data(), data.len());
+    return CHANGE;
+  }
+
+ private:
+  size_t expansion_;
+};
+
+// Tweak the plaintext of server records so that they exceed the client's limit.
+TEST_P(TlsConnectTls13, RecordSizePlaintextExceed) {
+  EnsureTlsSetup();
+  auto server_expand = MakeTlsFilter<TlsRecordExpander>(server_, 1);
+  server_expand->EnableDecryption();
+
+  client_->SetOption(SSL_RECORD_SIZE_LIMIT, 64);
+  Connect();
+
+  server_->SendData(100);
+
+  client_->ExpectReadWriteError();
+  ExpectAlert(client_, kTlsAlertRecordOverflow);
+  client_->ReadBytes(100);
+  EXPECT_EQ(SSL_ERROR_RX_RECORD_TOO_LONG, client_->error_code());
+
+  // Consume the alert at the server.
+  server_->Handshake();
+  server_->CheckErrorCode(SSL_ERROR_RECORD_OVERFLOW_ALERT);
+}
+
+// Tweak the ciphertext of server records so that they greatly exceed the limit.
+// This requires a much larger expansion than for plaintext to trigger the
+// guard, which runs before decryption (current allowance is 304 octets).
+TEST_P(TlsConnectTls13, RecordSizeCiphertextExceed) {
+  EnsureTlsSetup();
+
+  client_->SetOption(SSL_RECORD_SIZE_LIMIT, 64);
+  Connect();
+
+  auto server_expand = MakeTlsFilter<TlsRecordExpander>(server_, 320);
+  server_->SendData(100);
+
+  client_->ExpectReadWriteError();
+  ExpectAlert(client_, kTlsAlertRecordOverflow);
+  client_->ReadBytes(100);
+  EXPECT_EQ(SSL_ERROR_RX_RECORD_TOO_LONG, client_->error_code());
+
+  // Consume the alert at the server.
+  server_->Handshake();
+  server_->CheckErrorCode(SSL_ERROR_RECORD_OVERFLOW_ALERT);
+}
+
+// This indiscriminately adds padding to application data records.
+class TlsRecordPadder : public TlsRecordFilter {
+ public:
+  TlsRecordPadder(const std::shared_ptr<TlsAgent>& a, size_t padding)
+      : TlsRecordFilter(a), padding_(padding) {}
+
+ protected:
+  PacketFilter::Action FilterRecord(const TlsRecordHeader& header,
+                                    const DataBuffer& record, size_t* offset,
+                                    DataBuffer* output) override {
+    if (header.content_type() != kTlsApplicationDataType) {
+      return KEEP;
+    }
+
+    uint8_t inner_content_type;
+    DataBuffer plaintext;
+    if (!Unprotect(header, record, &inner_content_type, &plaintext)) {
+      return KEEP;
+    }
+
+    if (inner_content_type != kTlsApplicationDataType) {
+      return KEEP;
+    }
+
+    DataBuffer ciphertext;
+    bool ok =
+        Protect(header, inner_content_type, plaintext, &ciphertext, padding_);
+    EXPECT_TRUE(ok);
+    if (!ok) {
+      return KEEP;
+    }
+    *offset = header.Write(output, *offset, ciphertext);
+    return CHANGE;
+  }
+
+ private:
+  size_t padding_;
+};
+
+TEST_P(TlsConnectTls13, RecordSizeExceedPad) {
+  EnsureTlsSetup();
+  auto server_max = std::make_shared<TlsRecordMaximum>(server_);
+  auto server_expand = std::make_shared<TlsRecordPadder>(server_, 1);
+  server_->SetFilter(std::make_shared<ChainedPacketFilter>(
+      ChainedPacketFilterInit({server_max, server_expand})));
+  server_expand->EnableDecryption();
+
+  client_->SetOption(SSL_RECORD_SIZE_LIMIT, 64);
+  Connect();
+
+  server_->SendData(100);
+
+  client_->ExpectReadWriteError();
+  ExpectAlert(client_, kTlsAlertRecordOverflow);
+  client_->ReadBytes(100);
+  EXPECT_EQ(SSL_ERROR_RX_RECORD_TOO_LONG, client_->error_code());
+
+  // Consume the alert at the server.
+  server_->Handshake();
+  server_->CheckErrorCode(SSL_ERROR_RECORD_OVERFLOW_ALERT);
+}
+
+TEST_P(TlsConnectGeneric, RecordSizeBadValues) {
+  EnsureTlsSetup();
+  EXPECT_EQ(SECFailure,
+            SSL_OptionSet(client_->ssl_fd(), SSL_RECORD_SIZE_LIMIT, 63));
+  EXPECT_EQ(SECFailure,
+            SSL_OptionSet(client_->ssl_fd(), SSL_RECORD_SIZE_LIMIT, -1));
+  EXPECT_EQ(SECFailure,
+            SSL_OptionSet(server_->ssl_fd(), SSL_RECORD_SIZE_LIMIT, 16386));
+  Connect();
+}
+
+TEST_P(TlsConnectGeneric, RecordSizeGetValues) {
+  EnsureTlsSetup();
+  int v;
+  EXPECT_EQ(SECSuccess,
+            SSL_OptionGet(client_->ssl_fd(), SSL_RECORD_SIZE_LIMIT, &v));
+  EXPECT_EQ(16385, v);
+  client_->SetOption(SSL_RECORD_SIZE_LIMIT, 300);
+  EXPECT_EQ(SECSuccess,
+            SSL_OptionGet(client_->ssl_fd(), SSL_RECORD_SIZE_LIMIT, &v));
+  EXPECT_EQ(300, v);
+  Connect();
+}
+
+// The value of the extension is capped by the maximum version of the client.
+TEST_P(TlsConnectGeneric, RecordSizeCapExtensionClient) {
+  EnsureTlsSetup();
+  client_->SetOption(SSL_RECORD_SIZE_LIMIT, 16385);
+  auto capture =
+      MakeTlsFilter<TlsExtensionCapture>(client_, ssl_record_size_limit_xtn);
+  capture->EnableDecryption();
+  Connect();
+
+  uint64_t val = 0;
+  EXPECT_TRUE(capture->extension().Read(0, 2, &val));
+  if (version_ < SSL_LIBRARY_VERSION_TLS_1_3) {
+    EXPECT_EQ(16384U, val) << "Extension should be capped";
+  } else {
+    EXPECT_EQ(16385U, val);
+  }
+}
+
+// The value of the extension is capped by the maximum version of the server.
+TEST_P(TlsConnectGeneric, RecordSizeCapExtensionServer) {
+  EnsureTlsSetup();
+  server_->SetOption(SSL_RECORD_SIZE_LIMIT, 16385);
+  auto capture =
+      MakeTlsFilter<TlsExtensionCapture>(server_, ssl_record_size_limit_xtn);
+  capture->EnableDecryption();
+  Connect();
+
+  uint64_t val = 0;
+  EXPECT_TRUE(capture->extension().Read(0, 2, &val));
+  if (version_ < SSL_LIBRARY_VERSION_TLS_1_3) {
+    EXPECT_EQ(16384U, val) << "Extension should be capped";
+  } else {
+    EXPECT_EQ(16385U, val);
+  }
+}
+
+// Damage the client extension and the handshake fails, but the server
+// doesn't generate a validation error.
+TEST_P(TlsConnectGenericPre13, RecordSizeClientExtensionInvalid) {
+  EnsureTlsSetup();
+  client_->SetOption(SSL_RECORD_SIZE_LIMIT, 1000);
+  static const uint8_t v[] = {0xf4, 0x1f};
+  MakeTlsFilter<TlsExtensionReplacer>(client_, ssl_record_size_limit_xtn,
+                                      DataBuffer(v, sizeof(v)));
+  ConnectExpectAlert(server_, kTlsAlertDecryptError);
+}
+
+// Special handling for TLS 1.3, where the alert isn't read.
+TEST_F(TlsConnectStreamTls13, RecordSizeClientExtensionInvalid) {
+  EnsureTlsSetup();
+  client_->SetOption(SSL_RECORD_SIZE_LIMIT, 1000);
+  static const uint8_t v[] = {0xf4, 0x1f};
+  MakeTlsFilter<TlsExtensionReplacer>(client_, ssl_record_size_limit_xtn,
+                                      DataBuffer(v, sizeof(v)));
+  client_->ExpectSendAlert(kTlsAlertBadRecordMac);
+  server_->ExpectSendAlert(kTlsAlertBadRecordMac);
+  ConnectExpectFail();
+}
+
+TEST_P(TlsConnectGeneric, RecordSizeServerExtensionInvalid) {
+  EnsureTlsSetup();
+  server_->SetOption(SSL_RECORD_SIZE_LIMIT, 1000);
+  static const uint8_t v[] = {0xf4, 0x1f};
+  auto replace = MakeTlsFilter<TlsExtensionReplacer>(
+      server_, ssl_record_size_limit_xtn, DataBuffer(v, sizeof(v)));
+  replace->EnableDecryption();
+  ConnectExpectAlert(client_, kTlsAlertIllegalParameter);
+}
+
+class RecordSizeDefaultsTest : public ::testing::Test {
+ public:
+  void SetUp() {
+    EXPECT_EQ(SECSuccess,
+              SSL_OptionGetDefault(SSL_RECORD_SIZE_LIMIT, &default_));
+  }
+  void TearDown() {
+    // Make sure to restore the default value at the end.
+    EXPECT_EQ(SECSuccess,
+              SSL_OptionSetDefault(SSL_RECORD_SIZE_LIMIT, default_));
+  }
+
+ private:
+  PRIntn default_;
+};
+
+TEST_F(RecordSizeDefaultsTest, RecordSizeBadValues) {
+  EXPECT_EQ(SECFailure, SSL_OptionSetDefault(SSL_RECORD_SIZE_LIMIT, 63));
+  EXPECT_EQ(SECFailure, SSL_OptionSetDefault(SSL_RECORD_SIZE_LIMIT, -1));
+  EXPECT_EQ(SECFailure, SSL_OptionSetDefault(SSL_RECORD_SIZE_LIMIT, 16386));
+}
+
+TEST_F(RecordSizeDefaultsTest, RecordSizeGetValue) {
+  int v;
+  EXPECT_EQ(SECSuccess, SSL_OptionGetDefault(SSL_RECORD_SIZE_LIMIT, &v));
+  EXPECT_EQ(16385, v);
+  EXPECT_EQ(SECSuccess, SSL_OptionSetDefault(SSL_RECORD_SIZE_LIMIT, 3000));
+  EXPECT_EQ(SECSuccess, SSL_OptionGetDefault(SSL_RECORD_SIZE_LIMIT, &v));
+  EXPECT_EQ(3000, v);
+}
+
+}  // namespace nss_test
diff --git a/gtests/ssl_gtest/tls_agent.cc b/gtests/ssl_gtest/tls_agent.cc
index 1cd6fe396..9bed1ce1b 100644
--- a/gtests/ssl_gtest/tls_agent.cc
+++ b/gtests/ssl_gtest/tls_agent.cc
@@ -939,9 +939,9 @@ static bool ErrorIsNonFatal(PRErrorCode code) {
 }
 
 void TlsAgent::SendData(size_t bytes, size_t blocksize) {
-  uint8_t block[4096];
+  uint8_t block[16385];  // One larger than the maximum record size.
 
-  ASSERT_LT(blocksize, sizeof(block));
+  ASSERT_LE(blocksize, sizeof(block));
 
   while (bytes) {
     size_t tosend = std::min(blocksize, bytes);
diff --git a/gtests/ssl_gtest/tls_filter.cc b/gtests/ssl_gtest/tls_filter.cc
index 5b49d9851..aa03cba70 100644
--- a/gtests/ssl_gtest/tls_filter.cc
+++ b/gtests/ssl_gtest/tls_filter.cc
@@ -410,7 +410,7 @@ bool TlsRecordFilter::Unprotect(const TlsRecordHeader& header,
 bool TlsRecordFilter::Protect(const TlsRecordHeader& header,
                               uint8_t inner_content_type,
                               const DataBuffer& plaintext,
-                              DataBuffer* ciphertext) {
+                              DataBuffer* ciphertext, size_t padding) {
   if (!cipher_spec_ || header.content_type() != kTlsApplicationDataType) {
     *ciphertext = plaintext;
     return true;
@@ -418,8 +418,10 @@ bool TlsRecordFilter::Protect(const TlsRecordHeader& header,
   if (g_ssl_gtest_verbose) {
     std::cerr << "protect: " << header.sequence_number() << std::endl;
   }
-  DataBuffer padded = plaintext;
-  padded.Write(padded.len(), inner_content_type, 1);
+  DataBuffer padded;
+  padded.Allocate(plaintext.len() + 1 + padding);
+  size_t offset = padded.Write(0, plaintext.data(), plaintext.len());
+  padded.Write(offset, inner_content_type, 1);
   return cipher_spec_->Protect(header, padded, ciphertext);
 }
 
diff --git a/gtests/ssl_gtest/tls_filter.h b/gtests/ssl_gtest/tls_filter.h
index 90520361c..80c60b42f 100644
--- a/gtests/ssl_gtest/tls_filter.h
+++ b/gtests/ssl_gtest/tls_filter.h
@@ -120,7 +120,8 @@ class TlsRecordFilter : public PacketFilter {
   bool Unprotect(const TlsRecordHeader& header, const DataBuffer& cipherText,
                  uint8_t* inner_content_type, DataBuffer* plaintext);
   bool Protect(const TlsRecordHeader& header, uint8_t inner_content_type,
-               const DataBuffer& plaintext, DataBuffer* ciphertext);
+               const DataBuffer& plaintext, DataBuffer* ciphertext,
+               size_t padding = 0);
 
  protected:
   // There are two filter functions which can be overriden. Both are
diff --git a/lib/ssl/dtlscon.c b/lib/ssl/dtlscon.c
index 6c3d7d24c..a82295c66 100644
--- a/lib/ssl/dtlscon.c
+++ b/lib/ssl/dtlscon.c
@@ -724,13 +724,16 @@ dtls_FragmentHandshake(sslSocket *ss, DTLSQueuedMessage *msg)
         PORT_Assert(end <= contentLen);
         fragmentLen = PR_MIN(end, contentLen) - fragmentOffset;
 
-        /* Reduce to the space remaining in the MTU.  Allow for any existing
-         * messages, record expansion, and the handshake header. */
+        /* Limit further by the record size limit.  Account for the header. */
+        fragmentLen = PR_MIN(fragmentLen,
+                             msg->cwSpec->recordSizeLimit - DTLS_HS_HDR_LEN);
+
+        /* Reduce to the space remaining in the MTU. */
         fragmentLen = PR_MIN(fragmentLen,
                              ss->ssl3.mtu -           /* MTU estimate. */
-                                 ss->pendingBuf.len - /* Less unsent records. */
+                                 ss->pendingBuf.len - /* Less any unsent records. */
                                  DTLS_MAX_EXPANSION - /* Allow for expansion. */
-                                 DTLS_HS_HDR_LEN);    /* + handshake header. */
+                                 DTLS_HS_HDR_LEN);    /* And the handshake header. */
         PORT_Assert(fragmentLen > 0 || fragmentOffset == 0);
 
         /* Make totally sure that we will fit in the buffer. This should be
diff --git a/lib/ssl/ssl.h b/lib/ssl/ssl.h
index 0280f73c3..ecc4f9506 100644
--- a/lib/ssl/ssl.h
+++ b/lib/ssl/ssl.h
@@ -243,6 +243,28 @@ SSL_IMPORT PRFileDesc *DTLS_ImportFD(PRFileDesc *model, PRFileDesc *fd);
  */
 #define SSL_ENABLE_0RTT_DATA 33
 
+/* Sets a limit to the size of encrypted records (see
+ * draft-ietf-tls-record-limit). This is the value that is advertised to peers,
+ * not a limit on the size of records that will be created.  Setting this value
+ * reduces the size of records that will be received (not sent).
+ *
+ * This limit applies to the plaintext, but the records that appear on the wire
+ * will be bigger.  This doesn't include record headers, IVs, block cipher
+ * padding, and authentication tags or MACs.
+ *
+ * NSS always advertises the record size limit extension.  If this option is not
+ * set, the extension will contain the maximum allowed size for the selected TLS
+ * version (currently this is 16384 or 2^14 for TLS 1.2 and lower and 16385 for
+ * TLS 1.3).
+ *
+ * By default, NSS creates records that are the maximum size possible, using all
+ * the data that was written by the application.  Writes larger than the maximum
+ * are split into maximum sized records, and any remainder (unless
+ * SSL_CBC_RANDOM_IV is enabled and active).  If a peer advertises a record size
+ * limit then that value is used instead.
+ */
+#define SSL_RECORD_SIZE_LIMIT 34
+
 /* Enables TLS 1.3 compatibility mode.  In this mode, the client includes a fake
  * session ID in the handshake and sends a ChangeCipherSpec.  A server will
  * always use the setting chosen by the client, so the value of this option has
diff --git a/lib/ssl/ssl3con.c b/lib/ssl/ssl3con.c
index dd280d2ef..466fc296f 100644
--- a/lib/ssl/ssl3con.c
+++ b/lib/ssl/ssl3con.c
@@ -1476,6 +1476,13 @@ ssl3_SetupBothPendingCipherSpecs(sslSocket *ss)
         goto loser;
     }
 
+    if (ssl3_ExtensionNegotiated(ss, ssl_record_size_limit_xtn)) {
+        ss->ssl3.prSpec->recordSizeLimit = PR_MIN(MAX_FRAGMENT_LENGTH,
+                                                  ss->opt.recordSizeLimit);
+        ss->ssl3.pwSpec->recordSizeLimit = PR_MIN(MAX_FRAGMENT_LENGTH,
+                                                  ss->xtnData.recordSizeLimit);
+    }
+
     ssl_ReleaseSpecWriteLock(ss); /*******************************/
     return SECSuccess;
 
@@ -2272,7 +2279,7 @@ ssl_ProtectNextRecord(sslSocket *ss, ssl3CipherSpec *spec, SSL3ContentType type,
     unsigned int spaceNeeded;
     SECStatus rv;
 
-    contentLen = PR_MIN(nIn, MAX_FRAGMENT_LENGTH);
+    contentLen = PR_MIN(nIn, spec->recordSizeLimit);
     spaceNeeded = contentLen + SSL3_BUFFER_FUDGE;
     if (spec->version >= SSL_LIBRARY_VERSION_TLS_1_1 &&
         spec->cipherDef->type == type_block) {
@@ -12147,6 +12154,11 @@ ssl3_GetCipherSpec(sslSocket *ss, SSL3Ciphertext *cText)
     return NULL;
 }
 
+/* MAX_EXPANSION is the amount by which a record might plausibly be expanded
+ * when protected.  It's the worst case estimate, so the sum of block cipher
+ * padding (up to 256 octets) and HMAC (48 octets for SHA-384). */
+#define MAX_EXPANSION (256 + 48)
+
 /* if cText is non-null, then decipher and check the MAC of the
  * SSL record from cText->buf (typically gs->inbuf)
  * into databuf (typically gs->buf), and any previous contents of databuf
@@ -12176,6 +12188,7 @@ ssl3_HandleRecord(sslSocket *ss, SSL3Ciphertext *cText)
     PRBool isTLS;
     DTLSEpoch epoch;
     ssl3CipherSpec *spec = NULL;
+    PRUint16 recordSizeLimit;
     PRBool outOfOrderSpec = PR_FALSE;
     SSL3ContentType rType;
     sslBuffer *plaintext = &ss->gs.buf;
@@ -12230,12 +12243,20 @@ ssl3_HandleRecord(sslSocket *ss, SSL3Ciphertext *cText)
         return SECFailure;
     }
 
-    if (plaintext->space < MAX_FRAGMENT_LENGTH) {
-        rv = sslBuffer_Grow(plaintext, MAX_FRAGMENT_LENGTH + 2048);
+    recordSizeLimit = spec->recordSizeLimit;
+    if (cText->buf->len > recordSizeLimit + MAX_EXPANSION) {
+        ssl_ReleaseSpecReadLock(ss); /*****************************/
+        SSL3_SendAlert(ss, alert_fatal, record_overflow);
+        PORT_SetError(SSL_ERROR_RX_RECORD_TOO_LONG);
+        return SECFailure;
+    }
+
+    if (plaintext->space < recordSizeLimit + MAX_EXPANSION) {
+        rv = sslBuffer_Grow(plaintext, recordSizeLimit + MAX_EXPANSION);
         if (rv != SECSuccess) {
             ssl_ReleaseSpecReadLock(ss); /*************************/
             SSL_DBG(("%d: SSL3[%d]: HandleRecord, tried to get %d bytes",
-                     SSL_GETPID(), ss->fd, MAX_FRAGMENT_LENGTH + 2048));
+                     SSL_GETPID(), ss->fd, recordSizeLimit + MAX_EXPANSION));
             /* sslBuffer_Grow has set a memory error code. */
             /* Perhaps we should send an alert. (but we have no memory!) */
             return SECFailure;
@@ -12294,7 +12315,10 @@ ssl3_HandleRecord(sslSocket *ss, SSL3Ciphertext *cText)
         if (IS_DTLS(ss) ||
             (ss->sec.isServer &&
              ss->ssl3.hs.zeroRttIgnore == ssl_0rtt_ignore_trial)) {
-            /* Silently drop the packet */
+            /* Silently drop the packet unless we sent a fatal alert. */
+            if (ss->ssl3.fatalAlertSent) {
+                return SECFailure;
+            }
             return SECSuccess;
         }
 
@@ -12330,7 +12354,7 @@ ssl3_HandleRecord(sslSocket *ss, SSL3Ciphertext *cText)
     }
 
     /* Check the length of the plaintext. */
-    if (isTLS && plaintext->len > MAX_FRAGMENT_LENGTH) {
+    if (isTLS && plaintext->len > recordSizeLimit) {
         plaintext->len = 0;
         SSL3_SendAlert(ss, alert_fatal, record_overflow);
         PORT_SetError(SSL_ERROR_RX_RECORD_TOO_LONG);
diff --git a/lib/ssl/ssl3ext.c b/lib/ssl/ssl3ext.c
index 782425e57..9b6c719f8 100644
--- a/lib/ssl/ssl3ext.c
+++ b/lib/ssl/ssl3ext.c
@@ -50,6 +50,7 @@ static const ssl3ExtensionHandler clientHelloHandlers[] = {
     { ssl_tls13_early_data_xtn, &tls13_ServerHandleEarlyDataXtn },
     { ssl_tls13_psk_key_exchange_modes_xtn, &tls13_ServerHandlePskModesXtn },
     { ssl_tls13_cookie_xtn, &tls13_ServerHandleCookieXtn },
+    { ssl_record_size_limit_xtn, &ssl_HandleRecordSizeLimitXtn },
     { 0, NULL }
 };
 
@@ -68,6 +69,7 @@ static const ssl3ExtensionHandler serverHelloHandlersTLS[] = {
     { ssl_tls13_key_share_xtn, &tls13_ClientHandleKeyShareXtn },
     { ssl_tls13_pre_shared_key_xtn, &tls13_ClientHandlePreSharedKeyXtn },
     { ssl_tls13_early_data_xtn, &tls13_ClientHandleEarlyDataXtn },
+    { ssl_record_size_limit_xtn, &ssl_HandleRecordSizeLimitXtn },
     { 0, NULL }
 };
 
@@ -134,6 +136,7 @@ static const sslExtensionBuilder clientHelloSendersTLS[] =
       { ssl_signature_algorithms_xtn, &ssl3_SendSigAlgsXtn },
       { ssl_tls13_cookie_xtn, &tls13_ClientSendHrrCookieXtn },
       { ssl_tls13_psk_key_exchange_modes_xtn, &tls13_ClientSendPskModesXtn },
+      { ssl_record_size_limit_xtn, &ssl_SendRecordSizeLimitXtn },
       /* The pre_shared_key extension MUST be last. */
       { ssl_tls13_pre_shared_key_xtn, &tls13_ClientSendPreSharedKeyXtn },
       { 0, NULL }
diff --git a/lib/ssl/ssl3ext.h b/lib/ssl/ssl3ext.h
index d0f75a599..6d77c7459 100644
--- a/lib/ssl/ssl3ext.h
+++ b/lib/ssl/ssl3ext.h
@@ -98,6 +98,9 @@ struct TLSExtensionDataStr {
     /* The application token contains a value that was passed to the client via
      * a session ticket, or the cookie in a HelloRetryRequest. */
     SECItem applicationToken;
+
+    /* The record size limit set by the peer. Our value is kept in ss->opt. */
+    PRUint16 recordSizeLimit;
 };
 
 typedef struct TLSExtensionStr {
diff --git a/lib/ssl/ssl3exthandle.c b/lib/ssl/ssl3exthandle.c
index d08e5e435..d1f286dc3 100644
--- a/lib/ssl/ssl3exthandle.c
+++ b/lib/ssl/ssl3exthandle.c
@@ -1868,3 +1868,67 @@ ssl_HandleSupportedGroupsXtn(const sslSocket *ss, TLSExtensionData *xtnData,
 
     return SECSuccess;
 }
+
+SECStatus
+ssl_HandleRecordSizeLimitXtn(const sslSocket *ss, TLSExtensionData *xtnData,
+                             SECItem *data)
+{
+    SECStatus rv;
+    PRUint32 limit;
+    PRUint32 maxLimit = (ss->version >= SSL_LIBRARY_VERSION_TLS_1_3)
+                            ? (MAX_FRAGMENT_LENGTH + 1)
+                            : MAX_FRAGMENT_LENGTH;
+
+    rv = ssl3_ExtConsumeHandshakeNumber(ss, &limit, 2, &data->data, &data->len);
+    if (rv != SECSuccess) {
+        return SECFailure;
+    }
+    if (data->len != 0 || limit < 64) {
+        ssl3_ExtSendAlert(ss, alert_fatal, illegal_parameter);
+        PORT_SetError(SSL_ERROR_RX_MALFORMED_HANDSHAKE);
+        return SECFailure;
+    }
+
+    if (ss->sec.isServer) {
+        rv = ssl3_RegisterExtensionSender(ss, xtnData, ssl_record_size_limit_xtn,
+                                          &ssl_SendRecordSizeLimitXtn);
+        if (rv != SECSuccess) {
+            return SECFailure; /* error already set. */
+        }
+    } else if (limit > maxLimit) {
+        /* The client can sensibly check the maximum. */
+        ssl3_ExtSendAlert(ss, alert_fatal, illegal_parameter);
+        PORT_SetError(SSL_ERROR_RX_MALFORMED_HANDSHAKE);
+        return SECFailure;
+    }
+
+    /* We can't enforce the maximum on a server. But we do need to ensure
+     * that we don't apply a limit that is too large. */
+    xtnData->recordSizeLimit = PR_MIN(maxLimit, limit);
+    xtnData->negotiated[xtnData->numNegotiated++] = ssl_record_size_limit_xtn;
+    return SECSuccess;
+}
+
+SECStatus
+ssl_SendRecordSizeLimitXtn(const sslSocket *ss, TLSExtensionData *xtnData,
+                           sslBuffer *buf, PRBool *added)
+{
+    PRUint32 maxLimit;
+    if (ss->sec.isServer) {
+        maxLimit = (ss->version >= SSL_LIBRARY_VERSION_TLS_1_3)
+                       ? (MAX_FRAGMENT_LENGTH + 1)
+                       : MAX_FRAGMENT_LENGTH;
+    } else {
+        maxLimit = (ss->vrange.max >= SSL_LIBRARY_VERSION_TLS_1_3)
+                       ? (MAX_FRAGMENT_LENGTH + 1)
+                       : MAX_FRAGMENT_LENGTH;
+    }
+    PRUint32 limit = PR_MIN(ss->opt.recordSizeLimit, maxLimit);
+    SECStatus rv = sslBuffer_AppendNumber(buf, limit, 2);
+    if (rv != SECSuccess) {
+        return SECFailure;
+    }
+
+    *added = PR_TRUE;
+    return SECSuccess;
+}
diff --git a/lib/ssl/ssl3exthandle.h b/lib/ssl/ssl3exthandle.h
index b84bd074c..eaf7f0081 100644
--- a/lib/ssl/ssl3exthandle.h
+++ b/lib/ssl/ssl3exthandle.h
@@ -119,4 +119,11 @@ SECStatus ssl_SendSupportedGroupsXtn(const sslSocket *ss,
 SECStatus ssl3_SendSupportedPointFormatsXtn(const sslSocket *ss,
                                             TLSExtensionData *xtnData,
                                             sslBuffer *buf, PRBool *added);
+SECStatus ssl_HandleRecordSizeLimitXtn(const sslSocket *ss,
+                                       TLSExtensionData *xtnData,
+                                       SECItem *data);
+SECStatus ssl_SendRecordSizeLimitXtn(const sslSocket *ss,
+                                     TLSExtensionData *xtnData,
+                                     sslBuffer *buf, PRBool *added);
+
 #endif
diff --git a/lib/ssl/sslimpl.h b/lib/ssl/sslimpl.h
index b358546be..a2209e90a 100644
--- a/lib/ssl/sslimpl.h
+++ b/lib/ssl/sslimpl.h
@@ -236,6 +236,7 @@ typedef struct sslOptionsStr {
     /* If SSL_SetNextProtoNego has been called, then this contains the
      * list of supported protocols. */
     SECItem nextProtoNego;
+    PRUint16 recordSizeLimit;
 
     PRUint32 maxEarlyDataSize;
     unsigned int useSecurity : 1;
diff --git a/lib/ssl/sslsock.c b/lib/ssl/sslsock.c
index 286b35e7d..33595ffae 100644
--- a/lib/ssl/sslsock.c
+++ b/lib/ssl/sslsock.c
@@ -55,6 +55,7 @@ static const sslSocketOps ssl_secure_ops = { /* SSL. */
 static sslOptions ssl_defaults = {
     .nextProtoNego = { siBuffer, NULL, 0 },
     .maxEarlyDataSize = 1 << 16,
+    .recordSizeLimit = MAX_FRAGMENT_LENGTH + 1,
     .useSecurity = PR_TRUE,
     .useSocks = PR_FALSE,
     .requestCertificate = PR_FALSE,
@@ -803,6 +804,15 @@ SSL_OptionSet(PRFileDesc *fd, PRInt32 which, PRIntn val)
             ss->opt.enable0RttData = val;
             break;
 
+        case SSL_RECORD_SIZE_LIMIT:
+            if (val < 64 || val > (MAX_FRAGMENT_LENGTH + 1)) {
+                PORT_SetError(SEC_ERROR_INVALID_ARGS);
+                rv = SECFailure;
+            } else {
+                ss->opt.recordSizeLimit = val;
+            }
+            break;
+
         case SSL_ENABLE_TLS13_COMPAT_MODE:
             ss->opt.enableTls13CompatMode = val;
             break;
@@ -944,6 +954,9 @@ SSL_OptionGet(PRFileDesc *fd, PRInt32 which, PRIntn *pVal)
         case SSL_ENABLE_0RTT_DATA:
             val = ss->opt.enable0RttData;
             break;
+        case SSL_RECORD_SIZE_LIMIT:
+            val = ss->opt.recordSizeLimit;
+            break;
         case SSL_ENABLE_TLS13_COMPAT_MODE:
             val = ss->opt.enableTls13CompatMode;
             break;
@@ -1067,6 +1080,9 @@ SSL_OptionGetDefault(PRInt32 which, PRIntn *pVal)
         case SSL_ENABLE_0RTT_DATA:
             val = ssl_defaults.enable0RttData;
             break;
+        case SSL_RECORD_SIZE_LIMIT:
+            val = ssl_defaults.recordSizeLimit;
+            break;
         case SSL_ENABLE_TLS13_COMPAT_MODE:
             val = ssl_defaults.enableTls13CompatMode;
             break;
@@ -1252,6 +1268,14 @@ SSL_OptionSetDefault(PRInt32 which, PRIntn val)
             ssl_defaults.enable0RttData = val;
             break;
 
+        case SSL_RECORD_SIZE_LIMIT:
+            if (val < 64 || val > (MAX_FRAGMENT_LENGTH + 1)) {
+                PORT_SetError(SEC_ERROR_INVALID_ARGS);
+                return SECFailure;
+            }
+            ssl_defaults.recordSizeLimit = val;
+            break;
+
         case SSL_ENABLE_TLS13_COMPAT_MODE:
             ssl_defaults.enableTls13CompatMode = val;
             break;
diff --git a/lib/ssl/sslspec.c b/lib/ssl/sslspec.c
index 26c3eb546..7833eeab6 100644
--- a/lib/ssl/sslspec.c
+++ b/lib/ssl/sslspec.c
@@ -143,6 +143,7 @@ ssl_CreateCipherSpec(sslSocket *ss, CipherSpecDirection direction)
     spec->refCt = 1;
     spec->version = ss->version;
     spec->direction = direction;
+    spec->recordSizeLimit = MAX_FRAGMENT_LENGTH;
     SSL_TRC(10, ("%d: SSL[%d]: new %s spec %d ct=%d",
                  SSL_GETPID(), ss->fd, SPEC_DIR(spec), spec,
                  spec->refCt));
diff --git a/lib/ssl/sslspec.h b/lib/ssl/sslspec.h
index 207bd6ef6..b25601755 100644
--- a/lib/ssl/sslspec.h
+++ b/lib/ssl/sslspec.h
@@ -170,6 +170,10 @@ struct ssl3CipherSpecStr {
     /* The number of 0-RTT bytes that can be sent or received in TLS 1.3. This
      * will be zero for everything but 0-RTT. */
     PRUint32 earlyDataRemaining;
+    /* The maximum plaintext length.  This differs from the configured or
+     * negotiated value for TLS 1.3; it is reduced by one to account for the
+     * content type octet. */
+    PRUint16 recordSizeLimit;
 };
 
 typedef void (*sslCipherSpecChangedFunc)(void *arg,
diff --git a/lib/ssl/sslt.h b/lib/ssl/sslt.h
index e2b80fb43..bb1bec7a3 100644
--- a/lib/ssl/sslt.h
+++ b/lib/ssl/sslt.h
@@ -432,6 +432,7 @@ typedef enum {
     ssl_signed_cert_timestamp_xtn = 18,
     ssl_padding_xtn = 21,
     ssl_extended_master_secret_xtn = 23,
+    ssl_record_size_limit_xtn = 28,
     ssl_session_ticket_xtn = 35,
     /* 40 was used in draft versions of TLS 1.3; it is now reserved. */
     ssl_tls13_pre_shared_key_xtn = 41,
@@ -454,7 +455,7 @@ typedef enum {
 /* SSL_MAX_EXTENSIONS includes the maximum number of extensions that are
  * supported for any single message type.  That is, a ClientHello; ServerHello
  * and TLS 1.3 NewSessionTicket and HelloRetryRequest extensions have fewer. */
-#define SSL_MAX_EXTENSIONS 20
+#define SSL_MAX_EXTENSIONS 21
 
 /* Deprecated */
 typedef enum {
diff --git a/lib/ssl/tls13con.c b/lib/ssl/tls13con.c
index 3592f30f7..4d9170fb0 100644
--- a/lib/ssl/tls13con.c
+++ b/lib/ssl/tls13con.c
@@ -3254,6 +3254,17 @@ tls13_SetupPendingCipherSpec(sslSocket *ss, ssl3CipherSpec *spec)
     }
 
     tls13_SetSpecRecordVersion(ss, spec);
+
+    /* The record size limit is reduced by one so that the remainder of the
+     * record handling code can use the same checks for all versions. */
+    if (ssl3_ExtensionNegotiated(ss, ssl_record_size_limit_xtn)) {
+        spec->recordSizeLimit = ((spec->direction == CipherSpecRead)
+                                     ? ss->opt.recordSizeLimit
+                                     : ss->xtnData.recordSizeLimit) -
+                                1;
+    } else {
+        spec->recordSizeLimit = MAX_FRAGMENT_LENGTH;
+    }
     return SECSuccess;
 }
 
@@ -4746,7 +4757,8 @@ static const struct {
     { ssl_tls13_cookie_xtn, _M2(client_hello, hello_retry_request) },
     { ssl_tls13_certificate_authorities_xtn, _M1(certificate_request) },
     { ssl_tls13_supported_versions_xtn, _M3(client_hello, server_hello,
-                                            hello_retry_request) }
+                                            hello_retry_request) },
+    { ssl_record_size_limit_xtn, _M2(client_hello, encrypted_extensions) }
 };
 
 tls13ExtensionStatus
@@ -5015,6 +5027,16 @@ tls13_UnprotectRecord(sslSocket *ss,
         return SECFailure;
     }
 
+    /* There is a similar test in ssl3_HandleRecord, but this test is needed to
+     * account for padding.  It's safe to do this here (including the alert),
+     * because it only confirms that the record exceeded the size limit, which
+     * is apparent from the size of the ciphertext. */
+    if (plaintext->len > spec->recordSizeLimit + 1) {
+        SSL3_SendAlert(ss, alert_fatal, record_overflow);
+        PORT_SetError(SSL_ERROR_RX_RECORD_TOO_LONG);
+        return SECFailure;
+    }
+
     /* The record is right-padded with 0s, followed by the true
      * content type, so read from the right until we receive a
      * nonzero byte. */