net/quic/crypto/strike_register.cc

   1 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "net/quic/crypto/strike_register.h"
   6
   7 #include <limits>
   8
   9 #include "base/logging.h"
  10
  11 using std::max;
  12 using std::min;
  13 using std::pair;
  14 using std::set;
  15 using std::vector;
  16
  17 namespace net {
  18
  19 namespace {
  20
  21 uint32 GetInitialHorizon(uint32 current_time_internal,
  22                          uint32 window_secs,
  23                          StrikeRegister::StartupType startup) {
  24   if (startup == StrikeRegister::DENY_REQUESTS_AT_STARTUP) {
  25     // The horizon is initially set |window_secs| into the future because, if
  26     // we just crashed, then we may have accepted nonces in the span
  27     // [current_time...current_time+window_secs] and so we conservatively
  28     // reject the whole timespan unless |startup| tells us otherwise.
  29     return current_time_internal + window_secs + 1;
  30   } else {  // startup == StrikeRegister::NO_STARTUP_PERIOD_NEEDED
  31     // The orbit can be assumed to be globally unique.  Use a horizon
  32     // in the past.
  33     return 0;
  34   }
  35 }
  36
  37 }  // namespace
  38
  39 // static
  40 const uint32 StrikeRegister::kExternalNodeSize = 24;
  41 // static
  42 const uint32 StrikeRegister::kNil = (1u << 31) | 1;
  43 // static
  44 const uint32 StrikeRegister::kExternalFlag = 1 << 23;
  45
  46 // InternalNode represents a non-leaf node in the critbit tree. See the comment
  47 // in the .h file for details.
  48 class StrikeRegister::InternalNode {
  49  public:
  50   void SetChild(unsigned direction, uint32 child) {
  51     data_[direction] = (data_[direction] & 0xff) | (child << 8);
  52   }
  53
  54   void SetCritByte(uint8 critbyte) {
  55     data_[0] = (data_[0] & 0xffffff00) | critbyte;
  56   }
  57
  58   void SetOtherBits(uint8 otherbits) {
  59     data_[1] = (data_[1] & 0xffffff00) | otherbits;
  60   }
  61
  62   void SetNextPtr(uint32 next) { data_[0] = next; }
  63
  64   uint32 next() const { return data_[0]; }
  65
  66   uint32 child(unsigned n) const { return data_[n] >> 8; }
  67
  68   uint8 critbyte() const { return static_cast<uint8>(data_[0]); }
  69
  70   uint8 otherbits() const { return static_cast<uint8>(data_[1]); }
  71
  72   // These bytes are organised thus:
  73   //   <24 bits> left child
  74   //   <8 bits> crit-byte
  75   //   <24 bits> right child
  76   //   <8 bits> other-bits
  77   uint32 data_[2];
  78 };
  79
  80 // kCreationTimeFromInternalEpoch contains the number of seconds between the
  81 // start of the internal epoch and the creation time. This allows us
  82 // to consider times that are before the creation time.
  83 static const uint32 kCreationTimeFromInternalEpoch = 63115200;  // 2 years.
  84
  85 void StrikeRegister::ValidateStrikeRegisterConfig(unsigned max_entries) {
  86   // We only have 23 bits of index available.
  87   CHECK_LT(max_entries, 1u << 23);
  88   CHECK_GT(max_entries, 1u);           // There must be at least two entries.
  89   CHECK_EQ(sizeof(InternalNode), 8u);  // in case of compiler changes.
  90 }
  91
  92 StrikeRegister::StrikeRegister(unsigned max_entries,
  93                                uint32 current_time,
  94                                uint32 window_secs,
  95                                const uint8 orbit[8],
  96                                StartupType startup)
  97     : max_entries_(max_entries),
  98       window_secs_(window_secs),
  99       internal_epoch_(current_time > kCreationTimeFromInternalEpoch
 100                           ? current_time - kCreationTimeFromInternalEpoch
 101                           : 0),
 102       horizon_(GetInitialHorizon(
 103           ExternalTimeToInternal(current_time), window_secs, startup)) {
 104   memcpy(orbit_, orbit, sizeof(orbit_));
 105
 106   ValidateStrikeRegisterConfig(max_entries);
 107   internal_nodes_ = new InternalNode[max_entries];
 108   external_nodes_.reset(new uint8[kExternalNodeSize * max_entries]);
 109
 110   Reset();
 111 }
 112
 113 StrikeRegister::~StrikeRegister() { delete[] internal_nodes_; }
 114
 115 void StrikeRegister::Reset() {
 116   // Thread a free list through all of the internal nodes.
 117   internal_node_free_head_ = 0;
 118   for (unsigned i = 0; i < max_entries_ - 1; i++)
 119     internal_nodes_[i].SetNextPtr(i + 1);
 120   internal_nodes_[max_entries_ - 1].SetNextPtr(kNil);
 121
 122   // Also thread a free list through the external nodes.
 123   external_node_free_head_ = 0;
 124   for (unsigned i = 0; i < max_entries_ - 1; i++)
 125     external_node_next_ptr(i) = i + 1;
 126   external_node_next_ptr(max_entries_ - 1) = kNil;
 127
 128   // This is the root of the tree.
 129   internal_node_head_ = kNil;
 130 }
 131
 132 InsertStatus StrikeRegister::Insert(const uint8 nonce[32],
 133                                     uint32 current_time_external) {
 134   // Make space for the insertion if the strike register is full.
 135   while (external_node_free_head_ == kNil ||
 136          internal_node_free_head_ == kNil) {
 137     DropOldestNode();
 138   }
 139
 140   const uint32 current_time = ExternalTimeToInternal(current_time_external);
 141
 142   // Check to see if the orbit is correct.
 143   if (memcmp(nonce + sizeof(current_time), orbit_, sizeof(orbit_))) {
 144     return NONCE_INVALID_ORBIT_FAILURE;
 145   }
 146
 147   const uint32 nonce_time = ExternalTimeToInternal(TimeFromBytes(nonce));
 148
 149   // Check that the timestamp is in the valid range.
 150   pair<uint32, uint32> valid_range =
 151       StrikeRegister::GetValidRange(current_time);
 152   if (nonce_time < valid_range.first || nonce_time > valid_range.second) {
 153     return NONCE_INVALID_TIME_FAILURE;
 154   }
 155
 156   // We strip the orbit out of the nonce.
 157   uint8 value[24];
 158   memcpy(value, nonce, sizeof(nonce_time));
 159   memcpy(value + sizeof(nonce_time),
 160          nonce + sizeof(nonce_time) + sizeof(orbit_),
 161          sizeof(value) - sizeof(nonce_time));
 162
 163   // Find the best match to |value| in the crit-bit tree. The best match is
 164   // simply the value which /could/ match |value|, if any does, so we still
 165   // need a memcmp to check.
 166   uint32 best_match_index = BestMatch(value);
 167   if (best_match_index == kNil) {
 168     // Empty tree. Just insert the new value at the root.
 169     uint32 index = GetFreeExternalNode();
 170     memcpy(external_node(index), value, sizeof(value));
 171     internal_node_head_ = (index | kExternalFlag) << 8;
 172     DCHECK_LE(horizon_, nonce_time);
 173     return NONCE_OK;
 174   }
 175
 176   const uint8* best_match = external_node(best_match_index);
 177   if (memcmp(best_match, value, sizeof(value)) == 0) {
 178     // We found the value in the tree.
 179     return NONCE_NOT_UNIQUE_FAILURE;
 180   }
 181
 182   // We are going to insert a new entry into the tree, so get the nodes now.
 183   uint32 internal_node_index = GetFreeInternalNode();
 184   uint32 external_node_index = GetFreeExternalNode();
 185
 186   // If we just evicted the best match, then we have to try and match again.
 187   // We know that we didn't just empty the tree because we require that
 188   // max_entries_ >= 2. Also, we know that it doesn't match because, if it
 189   // did, it would have been returned previously.
 190   if (external_node_index == best_match_index) {
 191     best_match_index = BestMatch(value);
 192     best_match = external_node(best_match_index);
 193   }
 194
 195   // Now we need to find the first bit where we differ from |best_match|.
 196   uint8 differing_byte;
 197   uint8 new_other_bits;
 198   for (differing_byte = 0; differing_byte < arraysize(value);
 199        differing_byte++) {
 200     new_other_bits = value[differing_byte] ^ best_match[differing_byte];
 201     if (new_other_bits) {
 202       break;
 203     }
 204   }
 205
 206   // Once we have the XOR the of first differing byte in new_other_bits we need
 207   // to find the most significant differing bit. We could do this with a simple
 208   // for loop, testing bits 7..0. Instead we fold the bits so that we end up
 209   // with a byte where all the bits below the most significant one, are set.
 210   new_other_bits |= new_other_bits >> 1;
 211   new_other_bits |= new_other_bits >> 2;
 212   new_other_bits |= new_other_bits >> 4;
 213   // Now this bit trick results in all the bits set, except the original
 214   // most-significant one.
 215   new_other_bits = (new_other_bits & ~(new_other_bits >> 1)) ^ 255;
 216
 217   // Consider the effect of ORing against |new_other_bits|. If |value| did not
 218   // have the critical bit set, the result is the same as |new_other_bits|. If
 219   // it did, the result is all ones.
 220
 221   unsigned newdirection;
 222   if ((new_other_bits | value[differing_byte]) == 0xff) {
 223     newdirection = 1;
 224   } else {
 225     newdirection = 0;
 226   }
 227
 228   memcpy(external_node(external_node_index), value, sizeof(value));
 229   InternalNode* inode = &internal_nodes_[internal_node_index];
 230
 231   inode->SetChild(newdirection, external_node_index | kExternalFlag);
 232   inode->SetCritByte(differing_byte);
 233   inode->SetOtherBits(new_other_bits);
 234
 235   // |where_index| is a pointer to the uint32 which needs to be updated in
 236   // order to insert the new internal node into the tree. The internal nodes
 237   // store the child indexes in the top 24-bits of a 32-bit word and, to keep
 238   // the code simple, we define that |internal_node_head_| is organised the
 239   // same way.
 240   DCHECK_EQ(internal_node_head_ & 0xff, 0u);
 241   uint32* where_index = &internal_node_head_;
 242   while (((*where_index >> 8) & kExternalFlag) == 0) {
 243     InternalNode* node = &internal_nodes_[*where_index >> 8];
 244     if (node->critbyte() > differing_byte) {
 245       break;
 246     }
 247     if (node->critbyte() == differing_byte &&
 248         node->otherbits() > new_other_bits) {
 249       break;
 250     }
 251     if (node->critbyte() == differing_byte &&
 252         node->otherbits() == new_other_bits) {
 253       CHECK(false);
 254     }
 255
 256     uint8 c = value[node->critbyte()];
 257     const int direction =
 258         (1 + static_cast<unsigned>(node->otherbits() | c)) >> 8;
 259     where_index = &node->data_[direction];
 260   }
 261
 262   inode->SetChild(newdirection ^ 1, *where_index >> 8);
 263   *where_index = (*where_index & 0xff) | (internal_node_index << 8);
 264
 265   DCHECK_LE(horizon_, nonce_time);
 266   return NONCE_OK;
 267 }
 268
 269 const uint8* StrikeRegister::orbit() const {
 270   return orbit_;
 271 }
 272
 273 uint32 StrikeRegister::GetCurrentValidWindowSecs(
 274     uint32 current_time_external) const {
 275   uint32 current_time = ExternalTimeToInternal(current_time_external);
 276   pair<uint32, uint32> valid_range = StrikeRegister::GetValidRange(
 277       current_time);
 278   if (valid_range.second >= valid_range.first) {
 279     return valid_range.second - current_time + 1;
 280   } else {
 281     return 0;
 282   }
 283 }
 284
 285 void StrikeRegister::Validate() {
 286   set<uint32> free_internal_nodes;
 287   for (uint32 i = internal_node_free_head_; i != kNil;
 288        i = internal_nodes_[i].next()) {
 289     CHECK_LT(i, max_entries_);
 290     CHECK_EQ(free_internal_nodes.count(i), 0u);
 291     free_internal_nodes.insert(i);
 292   }
 293
 294   set<uint32> free_external_nodes;
 295   for (uint32 i = external_node_free_head_; i != kNil;
 296        i = external_node_next_ptr(i)) {
 297     CHECK_LT(i, max_entries_);
 298     CHECK_EQ(free_external_nodes.count(i), 0u);
 299     free_external_nodes.insert(i);
 300   }
 301
 302   set<uint32> used_external_nodes;
 303   set<uint32> used_internal_nodes;
 304
 305   if (internal_node_head_ != kNil &&
 306       ((internal_node_head_ >> 8) & kExternalFlag) == 0) {
 307     vector<pair<unsigned, bool> > bits;
 308     ValidateTree(internal_node_head_ >> 8, -1, bits, free_internal_nodes,
 309                  free_external_nodes, &used_internal_nodes,
 310                  &used_external_nodes);
 311   }
 312 }
 313
 314 // static
 315 uint32 StrikeRegister::TimeFromBytes(const uint8 d[4]) {
 316   return static_cast<uint32>(d[0]) << 24 |
 317          static_cast<uint32>(d[1]) << 16 |
 318          static_cast<uint32>(d[2]) << 8 |
 319          static_cast<uint32>(d[3]);
 320 }
 321
 322 pair<uint32, uint32> StrikeRegister::GetValidRange(
 323     uint32 current_time_internal) const {
 324   if (current_time_internal < horizon_) {
 325     // Empty valid range.
 326     return std::make_pair(std::numeric_limits<uint32>::max(), 0);
 327   }
 328
 329   uint32 lower_bound;
 330   if (current_time_internal >= window_secs_) {
 331     lower_bound = max(horizon_, current_time_internal - window_secs_);
 332   } else {
 333     lower_bound = horizon_;
 334   }
 335
 336   // Also limit the upper range based on horizon_.  This makes the
 337   // strike register reject inserts that are far in the future and
 338   // would consume strike register resources for a long time.  This
 339   // allows the strike server to degrade optimally in cases where the
 340   // insert rate exceeds |max_entries_ / (2 * window_secs_)| entries
 341   // per second.
 342   uint32 upper_bound =
 343       current_time_internal + min(current_time_internal - horizon_,
 344                                   window_secs_);
 345
 346   return std::make_pair(lower_bound, upper_bound);
 347 }
 348
 349 uint32 StrikeRegister::ExternalTimeToInternal(uint32 external_time) const {
 350   return external_time - internal_epoch_;
 351 }
 352
 353 uint32 StrikeRegister::BestMatch(const uint8 v[24]) const {
 354   if (internal_node_head_ == kNil) {
 355     return kNil;
 356   }
 357
 358   uint32 next = internal_node_head_ >> 8;
 359   while ((next & kExternalFlag) == 0) {
 360     InternalNode* node = &internal_nodes_[next];
 361     uint8 b = v[node->critbyte()];
 362     unsigned direction =
 363         (1 + static_cast<unsigned>(node->otherbits() | b)) >> 8;
 364     next = node->child(direction);
 365   }
 366
 367   return next & ~kExternalFlag;
 368 }
 369
 370 uint32& StrikeRegister::external_node_next_ptr(unsigned i) {
 371   return *reinterpret_cast<uint32*>(&external_nodes_[i * kExternalNodeSize]);
 372 }
 373
 374 uint8* StrikeRegister::external_node(unsigned i) {
 375   return &external_nodes_[i * kExternalNodeSize];
 376 }
 377
 378 uint32 StrikeRegister::GetFreeExternalNode() {
 379   uint32 index = external_node_free_head_;
 380   DCHECK(index != kNil);
 381   external_node_free_head_ = external_node_next_ptr(index);
 382   return index;
 383 }
 384
 385 uint32 StrikeRegister::GetFreeInternalNode() {
 386   uint32 index = internal_node_free_head_;
 387   DCHECK(index != kNil);
 388   internal_node_free_head_ = internal_nodes_[index].next();
 389   return index;
 390 }
 391
 392 void StrikeRegister::DropOldestNode() {
 393   // DropOldestNode should never be called on an empty tree.
 394   DCHECK(internal_node_head_ != kNil);
 395
 396   // An internal node in a crit-bit tree always has exactly two children.
 397   // This means that, if we are removing an external node (which is one of
 398   // those children), then we also need to remove an internal node. In order
 399   // to do that we keep pointers to the parent (wherep) and grandparent
 400   // (whereq) when walking down the tree.
 401
 402   uint32 p = internal_node_head_ >> 8, *wherep = &internal_node_head_,
 403          *whereq = nullptr;
 404   while ((p & kExternalFlag) == 0) {
 405     whereq = wherep;
 406     InternalNode* inode = &internal_nodes_[p];
 407     // We always go left, towards the smallest element, exploiting the fact
 408     // that the timestamp is big-endian and at the start of the value.
 409     wherep = &inode->data_[0];
 410     p = (*wherep) >> 8;
 411   }
 412
 413   const uint32 ext_index = p & ~kExternalFlag;
 414   const uint8* ext_node = external_node(ext_index);
 415   uint32 new_horizon = ExternalTimeToInternal(TimeFromBytes(ext_node)) + 1;
 416   DCHECK_LE(horizon_, new_horizon);
 417   horizon_ = new_horizon;
 418
 419   if (!whereq) {
 420     // We are removing the last element in a tree.
 421     internal_node_head_ = kNil;
 422     FreeExternalNode(ext_index);
 423     return;
 424   }
 425
 426   // |wherep| points to the left child pointer in the parent so we can add
 427   // one and dereference to get the right child.
 428   const uint32 other_child = wherep[1];
 429   FreeInternalNode((*whereq) >> 8);
 430   *whereq = (*whereq & 0xff) | (other_child & 0xffffff00);
 431   FreeExternalNode(ext_index);
 432 }
 433
 434 void StrikeRegister::FreeExternalNode(uint32 index) {
 435   external_node_next_ptr(index) = external_node_free_head_;
 436   external_node_free_head_ = index;
 437 }
 438
 439 void StrikeRegister::FreeInternalNode(uint32 index) {
 440   internal_nodes_[index].SetNextPtr(internal_node_free_head_);
 441   internal_node_free_head_ = index;
 442 }
 443
 444 void StrikeRegister::ValidateTree(
 445     uint32 internal_node,
 446     int last_bit,
 447     const vector<pair<unsigned, bool> >& bits,
 448     const set<uint32>& free_internal_nodes,
 449     const set<uint32>& free_external_nodes,
 450     set<uint32>* used_internal_nodes,
 451     set<uint32>* used_external_nodes) {
 452   CHECK_LT(internal_node, max_entries_);
 453   const InternalNode* i = &internal_nodes_[internal_node];
 454   unsigned bit = 0;
 455   switch (i->otherbits()) {
 456     case 0xff & ~(1 << 7):
 457       bit = 0;
 458       break;
 459     case 0xff & ~(1 << 6):
 460       bit = 1;
 461       break;
 462     case 0xff & ~(1 << 5):
 463       bit = 2;
 464       break;
 465     case 0xff & ~(1 << 4):
 466       bit = 3;
 467       break;
 468     case 0xff & ~(1 << 3):
 469       bit = 4;
 470       break;
 471     case 0xff & ~(1 << 2):
 472       bit = 5;
 473       break;
 474     case 0xff & ~(1 << 1):
 475       bit = 6;
 476       break;
 477     case 0xff & ~1:
 478       bit = 7;
 479       break;
 480     default:
 481       CHECK(false);
 482   }
 483
 484   bit += 8 * i->critbyte();
 485   if (last_bit > -1) {
 486     CHECK_GT(bit, static_cast<unsigned>(last_bit));
 487   }
 488
 489   CHECK_EQ(free_internal_nodes.count(internal_node), 0u);
 490
 491   for (unsigned child = 0; child < 2; child++) {
 492     if (i->child(child) & kExternalFlag) {
 493       uint32 ext = i->child(child) & ~kExternalFlag;
 494       CHECK_EQ(free_external_nodes.count(ext), 0u);
 495       CHECK_EQ(used_external_nodes->count(ext), 0u);
 496       used_external_nodes->insert(ext);
 497       const uint8* bytes = external_node(ext);
 498       for (const pair<unsigned, bool>& pair : bits) {
 499         unsigned byte = pair.first / 8;
 500         DCHECK_LE(byte, 0xffu);
 501         unsigned bit_new = pair.first % 8;
 502         static const uint8 kMasks[8] =
 503             {0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, 0x01};
 504         CHECK_EQ((bytes[byte] & kMasks[bit_new]) != 0, pair.second);
 505       }
 506     } else {
 507       uint32 inter = i->child(child);
 508       vector<pair<unsigned, bool> > new_bits(bits);
 509       new_bits.push_back(pair<unsigned, bool>(bit, child != 0));
 510       CHECK_EQ(free_internal_nodes.count(inter), 0u);
 511       CHECK_EQ(used_internal_nodes->count(inter), 0u);
 512       used_internal_nodes->insert(inter);
 513       ValidateTree(inter, bit, bits, free_internal_nodes, free_external_nodes,
 514                    used_internal_nodes, used_external_nodes);
 515     }
 516   }
 517 }
 518
 519 }  // namespace net