adds the security cookie check hook transform.

syzygy/instrument/transforms/security_cookie_check_hook_transform.cc

+// Copyright 2017 Google Inc. All Rights Reserved.
+//
+// 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.
+//
+
+#include "syzygy/instrument/transforms/security_cookie_check_hook_transform.h"
+
+#include "syzygy/pe/pe_utils.h"
+
+namespace instrument {
+namespace transforms {
+
+using block_graph::Displacement;
+using block_graph::Operand;
+
+const char SecurityCookieCheckHookTransform::kTransformName[] =
+    "SecurityCookieCheckHookTransform";
+
+const char SecurityCookieCheckHookTransform::kReportGsFailure[] =
+    "__report_gsfailure";
+
+const char SecurityCookieCheckHookTransform::kSyzygyReportGsFailure[] =
+    "__syzygy_report_gsfailure";
+
+const uint32_t SecurityCookieCheckHookTransform::kInvalidUserAddress =
+    0xdeadbeef;
+
+bool SecurityCookieCheckHookTransform::TransformBlockGraph(
+    const TransformPolicyInterface* policy,
+    BlockGraph* block_graph,
+    BlockGraph::Block* header_block) {
+  BlockGraph::Block* report_gsfailure = nullptr;
+  BlockGraph::BlockMap& blocks = block_graph->blocks_mutable();
+  for (auto& block : blocks) {
+    std::string name(block.second.name());
+    if (name == kReportGsFailure) {
+      report_gsfailure = &block.second;
+      break;
+    }
+  }
+
+  if (report_gsfailure == nullptr) {
+    LOG(ERROR) << "Could not find " << kReportGsFailure << ".";
+    return false;
+  }
+
+  DCHECK_EQ(1u, report_gsfailure->referrers().size());
+
+  if (report_gsfailure->referrers().size() != 1) {

[chrisha, 2017/05/10 20:18:42]

(Sorry, didn't catch this before.)

No need for both a DCHECK and a LOG(ERROR). Since this is a condition that can
occur due to external data provided to the program, the LOG(ERROR) is
appropriate.

A DCHECK is appropriate for assertions about the behaviour of our own code that
would indicate a logical error and/or programmer misuse of an API.

So please remove the DCHECK.

[0vercl0k, 2017/05/10 20:51:46]

No worries - I was a bit confused about it too :). Done!

+    // We bail out if we don't have a single referrer as the only
+    // expected referrer is supposed to be __security_cookie_check.
+    // If there is more than one, we would rather bail out than take
+    // a chance at modifying the behavior of the PE image.
+    LOG(ERROR) << "Only a single referrer to " << kReportGsFailure
+               << " is expected.";
+    return false;
+  }
+
+  LOG(INFO) << "Found a " << kReportGsFailure
+            << " implementation, hooking it now.";
+  BlockGraph::Section* section_text = block_graph->FindOrAddSection(
+      pe::kCodeSectionName, pe::kCodeCharacteristics);
+
+  // All of the below is needed to build the instrumentation via the assembler.
+  BasicBlockSubGraph bbsg;
+  BasicBlockSubGraph::BlockDescription* block_desc = bbsg.AddBlockDescription(
+      kSyzygyReportGsFailure, nullptr, BlockGraph::CODE_BLOCK,
+      section_text->id(), 1, 0);
+
+  BasicCodeBlock* bb = bbsg.AddBasicCodeBlock(kSyzygyReportGsFailure);
+  block_desc->basic_block_order.push_back(bb);
+  BasicBlockAssembler assm(bb->instructions().begin(), &bb->instructions());
+  assm.mov(Operand(Displacement(kInvalidUserAddress)), assm::eax);
+
+  // Condense into a block.
+  BlockBuilder block_builder(block_graph);
+  if (!block_builder.Merge(&bbsg)) {
+    LOG(ERROR) << "Failed to build " << kSyzygyReportGsFailure << " block.";
+    return false;
+  }
+
+  DCHECK_EQ(1u, block_builder.new_blocks().size());
+
+  // Transfer the referrers to the new block, and delete the old one.
+  BlockGraph::Block* syzygy_report_gsfailure =
+      block_builder.new_blocks().front();
+  report_gsfailure->TransferReferrers(
+      0, syzygy_report_gsfailure,
+      BlockGraph::Block::kTransferInternalReferences);
+
+  report_gsfailure->RemoveAllReferences();
+  if (!block_graph->RemoveBlock(report_gsfailure)) {
+    LOG(ERROR) << "Removing " << kReportGsFailure << " failed.";
+    return false;
+  }
+
+  return true;
+}
+
+}  // namespace transforms
+}  // namespace instrument