1 //===---- X86IndirectBranchTracking.cpp - Enables CET IBT mechanism -------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file defines a pass that enables Indirect Branch Tracking (IBT) as part
10 // of Control-Flow Enforcement Technology (CET).
11 // The pass adds ENDBR (End Branch) machine instructions at the beginning of
12 // each basic block or function that is referenced by an indrect jump/call
13 // instruction.
14 // The ENDBR instructions have a NOP encoding and as such are ignored in
15 // targets that do not support CET IBT mechanism.
16 //===----------------------------------------------------------------------===//
17 
18 #include "X86.h"
19 #include "X86InstrInfo.h"
20 #include "X86Subtarget.h"
21 #include "X86TargetMachine.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/CodeGen/MachineFunctionPass.h"
24 #include "llvm/CodeGen/MachineInstrBuilder.h"
25 #include "llvm/CodeGen/MachineModuleInfo.h"
26 
27 using namespace llvm;
28 
29 #define DEBUG_TYPE "x86-indirect-branch-tracking"
30 
31 static cl::opt<bool> IndirectBranchTracking(
32     "x86-indirect-branch-tracking", cl::init(false), cl::Hidden,
33     cl::desc("Enable X86 indirect branch tracking pass."));
34 
35 STATISTIC(NumEndBranchAdded, "Number of ENDBR instructions added");
36 
37 namespace {
38 class X86IndirectBranchTrackingPass : public MachineFunctionPass {
39 public:
40   X86IndirectBranchTrackingPass() : MachineFunctionPass(ID) {}
41 
42   StringRef getPassName() const override {
43     return "X86 Indirect Branch Tracking";
44   }
45 
46   bool runOnMachineFunction(MachineFunction &MF) override;
47 
48 private:
49   static char ID;
50 
51   /// Machine instruction info used throughout the class.
52   const X86InstrInfo *TII = nullptr;
53 
54   /// Endbr opcode for the current machine function.
55   unsigned int EndbrOpcode = 0;
56 
57   /// Adds a new ENDBR instruction to the beginning of the MBB.
58   /// The function will not add it if already exists.
59   /// It will add ENDBR32 or ENDBR64 opcode, depending on the target.
60   /// \returns true if the ENDBR was added and false otherwise.
61   bool addENDBR(MachineBasicBlock &MBB, MachineBasicBlock::iterator I) const;
62 };
63 
64 } // end anonymous namespace
65 
66 char X86IndirectBranchTrackingPass::ID = 0;
67 
68 FunctionPass *llvm::createX86IndirectBranchTrackingPass() {
69   return new X86IndirectBranchTrackingPass();
70 }
71 
72 bool X86IndirectBranchTrackingPass::addENDBR(
73     MachineBasicBlock &MBB, MachineBasicBlock::iterator I) const {
74   assert(TII && "Target instruction info was not initialized");
75   assert((X86::ENDBR64 == EndbrOpcode || X86::ENDBR32 == EndbrOpcode) &&
76          "Unexpected Endbr opcode");
77 
78   // If the MBB/I is empty or the current instruction is not ENDBR,
79   // insert ENDBR instruction to the location of I.
80   if (I == MBB.end() || I->getOpcode() != EndbrOpcode) {
81     BuildMI(MBB, I, MBB.findDebugLoc(I), TII->get(EndbrOpcode));
82     ++NumEndBranchAdded;
83     return true;
84   }
85   return false;
86 }
87 
88 static bool IsCallReturnTwice(llvm::MachineOperand &MOp) {
89   if (!MOp.isGlobal())
90     return false;
91   auto *CalleeFn = dyn_cast<Function>(MOp.getGlobal());
92   if (!CalleeFn)
93     return false;
94   AttributeList Attrs = CalleeFn->getAttributes();
95   if (Attrs.hasAttribute(AttributeList::FunctionIndex, Attribute::ReturnsTwice))
96     return true;
97   return false;
98 }
99 
100 bool X86IndirectBranchTrackingPass::runOnMachineFunction(MachineFunction &MF) {
101   const X86Subtarget &SubTarget = MF.getSubtarget<X86Subtarget>();
102 
103   // Check that the cf-protection-branch is enabled.
104   Metadata *isCFProtectionSupported =
105       MF.getMMI().getModule()->getModuleFlag("cf-protection-branch");
106   // NB: We need to enable IBT in jitted code if JIT compiler is CET
107   // enabled.
108   const X86TargetMachine *TM =
109       static_cast<const X86TargetMachine *>(&MF.getTarget());
110 #ifdef __CET__
111   bool isJITwithCET = TM->isJIT();
112 #else
113   bool isJITwithCET = false;
114 #endif
115   if (!isCFProtectionSupported && !IndirectBranchTracking && !isJITwithCET)
116     return false;
117 
118   // True if the current MF was changed and false otherwise.
119   bool Changed = false;
120 
121   TII = SubTarget.getInstrInfo();
122   EndbrOpcode = SubTarget.is64Bit() ? X86::ENDBR64 : X86::ENDBR32;
123 
124   // Large code model, non-internal function or function whose address
125   // was taken, can be accessed through indirect calls. Mark the first
126   // BB with ENDBR instruction unless nocf_check attribute is used.
127   if ((TM->getCodeModel() == CodeModel::Large ||
128        MF.getFunction().hasAddressTaken() ||
129        !MF.getFunction().hasLocalLinkage()) &&
130       !MF.getFunction().doesNoCfCheck()) {
131     auto MBB = MF.begin();
132     Changed |= addENDBR(*MBB, MBB->begin());
133   }
134 
135   for (auto &MBB : MF) {
136     // Find all basic blocks that their address was taken (for example
137     // in the case of indirect jump) and add ENDBR instruction.
138     if (MBB.hasAddressTaken())
139       Changed |= addENDBR(MBB, MBB.begin());
140 
141     // Exception handle may indirectly jump to catch pad, So we should add
142     // ENDBR before catch pad instructions.
143     bool EHPadIBTNeeded = MBB.isEHPad();
144 
145     for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); ++I) {
146       if (I->isCall() && IsCallReturnTwice(I->getOperand(0)))
147         Changed |= addENDBR(MBB, std::next(I));
148 
149       if (EHPadIBTNeeded && I->isEHLabel()) {
150         Changed |= addENDBR(MBB, std::next(I));
151         EHPadIBTNeeded = false;
152       }
153     }
154   }
155   return Changed;
156 }
157