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 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   return Attrs.hasFnAttribute(Attribute::ReturnsTwice);
96 }
97 
98 bool X86IndirectBranchTrackingPass::runOnMachineFunction(MachineFunction &MF) {
99   const X86Subtarget &SubTarget = MF.getSubtarget<X86Subtarget>();
100 
101   // Check that the cf-protection-branch is enabled.
102   Metadata *isCFProtectionSupported =
103       MF.getMMI().getModule()->getModuleFlag("cf-protection-branch");
104   // NB: We need to enable IBT in jitted code if JIT compiler is CET
105   // enabled.
106   const X86TargetMachine *TM =
107       static_cast<const X86TargetMachine *>(&MF.getTarget());
108 #ifdef __CET__
109   bool isJITwithCET = TM->isJIT();
110 #else
111   bool isJITwithCET = false;
112 #endif
113   if (!isCFProtectionSupported && !IndirectBranchTracking && !isJITwithCET)
114     return false;
115 
116   // True if the current MF was changed and false otherwise.
117   bool Changed = false;
118 
119   TII = SubTarget.getInstrInfo();
120   EndbrOpcode = SubTarget.is64Bit() ? X86::ENDBR64 : X86::ENDBR32;
121 
122   // Large code model, non-internal function or function whose address
123   // was taken, can be accessed through indirect calls. Mark the first
124   // BB with ENDBR instruction unless nocf_check attribute is used.
125   if ((TM->getCodeModel() == CodeModel::Large ||
126        MF.getFunction().hasAddressTaken() ||
127        !MF.getFunction().hasLocalLinkage()) &&
128       !MF.getFunction().doesNoCfCheck()) {
129     auto MBB = MF.begin();
130     Changed |= addENDBR(*MBB, MBB->begin());
131   }
132 
133   for (auto &MBB : MF) {
134     // Find all basic blocks that their address was taken (for example
135     // in the case of indirect jump) and add ENDBR instruction.
136     if (MBB.hasAddressTaken())
137       Changed |= addENDBR(MBB, MBB.begin());
138 
139     for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); ++I) {
140       if (I->isCall() && IsCallReturnTwice(I->getOperand(0)))
141         Changed |= addENDBR(MBB, std::next(I));
142     }
143 
144     // Exception handle may indirectly jump to catch pad, So we should add
145     // ENDBR before catch pad instructions. For SjLj exception model, it will
146     // create a new BB(new landingpad) indirectly jump to the old landingpad.
147     if (TM->Options.ExceptionModel == ExceptionHandling::SjLj) {
148       for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); ++I) {
149         // New Landingpad BB without EHLabel.
150         if (MBB.isEHPad()) {
151           if (I->isDebugInstr())
152             continue;
153           Changed |= addENDBR(MBB, I);
154           break;
155         } else if (I->isEHLabel()) {
156           // Old Landingpad BB (is not Landingpad now) with
157           // the the old "callee" EHLabel.
158           MCSymbol *Sym = I->getOperand(0).getMCSymbol();
159           if (!MF.hasCallSiteLandingPad(Sym))
160             continue;
161           Changed |= addENDBR(MBB, std::next(I));
162           break;
163         }
164       }
165     } else if (MBB.isEHPad()){
166       for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); ++I) {
167         if (!I->isEHLabel())
168           continue;
169         Changed |= addENDBR(MBB, std::next(I));
170         break;
171       }
172     }
173   }
174   return Changed;
175 }
176