1//===---- KCFI.cpp - Implements Kernel Control-Flow Integrity (KCFI) ------===//
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 pass implements Kernel Control-Flow Integrity (KCFI) indirect call
10// check lowering. For each call instruction with a cfi-type attribute, it
11// emits an arch-specific check before the call, and bundles the check and
12// the call to prevent unintentional modifications.
13//
14//===----------------------------------------------------------------------===//
15
16#include "llvm/CodeGen/KCFI.h"
17#include "llvm/ADT/Statistic.h"
18#include "llvm/CodeGen/MachineFunctionPass.h"
19#include "llvm/CodeGen/MachineInstrBundle.h"
20#include "llvm/CodeGen/MachineModuleInfo.h"
21#include "llvm/CodeGen/TargetInstrInfo.h"
22#include "llvm/CodeGen/TargetLowering.h"
23#include "llvm/CodeGen/TargetSubtargetInfo.h"
24#include "llvm/IR/Module.h"
25#include "llvm/InitializePasses.h"
26
27using namespace llvm;
28
29#define DEBUG_TYPE "kcfi"
30#define KCFI_PASS_NAME "Insert KCFI indirect call checks"
31
32STATISTIC(NumKCFIChecksAdded, "Number of indirect call checks added");
33
34namespace {
35class KCFI {
36public:
37 bool run(MachineFunction &MF);
38
39private:
40 /// Machine instruction info used throughout the class.
41 const TargetInstrInfo *TII = nullptr;
42
43 /// Target lowering for arch-specific parts.
44 const TargetLowering *TLI = nullptr;
45
46 /// Emits a KCFI check before an indirect call.
47 /// \returns true if the check was added and false otherwise.
48 bool emitCheck(MachineBasicBlock &MBB,
49 MachineBasicBlock::instr_iterator I) const;
50};
51
52class MachineKCFILegacy : public MachineFunctionPass {
53public:
54 static char ID;
55
56 MachineKCFILegacy() : MachineFunctionPass(ID) {}
57
58 StringRef getPassName() const override { return KCFI_PASS_NAME; }
59 bool runOnMachineFunction(MachineFunction &MF) override {
60 return KCFI().run(MF);
61 }
62};
63
64char MachineKCFILegacy::ID = 0;
65} // end anonymous namespace
66
67INITIALIZE_PASS(MachineKCFILegacy, DEBUG_TYPE, KCFI_PASS_NAME, false, false)
68
69FunctionPass *llvm::createKCFIPass() { return new MachineKCFILegacy(); }
70
71PreservedAnalyses MachineKCFIPass::run(MachineFunction &MF,
72 MachineFunctionAnalysisManager &MFAM) {
73 if (!KCFI().run(MF))
74 return PreservedAnalyses::all();
75
76 PreservedAnalyses PA = getMachineFunctionPassPreservedAnalyses();
77 PA.preserveSet<CFGAnalyses>();
78 return PA;
79}
80
81bool KCFI::emitCheck(MachineBasicBlock &MBB,
82 MachineBasicBlock::instr_iterator MBBI) const {
83 assert(TII && "Target instruction info was not initialized");
84 assert(TLI && "Target lowering was not initialized");
85
86 // If the call instruction is bundled, we can only emit a check safely if
87 // it's the first instruction in the bundle.
88 if (MBBI->isBundled() && !std::prev(x: MBBI)->isBundle())
89 report_fatal_error(reason: "Cannot emit a KCFI check for a bundled call");
90
91 // Emit a KCFI check for the call instruction at MBBI. The implementation
92 // must unfold memory operands if applicable.
93 MachineInstr *Check = TLI->EmitKCFICheck(MBB, MBBI, TII);
94
95 // Clear the original call's CFI type.
96 assert(MBBI->isCall() && "Unexpected instruction type");
97 MBBI->setCFIType(MF&: *MBB.getParent(), Type: 0);
98
99 // If not already bundled, bundle the check and the call to prevent
100 // further changes.
101 if (!MBBI->isBundled())
102 finalizeBundle(MBB, FirstMI: Check->getIterator(), LastMI: std::next(x: MBBI->getIterator()));
103
104 ++NumKCFIChecksAdded;
105 return true;
106}
107
108bool KCFI::run(MachineFunction &MF) {
109 const Module *M = MF.getFunction().getParent();
110 if (!M->getModuleFlag(Key: "kcfi"))
111 return false;
112
113 const auto &SubTarget = MF.getSubtarget();
114 TII = SubTarget.getInstrInfo();
115 TLI = SubTarget.getTargetLowering();
116
117 bool Changed = false;
118 for (MachineBasicBlock &MBB : MF) {
119 // Use instr_iterator because we don't want to skip bundles.
120 for (MachineBasicBlock::instr_iterator MII = MBB.instr_begin(),
121 MIE = MBB.instr_end();
122 MII != MIE; ++MII) {
123 if (MII->isCall() && MII->getCFIType())
124 Changed |= emitCheck(MBB, MBBI: MII);
125 }
126 }
127
128 return Changed;
129}
130