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