llvm/lib/CodeGen/KCFI.cpp

   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/MachineInstrBuilder.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/InitializePasses.h"
  25
  26 using namespace llvm;
  27
  28 #define DEBUG_TYPE "kcfi"
  29 #define KCFI_PASS_NAME "Insert KCFI indirect call checks"
  30
  31 STATISTIC(NumKCFIChecksAdded, "Number of indirect call checks added");
  32
  33 namespace {
  34 class KCFI : public MachineFunctionPass {
  35 public:
  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
  43 private:
  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
  56 char KCFI::ID = 0;
  57 } // end anonymous namespace
  58
  59 INITIALIZE_PASS(KCFI, DEBUG_TYPE, KCFI_PASS_NAME, false, false)
  60
  61 FunctionPass *llvm::createKCFIPass() { return new KCFI(); }
  62
  63 bool 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(MBBI)->isBundle())
  71     report_fatal_error("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(*MBB.getParent(), 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, Check->getIterator(), std::next(MBBI->getIterator()));
  85
  86   ++NumKCFIChecksAdded;
  87   return true;
  88 }
  89
  90 bool KCFI::runOnMachineFunction(MachineFunction &MF) {
  91   const Module *M = MF.getMMI().getModule();
  92   if (!M->getModuleFlag("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, MII);
 107     }
 108   }
 109
 110   return Changed;
 111 }