ARMBranchTargets.cpp

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//===-- ARMBranchTargets.cpp -- Harden code using v8.1-M BTI extension -----==//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This pass inserts BTI instructions at the start of every function and basic
// block which could be indirectly called. The hardware will (when enabled)
// trap when an indirect branch or call instruction targets an instruction
// which is not a valid BTI instruction. This is intended to guard against
// control-flow hijacking attacks.
//
//===----------------------------------------------------------------------===//
 
#include "ARM.h"
#include "ARMInstrInfo.h"
#include "ARMMachineFunctionInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineJumpTableInfo.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/Support/Debug.h"
 
using namespace llvm;
 
#define DEBUG_TYPE "arm-branch-targets"
#define ARM_BRANCH_TARGETS_NAME "ARM Branch Targets"
 
namespace {
class ARMBranchTargets : public MachineFunctionPass {
public:
  static char ID;
  ARMBranchTargets() : MachineFunctionPass(ID) {}
  void getAnalysisUsage(AnalysisUsage &AU) const override;
  bool runOnMachineFunction(MachineFunction &MF) override;
  StringRef getPassName() const override { return ARM_BRANCH_TARGETS_NAME; }
 
private:
  void addBTI(const ARMInstrInfo &TII, MachineBasicBlock &MBB, bool IsFirstBB);
};
} // end anonymous namespace
 
char ARMBranchTargets::ID = 0;
 
INITIALIZE_PASS(ARMBranchTargets, "arm-branch-targets", ARM_BRANCH_TARGETS_NAME,
                false, false)
 
void ARMBranchTargets::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesCFG();
  MachineFunctionPass::getAnalysisUsage(AU);
}
 
FunctionPass *llvm::createARMBranchTargetsPass() {
  return new ARMBranchTargets();
}
 
bool ARMBranchTargets::runOnMachineFunction(MachineFunction &MF) {
  if (!MF.getInfo<ARMFunctionInfo>()->branchTargetEnforcement())
    return false;
 
  LLVM_DEBUG(dbgs() << "********** ARM Branch Targets  **********\n"
                    << "********** Function: " << MF.getName() << '\n');
  const ARMInstrInfo &TII =
      *static_cast<const ARMInstrInfo *>(MF.getSubtarget().getInstrInfo());
 
  // LLVM does not consider basic blocks which are the targets of jump tables
  // to be address-taken (the address can't escape anywhere else), but they are
  // used for indirect branches, so need BTI instructions.
  SmallPtrSet<const MachineBasicBlock *, 8> JumpTableTargets;
  if (const MachineJumpTableInfo *JTI = MF.getJumpTableInfo())
    for (const MachineJumpTableEntry &JTE : JTI->getJumpTables())
      for (const MachineBasicBlock *MBB : JTE.MBBs)
        JumpTableTargets.insert(MBB);
 
  bool MadeChange = false;
  for (MachineBasicBlock &MBB : MF) {
    bool NeedBTI = false;
    bool IsFirstBB = &MBB == &MF.front();
 
    // Every function can potentially be called indirectly (even if it has
    // static linkage, due to linker-generated veneers).
    if (IsFirstBB)
      NeedBTI = true;
 
    // If the block itself is address-taken, or is an exception landing pad, it
    // could be indirectly branched to.
    if (MBB.hasAddressTaken() || MBB.isEHPad() || JumpTableTargets.count(&MBB))
      NeedBTI = true;
 
    if (NeedBTI) {
      addBTI(TII, MBB, IsFirstBB);
      MadeChange = true;
    }
  }
 
  return MadeChange;
}
 
/// Insert a BTI/PACBTI instruction into a given basic block \c MBB. If
/// \c IsFirstBB is true (meaning that this is the first BB in a function) try
/// to find a PAC instruction and replace it with PACBTI. Otherwise just insert
/// a BTI instruction.
/// The point of insertion is in the beginning of the BB, immediately after meta
/// instructions (such labels in exception handling landing pads).
void ARMBranchTargets::addBTI(const ARMInstrInfo &TII, MachineBasicBlock &MBB,
                              bool IsFirstBB) {
  // Which instruction to insert: BTI or PACBTI
  unsigned OpCode = ARM::t2BTI;
  unsigned MIFlags = 0;
 
  // Skip meta instructions, including EH labels
  auto MBBI = llvm::find_if_not(MBB.instrs(), [](const MachineInstr &MI) {
    return MI.isMetaInstruction();
  });
 
  // If this is the first BB in a function, check if it starts with a PAC
  // instruction and in that case remove the PAC instruction.
  if (IsFirstBB) {
    if (MBBI != MBB.instr_end() && MBBI->getOpcode() == ARM::t2PAC) {
      LLVM_DEBUG(dbgs() << "Removing a 'PAC' instr from BB '" << MBB.getName()
                        << "' to replace with PACBTI\n");
      OpCode = ARM::t2PACBTI;
      MIFlags = MachineInstr::FrameSetup;
      auto NextMBBI = std::next(MBBI);
      MBBI->eraseFromParent();
      MBBI = NextMBBI;
    }
  }
 
  LLVM_DEBUG(dbgs() << "Inserting a '"
                    << (OpCode == ARM::t2BTI ? "BTI" : "PACBTI")
                    << "' instr into BB '" << MBB.getName() << "'\n");
  // Finally, insert a new instruction (either PAC or PACBTI)
  BuildMI(MBB, MBBI, MBB.findDebugLoc(MBBI), TII.get(OpCode))
      .setMIFlags(MIFlags);
}