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//======- X86RetpolineThunks.cpp - Construct retpoline thunks for x86 --=====//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
/// \file
/// Pass that injects an MI thunk implementing a "retpoline". This is
/// a RET-implemented trampoline that is used to lower indirect calls in a way
/// that prevents speculation on some x86 processors and can be used to mitigate
/// security vulnerabilities due to targeted speculative execution and side
/// channels such as CVE-2017-5715.
/// TODO(chandlerc): All of this code could use better comments and
/// documentation.
#include "X86.h"
#include "X86InstrBuilder.h"
#include "X86Subtarget.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "x86-retpoline-thunks"
static const char ThunkNamePrefix[] = "__llvm_retpoline_";
static const char R11ThunkName[] = "__llvm_retpoline_r11";
static const char EAXThunkName[] = "__llvm_retpoline_eax";
static const char ECXThunkName[] = "__llvm_retpoline_ecx";
static const char EDXThunkName[] = "__llvm_retpoline_edx";
static const char EDIThunkName[] = "__llvm_retpoline_edi";
namespace {
class X86RetpolineThunks : public MachineFunctionPass {
static char ID;
X86RetpolineThunks() : MachineFunctionPass(ID) {}
StringRef getPassName() const override { return "X86 Retpoline Thunks"; }
bool doInitialization(Module &M) override;
bool runOnMachineFunction(MachineFunction &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineModuleInfo *MMI;
const TargetMachine *TM;
bool Is64Bit;
const X86Subtarget *STI;
const X86InstrInfo *TII;
bool InsertedThunks;
void createThunkFunction(Module &M, StringRef Name);
void insertRegReturnAddrClobber(MachineBasicBlock &MBB, unsigned Reg);
void populateThunk(MachineFunction &MF, unsigned Reg);
} // end anonymous namespace
FunctionPass *llvm::createX86RetpolineThunksPass() {
return new X86RetpolineThunks();
char X86RetpolineThunks::ID = 0;
bool X86RetpolineThunks::doInitialization(Module &M) {
InsertedThunks = false;
return false;
bool X86RetpolineThunks::runOnMachineFunction(MachineFunction &MF) {
LLVM_DEBUG(dbgs() << getPassName() << '\n');
TM = &MF.getTarget();;
STI = &MF.getSubtarget<X86Subtarget>();
TII = STI->getInstrInfo();
Is64Bit = TM->getTargetTriple().getArch() == Triple::x86_64;
MMI = &getAnalysis<MachineModuleInfoWrapperPass>().getMMI();
Module &M = const_cast<Module &>(*MMI->getModule());
// If this function is not a thunk, check to see if we need to insert
// a thunk.
if (!MF.getName().startswith(ThunkNamePrefix)) {
// If we've already inserted a thunk, nothing else to do.
if (InsertedThunks)
return false;
// Only add a thunk if one of the functions has the retpoline feature
// enabled in its subtarget, and doesn't enable external thunks.
// FIXME: Conditionalize on indirect calls so we don't emit a thunk when
// nothing will end up calling it.
// FIXME: It's a little silly to look at every function just to enumerate
// the subtargets, but eventually we'll want to look at them for indirect
// calls, so maybe this is OK.
if ((!STI->useRetpolineIndirectCalls() &&
!STI->useRetpolineIndirectBranches()) ||
return false;
// Otherwise, we need to insert the thunk.
// WARNING: This is not really a well behaving thing to do in a function
// pass. We extract the module and insert a new function (and machine
// function) directly into the module.
if (Is64Bit)
createThunkFunction(M, R11ThunkName);
for (StringRef Name :
{EAXThunkName, ECXThunkName, EDXThunkName, EDIThunkName})
createThunkFunction(M, Name);
InsertedThunks = true;
return true;
// If this *is* a thunk function, we need to populate it with the correct MI.
if (Is64Bit) {
assert(MF.getName() == "__llvm_retpoline_r11" &&
"Should only have an r11 thunk on 64-bit targets");
// __llvm_retpoline_r11:
// callq .Lr11_call_target
// .Lr11_capture_spec:
// pause
// lfence
// jmp .Lr11_capture_spec
// .align 16
// .Lr11_call_target:
// movq %r11, (%rsp)
// retq
populateThunk(MF, X86::R11);
} else {
// For 32-bit targets we need to emit a collection of thunks for various
// possible scratch registers as well as a fallback that uses EDI, which is
// normally callee saved.
// __llvm_retpoline_eax:
// calll .Leax_call_target
// .Leax_capture_spec:
// pause
// jmp .Leax_capture_spec
// .align 16
// .Leax_call_target:
// movl %eax, (%esp) # Clobber return addr
// retl
// __llvm_retpoline_ecx:
// ... # Same setup
// movl %ecx, (%esp)
// retl
// __llvm_retpoline_edx:
// ... # Same setup
// movl %edx, (%esp)
// retl
// __llvm_retpoline_edi:
// ... # Same setup
// movl %edi, (%esp)
// retl
if (MF.getName() == EAXThunkName)
populateThunk(MF, X86::EAX);
else if (MF.getName() == ECXThunkName)
populateThunk(MF, X86::ECX);
else if (MF.getName() == EDXThunkName)
populateThunk(MF, X86::EDX);
else if (MF.getName() == EDIThunkName)
populateThunk(MF, X86::EDI);
llvm_unreachable("Invalid thunk name on x86-32!");
return true;
void X86RetpolineThunks::createThunkFunction(Module &M, StringRef Name) {
assert(Name.startswith(ThunkNamePrefix) &&
"Created a thunk with an unexpected prefix!");
LLVMContext &Ctx = M.getContext();
auto Type = FunctionType::get(Type::getVoidTy(Ctx), false);
Function *F =
Function::Create(Type, GlobalValue::LinkOnceODRLinkage, Name, &M);
// Add Attributes so that we don't create a frame, unwind information, or
// inline.
AttrBuilder B;
F->addAttributes(llvm::AttributeList::FunctionIndex, B);
// Populate our function a bit so that we can verify.
BasicBlock *Entry = BasicBlock::Create(Ctx, "entry", F);
IRBuilder<> Builder(Entry);
// MachineFunctions/MachineBasicBlocks aren't created automatically for the
// IR-level constructs we already made. Create them and insert them into the
// module.
MachineFunction &MF = MMI->getOrCreateMachineFunction(*F);
MachineBasicBlock *EntryMBB = MF.CreateMachineBasicBlock(Entry);
// Insert EntryMBB into MF. It's not in the module until we do this.
MF.insert(MF.end(), EntryMBB);
void X86RetpolineThunks::insertRegReturnAddrClobber(MachineBasicBlock &MBB,
unsigned Reg) {
const unsigned MovOpc = Is64Bit ? X86::MOV64mr : X86::MOV32mr;
const unsigned SPReg = Is64Bit ? X86::RSP : X86::ESP;
addRegOffset(BuildMI(&MBB, DebugLoc(), TII->get(MovOpc)), SPReg, false, 0)
void X86RetpolineThunks::populateThunk(MachineFunction &MF,
unsigned Reg) {
// Set MF properties. We never use vregs...
// Grab the entry MBB and erase any other blocks. O0 codegen appears to
// generate two bbs for the entry block.
MachineBasicBlock *Entry = &MF.front();
while (MF.size() > 1)
MachineBasicBlock *CaptureSpec = MF.CreateMachineBasicBlock(Entry->getBasicBlock());
MachineBasicBlock *CallTarget = MF.CreateMachineBasicBlock(Entry->getBasicBlock());
MCSymbol *TargetSym = MF.getContext().createTempSymbol();
const unsigned CallOpc = Is64Bit ? X86::CALL64pcrel32 : X86::CALLpcrel32;
const unsigned RetOpc = Is64Bit ? X86::RETQ : X86::RETL;
BuildMI(Entry, DebugLoc(), TII->get(CallOpc)).addSym(TargetSym);
// The MIR verifier thinks that the CALL in the entry block will fall through
// to CaptureSpec, so mark it as the successor. Technically, CaptureTarget is
// the successor, but the MIR verifier doesn't know how to cope with that.
// In the capture loop for speculation, we want to stop the processor from
// speculating as fast as possible. On Intel processors, the PAUSE instruction
// will block speculation without consuming any execution resources. On AMD
// processors, the PAUSE instruction is (essentially) a nop, so we also use an
// LFENCE instruction which they have advised will stop speculation as well
// with minimal resource utilization. We still end the capture with a jump to
// form an infinite loop to fully guarantee that no matter what implementation
// of the x86 ISA, speculating this code path never escapes.
BuildMI(CaptureSpec, DebugLoc(), TII->get(X86::PAUSE));
BuildMI(CaptureSpec, DebugLoc(), TII->get(X86::LFENCE));
BuildMI(CaptureSpec, DebugLoc(), TII->get(X86::JMP_1)).addMBB(CaptureSpec);
insertRegReturnAddrClobber(*CallTarget, Reg);
CallTarget->back().setPreInstrSymbol(MF, TargetSym);
BuildMI(CallTarget, DebugLoc(), TII->get(RetOpc));