LLVM  8.0.0svn
X86RetpolineThunks.cpp
Go to the documentation of this file.
1 //======- X86RetpolineThunks.cpp - Construct retpoline thunks for x86 --=====//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 /// \file
10 ///
11 /// Pass that injects an MI thunk implementing a "retpoline". This is
12 /// a RET-implemented trampoline that is used to lower indirect calls in a way
13 /// that prevents speculation on some x86 processors and can be used to mitigate
14 /// security vulnerabilities due to targeted speculative execution and side
15 /// channels such as CVE-2017-5715.
16 ///
17 /// TODO(chandlerc): All of this code could use better comments and
18 /// documentation.
19 ///
20 //===----------------------------------------------------------------------===//
21 
22 #include "X86.h"
23 #include "X86InstrBuilder.h"
24 #include "X86Subtarget.h"
28 #include "llvm/CodeGen/Passes.h"
30 #include "llvm/IR/IRBuilder.h"
31 #include "llvm/IR/Instructions.h"
32 #include "llvm/IR/Module.h"
34 #include "llvm/Support/Debug.h"
36 
37 using namespace llvm;
38 
39 #define DEBUG_TYPE "x86-retpoline-thunks"
40 
41 static const char ThunkNamePrefix[] = "__llvm_retpoline_";
42 static const char R11ThunkName[] = "__llvm_retpoline_r11";
43 static const char EAXThunkName[] = "__llvm_retpoline_eax";
44 static const char ECXThunkName[] = "__llvm_retpoline_ecx";
45 static const char EDXThunkName[] = "__llvm_retpoline_edx";
46 static const char EDIThunkName[] = "__llvm_retpoline_edi";
47 
48 namespace {
49 class X86RetpolineThunks : public MachineFunctionPass {
50 public:
51  static char ID;
52 
53  X86RetpolineThunks() : MachineFunctionPass(ID) {}
54 
55  StringRef getPassName() const override { return "X86 Retpoline Thunks"; }
56 
57  bool doInitialization(Module &M) override;
58  bool runOnMachineFunction(MachineFunction &F) override;
59 
60  void getAnalysisUsage(AnalysisUsage &AU) const override {
64  }
65 
66 private:
67  MachineModuleInfo *MMI;
68  const TargetMachine *TM;
69  bool Is64Bit;
70  const X86Subtarget *STI;
71  const X86InstrInfo *TII;
72 
73  bool InsertedThunks;
74 
75  void createThunkFunction(Module &M, StringRef Name);
76  void insertRegReturnAddrClobber(MachineBasicBlock &MBB, unsigned Reg);
77  void populateThunk(MachineFunction &MF, unsigned Reg);
78 };
79 
80 } // end anonymous namespace
81 
83  return new X86RetpolineThunks();
84 }
85 
86 char X86RetpolineThunks::ID = 0;
87 
88 bool X86RetpolineThunks::doInitialization(Module &M) {
89  InsertedThunks = false;
90  return false;
91 }
92 
93 bool X86RetpolineThunks::runOnMachineFunction(MachineFunction &MF) {
94  LLVM_DEBUG(dbgs() << getPassName() << '\n');
95 
96  TM = &MF.getTarget();;
97  STI = &MF.getSubtarget<X86Subtarget>();
98  TII = STI->getInstrInfo();
99  Is64Bit = TM->getTargetTriple().getArch() == Triple::x86_64;
100 
101  MMI = &getAnalysis<MachineModuleInfo>();
102  Module &M = const_cast<Module &>(*MMI->getModule());
103 
104  // If this function is not a thunk, check to see if we need to insert
105  // a thunk.
106  if (!MF.getName().startswith(ThunkNamePrefix)) {
107  // If we've already inserted a thunk, nothing else to do.
108  if (InsertedThunks)
109  return false;
110 
111  // Only add a thunk if one of the functions has the retpoline feature
112  // enabled in its subtarget, and doesn't enable external thunks.
113  // FIXME: Conditionalize on indirect calls so we don't emit a thunk when
114  // nothing will end up calling it.
115  // FIXME: It's a little silly to look at every function just to enumerate
116  // the subtargets, but eventually we'll want to look at them for indirect
117  // calls, so maybe this is OK.
118  if ((!STI->useRetpolineIndirectCalls() &&
119  !STI->useRetpolineIndirectBranches()) ||
120  STI->useRetpolineExternalThunk())
121  return false;
122 
123  // Otherwise, we need to insert the thunk.
124  // WARNING: This is not really a well behaving thing to do in a function
125  // pass. We extract the module and insert a new function (and machine
126  // function) directly into the module.
127  if (Is64Bit)
128  createThunkFunction(M, R11ThunkName);
129  else
130  for (StringRef Name :
131  {EAXThunkName, ECXThunkName, EDXThunkName, EDIThunkName})
132  createThunkFunction(M, Name);
133  InsertedThunks = true;
134  return true;
135  }
136 
137  // If this *is* a thunk function, we need to populate it with the correct MI.
138  if (Is64Bit) {
139  assert(MF.getName() == "__llvm_retpoline_r11" &&
140  "Should only have an r11 thunk on 64-bit targets");
141 
142  // __llvm_retpoline_r11:
143  // callq .Lr11_call_target
144  // .Lr11_capture_spec:
145  // pause
146  // lfence
147  // jmp .Lr11_capture_spec
148  // .align 16
149  // .Lr11_call_target:
150  // movq %r11, (%rsp)
151  // retq
152  populateThunk(MF, X86::R11);
153  } else {
154  // For 32-bit targets we need to emit a collection of thunks for various
155  // possible scratch registers as well as a fallback that uses EDI, which is
156  // normally callee saved.
157  // __llvm_retpoline_eax:
158  // calll .Leax_call_target
159  // .Leax_capture_spec:
160  // pause
161  // jmp .Leax_capture_spec
162  // .align 16
163  // .Leax_call_target:
164  // movl %eax, (%esp) # Clobber return addr
165  // retl
166  //
167  // __llvm_retpoline_ecx:
168  // ... # Same setup
169  // movl %ecx, (%esp)
170  // retl
171  //
172  // __llvm_retpoline_edx:
173  // ... # Same setup
174  // movl %edx, (%esp)
175  // retl
176  //
177  // __llvm_retpoline_edi:
178  // ... # Same setup
179  // movl %edi, (%esp)
180  // retl
181  if (MF.getName() == EAXThunkName)
182  populateThunk(MF, X86::EAX);
183  else if (MF.getName() == ECXThunkName)
184  populateThunk(MF, X86::ECX);
185  else if (MF.getName() == EDXThunkName)
186  populateThunk(MF, X86::EDX);
187  else if (MF.getName() == EDIThunkName)
188  populateThunk(MF, X86::EDI);
189  else
190  llvm_unreachable("Invalid thunk name on x86-32!");
191  }
192 
193  return true;
194 }
195 
196 void X86RetpolineThunks::createThunkFunction(Module &M, StringRef Name) {
197  assert(Name.startswith(ThunkNamePrefix) &&
198  "Created a thunk with an unexpected prefix!");
199 
200  LLVMContext &Ctx = M.getContext();
201  auto Type = FunctionType::get(Type::getVoidTy(Ctx), false);
202  Function *F =
205  F->setComdat(M.getOrInsertComdat(Name));
206 
207  // Add Attributes so that we don't create a frame, unwind information, or
208  // inline.
209  AttrBuilder B;
210  B.addAttribute(llvm::Attribute::NoUnwind);
211  B.addAttribute(llvm::Attribute::Naked);
213 
214  // Populate our function a bit so that we can verify.
215  BasicBlock *Entry = BasicBlock::Create(Ctx, "entry", F);
216  IRBuilder<> Builder(Entry);
217 
218  Builder.CreateRetVoid();
219 
220  // MachineFunctions/MachineBasicBlocks aren't created automatically for the
221  // IR-level constructs we already made. Create them and insert them into the
222  // module.
223  MachineFunction &MF = MMI->getOrCreateMachineFunction(*F);
224  MachineBasicBlock *EntryMBB = MF.CreateMachineBasicBlock(Entry);
225 
226  // Insert EntryMBB into MF. It's not in the module until we do this.
227  MF.insert(MF.end(), EntryMBB);
228 }
229 
230 void X86RetpolineThunks::insertRegReturnAddrClobber(MachineBasicBlock &MBB,
231  unsigned Reg) {
232  const unsigned MovOpc = Is64Bit ? X86::MOV64mr : X86::MOV32mr;
233  const unsigned SPReg = Is64Bit ? X86::RSP : X86::ESP;
234  addRegOffset(BuildMI(&MBB, DebugLoc(), TII->get(MovOpc)), SPReg, false, 0)
235  .addReg(Reg);
236 }
237 
238 void X86RetpolineThunks::populateThunk(MachineFunction &MF,
239  unsigned Reg) {
240  // Set MF properties. We never use vregs...
242 
243  // Grab the entry MBB and erase any other blocks. O0 codegen appears to
244  // generate two bbs for the entry block.
245  MachineBasicBlock *Entry = &MF.front();
246  Entry->clear();
247  while (MF.size() > 1)
248  MF.erase(std::next(MF.begin()));
249 
250  MachineBasicBlock *CaptureSpec = MF.CreateMachineBasicBlock(Entry->getBasicBlock());
251  MachineBasicBlock *CallTarget = MF.CreateMachineBasicBlock(Entry->getBasicBlock());
252  MCSymbol *TargetSym = MF.getContext().createTempSymbol();
253  MF.push_back(CaptureSpec);
254  MF.push_back(CallTarget);
255 
256  const unsigned CallOpc = Is64Bit ? X86::CALL64pcrel32 : X86::CALLpcrel32;
257  const unsigned RetOpc = Is64Bit ? X86::RETQ : X86::RETL;
258 
259  Entry->addLiveIn(Reg);
260  BuildMI(Entry, DebugLoc(), TII->get(CallOpc)).addSym(TargetSym);
261 
262  // The MIR verifier thinks that the CALL in the entry block will fall through
263  // to CaptureSpec, so mark it as the successor. Technically, CaptureTarget is
264  // the successor, but the MIR verifier doesn't know how to cope with that.
265  Entry->addSuccessor(CaptureSpec);
266 
267  // In the capture loop for speculation, we want to stop the processor from
268  // speculating as fast as possible. On Intel processors, the PAUSE instruction
269  // will block speculation without consuming any execution resources. On AMD
270  // processors, the PAUSE instruction is (essentially) a nop, so we also use an
271  // LFENCE instruction which they have advised will stop speculation as well
272  // with minimal resource utilization. We still end the capture with a jump to
273  // form an infinite loop to fully guarantee that no matter what implementation
274  // of the x86 ISA, speculating this code path never escapes.
275  BuildMI(CaptureSpec, DebugLoc(), TII->get(X86::PAUSE));
276  BuildMI(CaptureSpec, DebugLoc(), TII->get(X86::LFENCE));
277  BuildMI(CaptureSpec, DebugLoc(), TII->get(X86::JMP_1)).addMBB(CaptureSpec);
278  CaptureSpec->setHasAddressTaken();
279  CaptureSpec->addSuccessor(CaptureSpec);
280 
281  CallTarget->addLiveIn(Reg);
282  CallTarget->setHasAddressTaken();
283  CallTarget->setAlignment(4);
284  insertRegReturnAddrClobber(*CallTarget, Reg);
285  CallTarget->back().setPreInstrSymbol(MF, TargetSym);
286  BuildMI(CallTarget, DebugLoc(), TII->get(RetOpc));
287 }
void setVisibility(VisibilityTypes V)
Definition: GlobalValue.h:239
static const char EAXThunkName[]
AnalysisUsage & addPreserved()
Add the specified Pass class to the set of analyses preserved by this pass.
This class represents lattice values for constants.
Definition: AllocatorList.h:24
MCSymbol - Instances of this class represent a symbol name in the MC file, and MCSymbols are created ...
Definition: MCSymbol.h:42
A Module instance is used to store all the information related to an LLVM module. ...
Definition: Module.h:65
static const char EDXThunkName[]
const MachineFunctionProperties & getProperties() const
Get the function properties.
unsigned size() const
unsigned Reg
A debug info location.
Definition: DebugLoc.h:34
F(f)
AttrBuilder & addAttribute(Attribute::AttrKind Val)
Add an attribute to the builder.
void setAlignment(unsigned Align)
Set alignment of the basic block.
AnalysisUsage & addRequired()
static const char ECXThunkName[]
amdgpu Simplify well known AMD library false Value Value const Twine & Name
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
const HexagonInstrInfo * TII
LLVMContext & getContext() const
Get the global data context.
Definition: Module.h:244
This provides a uniform API for creating instructions and inserting them into a basic block: either a...
Definition: IRBuilder.h:743
LLVM_NODISCARD LLVM_ATTRIBUTE_ALWAYS_INLINE bool startswith(StringRef Prefix) const
Check if this string starts with the given Prefix.
Definition: StringRef.h:267
void setComdat(Comdat *C)
Definition: GlobalObject.h:103
MachineBasicBlock * CreateMachineBasicBlock(const BasicBlock *bb=nullptr)
CreateMachineBasicBlock - Allocate a new MachineBasicBlock.
StringRef getName() const
getName - Return the name of the corresponding LLVM function.
MCContext & getContext() const
MachineInstrBuilder BuildMI(MachineFunction &MF, const DebugLoc &DL, const MCInstrDesc &MCID)
Builder interface. Specify how to create the initial instruction itself.
Same, but only replaced by something equivalent.
Definition: GlobalValue.h:52
void addLiveIn(MCPhysReg PhysReg, LaneBitmask LaneMask=LaneBitmask::getAll())
Adds the specified register as a live in.
static Function * Create(FunctionType *Ty, LinkageTypes Linkage, unsigned AddrSpace, const Twine &N="", Module *M=nullptr)
Definition: Function.h:136
MCSymbol * createTempSymbol(bool CanBeUnnamed=true)
Create and return a new assembler temporary symbol with a unique but unspecified name.
Definition: MCContext.cpp:217
static GCRegistry::Add< OcamlGC > B("ocaml", "ocaml 3.10-compatible GC")
static const char EDIThunkName[]
LLVM Basic Block Representation.
Definition: BasicBlock.h:58
The instances of the Type class are immutable: once they are created, they are never changed...
Definition: Type.h:46
const TargetSubtargetInfo & getSubtarget() const
getSubtarget - Return the subtarget for which this machine code is being compiled.
void getAnalysisUsage(AnalysisUsage &AU) const override
getAnalysisUsage - Subclasses that override getAnalysisUsage must call this.
This is an important class for using LLVM in a threaded context.
Definition: LLVMContext.h:69
Represent the analysis usage information of a pass.
static Type * getVoidTy(LLVMContext &C)
Definition: Type.cpp:161
static const MachineInstrBuilder & addRegOffset(const MachineInstrBuilder &MIB, unsigned Reg, bool isKill, int Offset)
addRegOffset - This function is used to add a memory reference of the form [Reg + Offset]...
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:285
static FunctionType * get(Type *Result, ArrayRef< Type *> Params, bool isVarArg)
This static method is the primary way of constructing a FunctionType.
Definition: Type.cpp:297
static BasicBlock * Create(LLVMContext &Context, const Twine &Name="", Function *Parent=nullptr, BasicBlock *InsertBefore=nullptr)
Creates a new BasicBlock.
Definition: BasicBlock.h:100
Comdat * getOrInsertComdat(StringRef Name)
Return the Comdat in the module with the specified name.
Definition: Module.cpp:478
const MachineBasicBlock & front() const
#define llvm_unreachable(msg)
Marks that the current location is not supposed to be reachable.
void addSuccessor(MachineBasicBlock *Succ, BranchProbability Prob=BranchProbability::getUnknown())
Add Succ as a successor of this MachineBasicBlock.
Module.h This file contains the declarations for the Module class.
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:133
void setHasAddressTaken()
Set this block to reflect that it potentially is the target of an indirect branch.
MachineFunctionProperties & set(Property P)
ReturnInst * CreateRetVoid()
Create a &#39;ret void&#39; instruction.
Definition: IRBuilder.h:824
void setPreInstrSymbol(MachineFunction &MF, MCSymbol *Symbol)
Set a symbol that will be emitted just prior to the instruction itself.
const BasicBlock * getBasicBlock() const
Return the LLVM basic block that this instance corresponded to originally.
const MachineInstrBuilder & addReg(unsigned RegNo, unsigned flags=0, unsigned SubReg=0) const
Add a new virtual register operand.
const LLVMTargetMachine & getTarget() const
getTarget - Return the target machine this machine code is compiled with
FunctionPass * createX86RetpolineThunksPass()
This pass creates the thunks for the retpoline feature.
void erase(iterator MBBI)
void addAttributes(unsigned i, const AttrBuilder &Attrs)
adds the attributes to the list of attributes.
Definition: Function.cpp:381
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
static const char R11ThunkName[]
void insert(iterator MBBI, MachineBasicBlock *MBB)
void push_back(MachineBasicBlock *MBB)
Primary interface to the complete machine description for the target machine.
Definition: TargetMachine.h:59
StringRef - Represent a constant reference to a string, i.e.
Definition: StringRef.h:49
#define LLVM_DEBUG(X)
Definition: Debug.h:123
This class contains meta information specific to a module.
static const char ThunkNamePrefix[]