1//==- X86IndirectThunks.cpp - Construct indirect call/jump thunks for x86 --=//
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/// \file
9///
10/// Pass that injects an MI thunk that is used to lower indirect calls in a way
11/// that prevents speculation on some x86 processors and can be used to mitigate
12/// security vulnerabilities due to targeted speculative execution and side
13/// channels such as CVE-2017-5715.
14///
15/// Currently supported thunks include:
16/// - Retpoline -- A RET-implemented trampoline that lowers indirect calls
17/// - LVI Thunk -- A CALL/JMP-implemented thunk that forces load serialization
18/// before making an indirect call/jump
19///
20/// Note that the reason that this is implemented as a MachineFunctionPass and
21/// not a ModulePass is that ModulePasses at this point in the LLVM X86 pipeline
22/// serialize all transformations, which can consume lots of memory.
23///
24/// TODO(chandlerc): All of this code could use better comments and
25/// documentation.
26///
27//===----------------------------------------------------------------------===//
28
29#include "X86.h"
30#include "X86InstrBuilder.h"
31#include "X86Subtarget.h"
32#include "llvm/CodeGen/IndirectThunks.h"
33#include "llvm/CodeGen/MachineFunction.h"
34#include "llvm/CodeGen/MachineFunctionPass.h"
35#include "llvm/CodeGen/MachineInstrBuilder.h"
36#include "llvm/CodeGen/MachineModuleInfo.h"
37#include "llvm/CodeGen/Passes.h"
38#include "llvm/CodeGen/TargetPassConfig.h"
39#include "llvm/IR/IRBuilder.h"
40#include "llvm/IR/Instructions.h"
41#include "llvm/IR/Module.h"
42#include "llvm/Support/CommandLine.h"
43#include "llvm/Support/Debug.h"
44#include "llvm/Support/raw_ostream.h"
45#include "llvm/Target/TargetMachine.h"
46
47using namespace llvm;
48
49#define DEBUG_TYPE "x86-retpoline-thunks"
50
51static const char RetpolineNamePrefix[] = "__llvm_retpoline_";
52static const char R11RetpolineName[] = "__llvm_retpoline_r11";
53static const char EAXRetpolineName[] = "__llvm_retpoline_eax";
54static const char ECXRetpolineName[] = "__llvm_retpoline_ecx";
55static const char EDXRetpolineName[] = "__llvm_retpoline_edx";
56static const char EDIRetpolineName[] = "__llvm_retpoline_edi";
57
58static const char LVIThunkNamePrefix[] = "__llvm_lvi_thunk_";
59static const char R11LVIThunkName[] = "__llvm_lvi_thunk_r11";
60
61namespace {
62struct RetpolineThunkInserter : ThunkInserter<RetpolineThunkInserter> {
63 const char *getThunkPrefix() { return RetpolineNamePrefix; }
64 bool mayUseThunk(const MachineFunction &MF) {
65 const auto &STI = MF.getSubtarget<X86Subtarget>();
66 return (STI.useRetpolineIndirectCalls() ||
67 STI.useRetpolineIndirectBranches()) &&
68 !STI.useRetpolineExternalThunk();
69 }
70 bool insertThunks(MachineModuleInfo &MMI, MachineFunction &MF,
71 bool ExistingThunks);
72 void populateThunk(MachineFunction &MF);
73};
74
75struct LVIThunkInserter : ThunkInserter<LVIThunkInserter> {
76 const char *getThunkPrefix() { return LVIThunkNamePrefix; }
77 bool mayUseThunk(const MachineFunction &MF) {
78 return MF.getSubtarget<X86Subtarget>().useLVIControlFlowIntegrity();
79 }
80 bool insertThunks(MachineModuleInfo &MMI, MachineFunction &MF,
81 bool ExistingThunks) {
82 if (ExistingThunks)
83 return false;
84 createThunkFunction(MMI, Name: R11LVIThunkName);
85 return true;
86 }
87 void populateThunk(MachineFunction &MF) {
88 assert (MF.size() == 1);
89 MachineBasicBlock *Entry = &MF.front();
90 Entry->clear();
91
92 // This code mitigates LVI by replacing each indirect call/jump with a
93 // direct call/jump to a thunk that looks like:
94 // ```
95 // lfence
96 // jmpq *%r11
97 // ```
98 // This ensures that if the value in register %r11 was loaded from memory,
99 // then the value in %r11 is (architecturally) correct prior to the jump.
100 const TargetInstrInfo *TII = MF.getSubtarget<X86Subtarget>().getInstrInfo();
101 BuildMI(BB: &MF.front(), MIMD: DebugLoc(), MCID: TII->get(Opcode: X86::LFENCE));
102 BuildMI(BB: &MF.front(), MIMD: DebugLoc(), MCID: TII->get(Opcode: X86::JMP64r)).addReg(RegNo: X86::R11);
103 MF.front().addLiveIn(PhysReg: X86::R11);
104 }
105};
106
107class X86IndirectThunks
108 : public ThunkInserterPass<RetpolineThunkInserter, LVIThunkInserter> {
109public:
110 static char ID;
111
112 X86IndirectThunks() : ThunkInserterPass(ID) {}
113
114 StringRef getPassName() const override { return "X86 Indirect Thunks"; }
115};
116
117} // end anonymous namespace
118
119bool RetpolineThunkInserter::insertThunks(MachineModuleInfo &MMI,
120 MachineFunction &MF,
121 bool ExistingThunks) {
122 if (ExistingThunks)
123 return false;
124 if (MMI.getTarget().getTargetTriple().getArch() == Triple::x86_64)
125 createThunkFunction(MMI, Name: R11RetpolineName);
126 else
127 for (StringRef Name : {EAXRetpolineName, ECXRetpolineName, EDXRetpolineName,
128 EDIRetpolineName})
129 createThunkFunction(MMI, Name);
130 return true;
131}
132
133void RetpolineThunkInserter::populateThunk(MachineFunction &MF) {
134 bool Is64Bit = MF.getTarget().getTargetTriple().getArch() == Triple::x86_64;
135 Register ThunkReg;
136 if (Is64Bit) {
137 assert(MF.getName() == "__llvm_retpoline_r11" &&
138 "Should only have an r11 thunk on 64-bit targets");
139
140 // __llvm_retpoline_r11:
141 // callq .Lr11_call_target
142 // .Lr11_capture_spec:
143 // pause
144 // lfence
145 // jmp .Lr11_capture_spec
146 // .align 16
147 // .Lr11_call_target:
148 // movq %r11, (%rsp)
149 // retq
150 ThunkReg = X86::R11;
151 } else {
152 // For 32-bit targets we need to emit a collection of thunks for various
153 // possible scratch registers as well as a fallback that uses EDI, which is
154 // normally callee saved.
155 // __llvm_retpoline_eax:
156 // calll .Leax_call_target
157 // .Leax_capture_spec:
158 // pause
159 // jmp .Leax_capture_spec
160 // .align 16
161 // .Leax_call_target:
162 // movl %eax, (%esp) # Clobber return addr
163 // retl
164 //
165 // __llvm_retpoline_ecx:
166 // ... # Same setup
167 // movl %ecx, (%esp)
168 // retl
169 //
170 // __llvm_retpoline_edx:
171 // ... # Same setup
172 // movl %edx, (%esp)
173 // retl
174 //
175 // __llvm_retpoline_edi:
176 // ... # Same setup
177 // movl %edi, (%esp)
178 // retl
179 if (MF.getName() == EAXRetpolineName)
180 ThunkReg = X86::EAX;
181 else if (MF.getName() == ECXRetpolineName)
182 ThunkReg = X86::ECX;
183 else if (MF.getName() == EDXRetpolineName)
184 ThunkReg = X86::EDX;
185 else if (MF.getName() == EDIRetpolineName)
186 ThunkReg = X86::EDI;
187 else
188 llvm_unreachable("Invalid thunk name on x86-32!");
189 }
190
191 const TargetInstrInfo *TII = MF.getSubtarget<X86Subtarget>().getInstrInfo();
192 assert (MF.size() == 1);
193 MachineBasicBlock *Entry = &MF.front();
194 Entry->clear();
195
196 MachineBasicBlock *CaptureSpec =
197 MF.CreateMachineBasicBlock(BB: Entry->getBasicBlock());
198 MachineBasicBlock *CallTarget =
199 MF.CreateMachineBasicBlock(BB: Entry->getBasicBlock());
200 MCSymbol *TargetSym = MF.getContext().createTempSymbol();
201 MF.push_back(MBB: CaptureSpec);
202 MF.push_back(MBB: CallTarget);
203
204 const unsigned CallOpc = Is64Bit ? X86::CALL64pcrel32 : X86::CALLpcrel32;
205 const unsigned RetOpc = Is64Bit ? X86::RET64 : X86::RET32;
206
207 Entry->addLiveIn(PhysReg: ThunkReg);
208 BuildMI(BB: Entry, MIMD: DebugLoc(), MCID: TII->get(Opcode: CallOpc)).addSym(Sym: TargetSym);
209
210 // The MIR verifier thinks that the CALL in the entry block will fall through
211 // to CaptureSpec, so mark it as the successor. Technically, CaptureTarget is
212 // the successor, but the MIR verifier doesn't know how to cope with that.
213 Entry->addSuccessor(Succ: CaptureSpec);
214
215 // In the capture loop for speculation, we want to stop the processor from
216 // speculating as fast as possible. On Intel processors, the PAUSE instruction
217 // will block speculation without consuming any execution resources. On AMD
218 // processors, the PAUSE instruction is (essentially) a nop, so we also use an
219 // LFENCE instruction which they have advised will stop speculation as well
220 // with minimal resource utilization. We still end the capture with a jump to
221 // form an infinite loop to fully guarantee that no matter what implementation
222 // of the x86 ISA, speculating this code path never escapes.
223 BuildMI(BB: CaptureSpec, MIMD: DebugLoc(), MCID: TII->get(Opcode: X86::PAUSE));
224 BuildMI(BB: CaptureSpec, MIMD: DebugLoc(), MCID: TII->get(Opcode: X86::LFENCE));
225 BuildMI(BB: CaptureSpec, MIMD: DebugLoc(), MCID: TII->get(Opcode: X86::JMP_1)).addMBB(MBB: CaptureSpec);
226 CaptureSpec->setMachineBlockAddressTaken();
227 CaptureSpec->addSuccessor(Succ: CaptureSpec);
228
229 CallTarget->addLiveIn(PhysReg: ThunkReg);
230 CallTarget->setMachineBlockAddressTaken();
231 CallTarget->setAlignment(Align(16));
232
233 // Insert return address clobber
234 const unsigned MovOpc = Is64Bit ? X86::MOV64mr : X86::MOV32mr;
235 const Register SPReg = Is64Bit ? X86::RSP : X86::ESP;
236 addRegOffset(MIB: BuildMI(BB: CallTarget, MIMD: DebugLoc(), MCID: TII->get(Opcode: MovOpc)), Reg: SPReg, isKill: false,
237 Offset: 0)
238 .addReg(RegNo: ThunkReg);
239
240 CallTarget->back().setPreInstrSymbol(MF, Symbol: TargetSym);
241 BuildMI(BB: CallTarget, MIMD: DebugLoc(), MCID: TII->get(Opcode: RetOpc));
242}
243
244FunctionPass *llvm::createX86IndirectThunksPass() {
245 return new X86IndirectThunks();
246}
247
248char X86IndirectThunks::ID = 0;
249