1 //===- AArch64SLSHardening.cpp - Harden Straight Line Missspeculation -----===// 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 file contains a pass to insert code to mitigate against side channel 10 // vulnerabilities that may happen under straight line miss-speculation. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "AArch64InstrInfo.h" 15 #include "AArch64Subtarget.h" 16 #include "Utils/AArch64BaseInfo.h" 17 #include "llvm/ADT/BitVector.h" 18 #include "llvm/ADT/SmallVector.h" 19 #include "llvm/CodeGen/IndirectThunks.h" 20 #include "llvm/CodeGen/MachineBasicBlock.h" 21 #include "llvm/CodeGen/MachineFunction.h" 22 #include "llvm/CodeGen/MachineFunctionPass.h" 23 #include "llvm/CodeGen/MachineInstr.h" 24 #include "llvm/CodeGen/MachineInstrBuilder.h" 25 #include "llvm/CodeGen/MachineOperand.h" 26 #include "llvm/CodeGen/MachineRegisterInfo.h" 27 #include "llvm/CodeGen/RegisterScavenging.h" 28 #include "llvm/IR/DebugLoc.h" 29 #include "llvm/Pass.h" 30 #include "llvm/Support/CodeGen.h" 31 #include "llvm/Support/Debug.h" 32 #include "llvm/Target/TargetMachine.h" 33 #include <cassert> 34 35 using namespace llvm; 36 37 #define DEBUG_TYPE "aarch64-sls-hardening" 38 39 #define AARCH64_SLS_HARDENING_NAME "AArch64 sls hardening pass" 40 41 namespace { 42 43 class AArch64SLSHardening : public MachineFunctionPass { 44 public: 45 const TargetInstrInfo *TII; 46 const TargetRegisterInfo *TRI; 47 const AArch64Subtarget *ST; 48 49 static char ID; 50 51 AArch64SLSHardening() : MachineFunctionPass(ID) { 52 initializeAArch64SLSHardeningPass(*PassRegistry::getPassRegistry()); 53 } 54 55 bool runOnMachineFunction(MachineFunction &Fn) override; 56 57 StringRef getPassName() const override { return AARCH64_SLS_HARDENING_NAME; } 58 59 private: 60 bool hardenReturnsAndBRs(MachineBasicBlock &MBB) const; 61 bool hardenBLRs(MachineBasicBlock &MBB) const; 62 MachineBasicBlock &ConvertBLRToBL(MachineBasicBlock &MBB, 63 MachineBasicBlock::instr_iterator) const; 64 }; 65 66 } // end anonymous namespace 67 68 char AArch64SLSHardening::ID = 0; 69 70 INITIALIZE_PASS(AArch64SLSHardening, "aarch64-sls-hardening", 71 AARCH64_SLS_HARDENING_NAME, false, false) 72 73 static void insertSpeculationBarrier(const AArch64Subtarget *ST, 74 MachineBasicBlock &MBB, 75 MachineBasicBlock::iterator MBBI, 76 DebugLoc DL, 77 bool AlwaysUseISBDSB = false) { 78 assert(MBBI != MBB.begin() && 79 "Must not insert SpeculationBarrierEndBB as only instruction in MBB."); 80 assert(std::prev(MBBI)->isBarrier() && 81 "SpeculationBarrierEndBB must only follow unconditional control flow " 82 "instructions."); 83 assert(std::prev(MBBI)->isTerminator() && 84 "SpeculationBarrierEndBB must only follow terminators."); 85 const TargetInstrInfo *TII = ST->getInstrInfo(); 86 unsigned BarrierOpc = ST->hasSB() && !AlwaysUseISBDSB 87 ? AArch64::SpeculationBarrierSBEndBB 88 : AArch64::SpeculationBarrierISBDSBEndBB; 89 if (MBBI == MBB.end() || 90 (MBBI->getOpcode() != AArch64::SpeculationBarrierSBEndBB && 91 MBBI->getOpcode() != AArch64::SpeculationBarrierISBDSBEndBB)) 92 BuildMI(MBB, MBBI, DL, TII->get(BarrierOpc)); 93 } 94 95 bool AArch64SLSHardening::runOnMachineFunction(MachineFunction &MF) { 96 ST = &MF.getSubtarget<AArch64Subtarget>(); 97 TII = MF.getSubtarget().getInstrInfo(); 98 TRI = MF.getSubtarget().getRegisterInfo(); 99 100 bool Modified = false; 101 for (auto &MBB : MF) { 102 Modified |= hardenReturnsAndBRs(MBB); 103 Modified |= hardenBLRs(MBB); 104 } 105 106 return Modified; 107 } 108 109 static bool isBLR(const MachineInstr &MI) { 110 switch (MI.getOpcode()) { 111 case AArch64::BLR: 112 case AArch64::BLRNoIP: 113 return true; 114 case AArch64::BLRAA: 115 case AArch64::BLRAB: 116 case AArch64::BLRAAZ: 117 case AArch64::BLRABZ: 118 llvm_unreachable("Currently, LLVM's code generator does not support " 119 "producing BLRA* instructions. Therefore, there's no " 120 "support in this pass for those instructions."); 121 } 122 return false; 123 } 124 125 bool AArch64SLSHardening::hardenReturnsAndBRs(MachineBasicBlock &MBB) const { 126 if (!ST->hardenSlsRetBr()) 127 return false; 128 bool Modified = false; 129 MachineBasicBlock::iterator MBBI = MBB.getFirstTerminator(), E = MBB.end(); 130 MachineBasicBlock::iterator NextMBBI; 131 for (; MBBI != E; MBBI = NextMBBI) { 132 MachineInstr &MI = *MBBI; 133 NextMBBI = std::next(MBBI); 134 if (MI.isReturn() || isIndirectBranchOpcode(MI.getOpcode())) { 135 assert(MI.isTerminator()); 136 insertSpeculationBarrier(ST, MBB, std::next(MBBI), MI.getDebugLoc()); 137 Modified = true; 138 } 139 } 140 return Modified; 141 } 142 143 static const char SLSBLRNamePrefix[] = "__llvm_slsblr_thunk_"; 144 145 static const struct ThunkNameAndReg { 146 const char* Name; 147 Register Reg; 148 } SLSBLRThunks[] = { 149 { "__llvm_slsblr_thunk_x0", AArch64::X0}, 150 { "__llvm_slsblr_thunk_x1", AArch64::X1}, 151 { "__llvm_slsblr_thunk_x2", AArch64::X2}, 152 { "__llvm_slsblr_thunk_x3", AArch64::X3}, 153 { "__llvm_slsblr_thunk_x4", AArch64::X4}, 154 { "__llvm_slsblr_thunk_x5", AArch64::X5}, 155 { "__llvm_slsblr_thunk_x6", AArch64::X6}, 156 { "__llvm_slsblr_thunk_x7", AArch64::X7}, 157 { "__llvm_slsblr_thunk_x8", AArch64::X8}, 158 { "__llvm_slsblr_thunk_x9", AArch64::X9}, 159 { "__llvm_slsblr_thunk_x10", AArch64::X10}, 160 { "__llvm_slsblr_thunk_x11", AArch64::X11}, 161 { "__llvm_slsblr_thunk_x12", AArch64::X12}, 162 { "__llvm_slsblr_thunk_x13", AArch64::X13}, 163 { "__llvm_slsblr_thunk_x14", AArch64::X14}, 164 { "__llvm_slsblr_thunk_x15", AArch64::X15}, 165 // X16 and X17 are deliberately missing, as the mitigation requires those 166 // register to not be used in BLR. See comment in ConvertBLRToBL for more 167 // details. 168 { "__llvm_slsblr_thunk_x18", AArch64::X18}, 169 { "__llvm_slsblr_thunk_x19", AArch64::X19}, 170 { "__llvm_slsblr_thunk_x20", AArch64::X20}, 171 { "__llvm_slsblr_thunk_x21", AArch64::X21}, 172 { "__llvm_slsblr_thunk_x22", AArch64::X22}, 173 { "__llvm_slsblr_thunk_x23", AArch64::X23}, 174 { "__llvm_slsblr_thunk_x24", AArch64::X24}, 175 { "__llvm_slsblr_thunk_x25", AArch64::X25}, 176 { "__llvm_slsblr_thunk_x26", AArch64::X26}, 177 { "__llvm_slsblr_thunk_x27", AArch64::X27}, 178 { "__llvm_slsblr_thunk_x28", AArch64::X28}, 179 { "__llvm_slsblr_thunk_x29", AArch64::FP}, 180 // X30 is deliberately missing, for similar reasons as X16 and X17 are 181 // missing. 182 { "__llvm_slsblr_thunk_x31", AArch64::XZR}, 183 }; 184 185 namespace { 186 struct SLSBLRThunkInserter : ThunkInserter<SLSBLRThunkInserter> { 187 const char *getThunkPrefix() { return SLSBLRNamePrefix; } 188 bool mayUseThunk(const MachineFunction &MF, bool InsertedThunks) { 189 if (InsertedThunks) 190 return false; 191 ComdatThunks &= !MF.getSubtarget<AArch64Subtarget>().hardenSlsNoComdat(); 192 // FIXME: This could also check if there are any BLRs in the function 193 // to more accurately reflect if a thunk will be needed. 194 return MF.getSubtarget<AArch64Subtarget>().hardenSlsBlr(); 195 } 196 bool insertThunks(MachineModuleInfo &MMI, MachineFunction &MF); 197 void populateThunk(MachineFunction &MF); 198 199 private: 200 bool ComdatThunks = true; 201 }; 202 } // namespace 203 204 bool SLSBLRThunkInserter::insertThunks(MachineModuleInfo &MMI, 205 MachineFunction &MF) { 206 // FIXME: It probably would be possible to filter which thunks to produce 207 // based on which registers are actually used in BLR instructions in this 208 // function. But would that be a worthwhile optimization? 209 for (auto T : SLSBLRThunks) 210 createThunkFunction(MMI, T.Name, ComdatThunks); 211 return true; 212 } 213 214 void SLSBLRThunkInserter::populateThunk(MachineFunction &MF) { 215 // FIXME: How to better communicate Register number, rather than through 216 // name and lookup table? 217 assert(MF.getName().startswith(getThunkPrefix())); 218 auto ThunkIt = llvm::find_if( 219 SLSBLRThunks, [&MF](auto T) { return T.Name == MF.getName(); }); 220 assert(ThunkIt != std::end(SLSBLRThunks)); 221 Register ThunkReg = ThunkIt->Reg; 222 223 const TargetInstrInfo *TII = 224 MF.getSubtarget<AArch64Subtarget>().getInstrInfo(); 225 assert (MF.size() == 1); 226 MachineBasicBlock *Entry = &MF.front(); 227 Entry->clear(); 228 229 // These thunks need to consist of the following instructions: 230 // __llvm_slsblr_thunk_xN: 231 // BR xN 232 // barrierInsts 233 Entry->addLiveIn(ThunkReg); 234 // MOV X16, ThunkReg == ORR X16, XZR, ThunkReg, LSL #0 235 BuildMI(Entry, DebugLoc(), TII->get(AArch64::ORRXrs), AArch64::X16) 236 .addReg(AArch64::XZR) 237 .addReg(ThunkReg) 238 .addImm(0); 239 BuildMI(Entry, DebugLoc(), TII->get(AArch64::BR)).addReg(AArch64::X16); 240 // Make sure the thunks do not make use of the SB extension in case there is 241 // a function somewhere that will call to it that for some reason disabled 242 // the SB extension locally on that function, even though it's enabled for 243 // the module otherwise. Therefore set AlwaysUseISBSDB to true. 244 insertSpeculationBarrier(&MF.getSubtarget<AArch64Subtarget>(), *Entry, 245 Entry->end(), DebugLoc(), true /*AlwaysUseISBDSB*/); 246 } 247 248 MachineBasicBlock &AArch64SLSHardening::ConvertBLRToBL( 249 MachineBasicBlock &MBB, MachineBasicBlock::instr_iterator MBBI) const { 250 // Transform a BLR to a BL as follows: 251 // Before: 252 // |-----------------------------| 253 // | ... | 254 // | instI | 255 // | BLR xN | 256 // | instJ | 257 // | ... | 258 // |-----------------------------| 259 // 260 // After: 261 // |-----------------------------| 262 // | ... | 263 // | instI | 264 // | BL __llvm_slsblr_thunk_xN | 265 // | instJ | 266 // | ... | 267 // |-----------------------------| 268 // 269 // __llvm_slsblr_thunk_xN: 270 // |-----------------------------| 271 // | BR xN | 272 // | barrierInsts | 273 // |-----------------------------| 274 // 275 // The __llvm_slsblr_thunk_xN thunks are created by the SLSBLRThunkInserter. 276 // This function merely needs to transform BLR xN into BL 277 // __llvm_slsblr_thunk_xN. 278 // 279 // Since linkers are allowed to clobber X16 and X17 on function calls, the 280 // above mitigation only works if the original BLR instruction was not 281 // BLR X16 nor BLR X17. Code generation before must make sure that no BLR 282 // X16|X17 was produced if the mitigation is enabled. 283 284 MachineInstr &BLR = *MBBI; 285 assert(isBLR(BLR)); 286 unsigned BLOpcode; 287 Register Reg; 288 bool RegIsKilled; 289 switch (BLR.getOpcode()) { 290 case AArch64::BLR: 291 case AArch64::BLRNoIP: 292 BLOpcode = AArch64::BL; 293 Reg = BLR.getOperand(0).getReg(); 294 assert(Reg != AArch64::X16 && Reg != AArch64::X17 && Reg != AArch64::LR); 295 RegIsKilled = BLR.getOperand(0).isKill(); 296 break; 297 case AArch64::BLRAA: 298 case AArch64::BLRAB: 299 case AArch64::BLRAAZ: 300 case AArch64::BLRABZ: 301 llvm_unreachable("BLRA instructions cannot yet be produced by LLVM, " 302 "therefore there is no need to support them for now."); 303 default: 304 llvm_unreachable("unhandled BLR"); 305 } 306 DebugLoc DL = BLR.getDebugLoc(); 307 308 // If we'd like to support also BLRAA and BLRAB instructions, we'd need 309 // a lot more different kind of thunks. 310 // For example, a 311 // 312 // BLRAA xN, xM 313 // 314 // instruction probably would need to be transformed to something like: 315 // 316 // BL __llvm_slsblraa_thunk_x<N>_x<M> 317 // 318 // __llvm_slsblraa_thunk_x<N>_x<M>: 319 // BRAA x<N>, x<M> 320 // barrierInsts 321 // 322 // Given that about 30 different values of N are possible and about 30 323 // different values of M are possible in the above, with the current way 324 // of producing indirect thunks, we'd be producing about 30 times 30, i.e. 325 // about 900 thunks (where most might not be actually called). This would 326 // multiply further by two to support both BLRAA and BLRAB variants of those 327 // instructions. 328 // If we'd want to support this, we'd probably need to look into a different 329 // way to produce thunk functions, based on which variants are actually 330 // needed, rather than producing all possible variants. 331 // So far, LLVM does never produce BLRA* instructions, so let's leave this 332 // for the future when LLVM can start producing BLRA* instructions. 333 MachineFunction &MF = *MBBI->getMF(); 334 MCContext &Context = MBB.getParent()->getContext(); 335 auto ThunkIt = 336 llvm::find_if(SLSBLRThunks, [Reg](auto T) { return T.Reg == Reg; }); 337 assert (ThunkIt != std::end(SLSBLRThunks)); 338 MCSymbol *Sym = Context.getOrCreateSymbol(ThunkIt->Name); 339 340 MachineInstr *BL = BuildMI(MBB, MBBI, DL, TII->get(BLOpcode)).addSym(Sym); 341 342 // Now copy the implicit operands from BLR to BL and copy other necessary 343 // info. 344 // However, both BLR and BL instructions implictly use SP and implicitly 345 // define LR. Blindly copying implicit operands would result in SP and LR 346 // operands to be present multiple times. While this may not be too much of 347 // an issue, let's avoid that for cleanliness, by removing those implicit 348 // operands from the BL created above before we copy over all implicit 349 // operands from the BLR. 350 int ImpLROpIdx = -1; 351 int ImpSPOpIdx = -1; 352 for (unsigned OpIdx = BL->getNumExplicitOperands(); 353 OpIdx < BL->getNumOperands(); OpIdx++) { 354 MachineOperand Op = BL->getOperand(OpIdx); 355 if (!Op.isReg()) 356 continue; 357 if (Op.getReg() == AArch64::LR && Op.isDef()) 358 ImpLROpIdx = OpIdx; 359 if (Op.getReg() == AArch64::SP && !Op.isDef()) 360 ImpSPOpIdx = OpIdx; 361 } 362 assert(ImpLROpIdx != -1); 363 assert(ImpSPOpIdx != -1); 364 int FirstOpIdxToRemove = std::max(ImpLROpIdx, ImpSPOpIdx); 365 int SecondOpIdxToRemove = std::min(ImpLROpIdx, ImpSPOpIdx); 366 BL->removeOperand(FirstOpIdxToRemove); 367 BL->removeOperand(SecondOpIdxToRemove); 368 // Now copy over the implicit operands from the original BLR 369 BL->copyImplicitOps(MF, BLR); 370 MF.moveCallSiteInfo(&BLR, BL); 371 // Also add the register called in the BLR as being used in the called thunk. 372 BL->addOperand(MachineOperand::CreateReg(Reg, false /*isDef*/, true /*isImp*/, 373 RegIsKilled /*isKill*/)); 374 // Remove BLR instruction 375 MBB.erase(MBBI); 376 377 return MBB; 378 } 379 380 bool AArch64SLSHardening::hardenBLRs(MachineBasicBlock &MBB) const { 381 if (!ST->hardenSlsBlr()) 382 return false; 383 bool Modified = false; 384 MachineBasicBlock::instr_iterator MBBI = MBB.instr_begin(), 385 E = MBB.instr_end(); 386 MachineBasicBlock::instr_iterator NextMBBI; 387 for (; MBBI != E; MBBI = NextMBBI) { 388 MachineInstr &MI = *MBBI; 389 NextMBBI = std::next(MBBI); 390 if (isBLR(MI)) { 391 ConvertBLRToBL(MBB, MBBI); 392 Modified = true; 393 } 394 } 395 return Modified; 396 } 397 398 FunctionPass *llvm::createAArch64SLSHardeningPass() { 399 return new AArch64SLSHardening(); 400 } 401 402 namespace { 403 class AArch64IndirectThunks : public MachineFunctionPass { 404 public: 405 static char ID; 406 407 AArch64IndirectThunks() : MachineFunctionPass(ID) {} 408 409 StringRef getPassName() const override { return "AArch64 Indirect Thunks"; } 410 411 bool doInitialization(Module &M) override; 412 bool runOnMachineFunction(MachineFunction &MF) override; 413 414 private: 415 std::tuple<SLSBLRThunkInserter> TIs; 416 417 // FIXME: When LLVM moves to C++17, these can become folds 418 template <typename... ThunkInserterT> 419 static void initTIs(Module &M, 420 std::tuple<ThunkInserterT...> &ThunkInserters) { 421 (void)std::initializer_list<int>{ 422 (std::get<ThunkInserterT>(ThunkInserters).init(M), 0)...}; 423 } 424 template <typename... ThunkInserterT> 425 static bool runTIs(MachineModuleInfo &MMI, MachineFunction &MF, 426 std::tuple<ThunkInserterT...> &ThunkInserters) { 427 bool Modified = false; 428 (void)std::initializer_list<int>{ 429 Modified |= std::get<ThunkInserterT>(ThunkInserters).run(MMI, MF)...}; 430 return Modified; 431 } 432 }; 433 434 } // end anonymous namespace 435 436 char AArch64IndirectThunks::ID = 0; 437 438 FunctionPass *llvm::createAArch64IndirectThunks() { 439 return new AArch64IndirectThunks(); 440 } 441 442 bool AArch64IndirectThunks::doInitialization(Module &M) { 443 initTIs(M, TIs); 444 return false; 445 } 446 447 bool AArch64IndirectThunks::runOnMachineFunction(MachineFunction &MF) { 448 LLVM_DEBUG(dbgs() << getPassName() << '\n'); 449 auto &MMI = getAnalysis<MachineModuleInfoWrapperPass>().getMMI(); 450 return runTIs(MMI, MF, TIs); 451 } 452