| //===- AArch64SpeculationHardening.cpp - Harden Against Missspeculation --===// |
| // |
| // 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 file contains a pass to insert code to mitigate against side channel |
| // vulnerabilities that may happen under control flow miss-speculation. |
| // |
| // The pass implements tracking of control flow miss-speculation into a "taint" |
| // register. That taint register can then be used to mask off registers with |
| // sensitive data when executing under miss-speculation, a.k.a. "transient |
| // execution". |
| // This pass is aimed at mitigating against SpectreV1-style vulnarabilities. |
| // |
| // It also implements speculative load hardening, i.e. using the taint register |
| // to automatically mask off loaded data. |
| // |
| // As a possible follow-on improvement, also an intrinsics-based approach as |
| // explained at https://lwn.net/Articles/759423/ could be implemented on top of |
| // the current design. |
| // |
| // For AArch64, the following implementation choices are made to implement the |
| // tracking of control flow miss-speculation into a taint register: |
| // Some of these are different than the implementation choices made in |
| // the similar pass implemented in X86SpeculativeLoadHardening.cpp, as |
| // the instruction set characteristics result in different trade-offs. |
| // - The speculation hardening is done after register allocation. With a |
| // relative abundance of registers, one register is reserved (X16) to be |
| // the taint register. X16 is expected to not clash with other register |
| // reservation mechanisms with very high probability because: |
| // . The AArch64 ABI doesn't guarantee X16 to be retained across any call. |
| // . The only way to request X16 to be used as a programmer is through |
| // inline assembly. In the rare case a function explicitly demands to |
| // use X16/W16, this pass falls back to hardening against speculation |
| // by inserting a DSB SYS/ISB barrier pair which will prevent control |
| // flow speculation. |
| // - It is easy to insert mask operations at this late stage as we have |
| // mask operations available that don't set flags. |
| // - The taint variable contains all-ones when no miss-speculation is detected, |
| // and contains all-zeros when miss-speculation is detected. Therefore, when |
| // masking, an AND instruction (which only changes the register to be masked, |
| // no other side effects) can easily be inserted anywhere that's needed. |
| // - The tracking of miss-speculation is done by using a data-flow conditional |
| // select instruction (CSEL) to evaluate the flags that were also used to |
| // make conditional branch direction decisions. Speculation of the CSEL |
| // instruction can be limited with a CSDB instruction - so the combination of |
| // CSEL + a later CSDB gives the guarantee that the flags as used in the CSEL |
| // aren't speculated. When conditional branch direction gets miss-speculated, |
| // the semantics of the inserted CSEL instruction is such that the taint |
| // register will contain all zero bits. |
| // One key requirement for this to work is that the conditional branch is |
| // followed by an execution of the CSEL instruction, where the CSEL |
| // instruction needs to use the same flags status as the conditional branch. |
| // This means that the conditional branches must not be implemented as one |
| // of the AArch64 conditional branches that do not use the flags as input |
| // (CB(N)Z and TB(N)Z). This is implemented by ensuring in the instruction |
| // selectors to not produce these instructions when speculation hardening |
| // is enabled. This pass will assert if it does encounter such an instruction. |
| // - On function call boundaries, the miss-speculation state is transferred from |
| // the taint register X16 to be encoded in the SP register as value 0. |
| // |
| // For the aspect of automatically hardening loads, using the taint register, |
| // (a.k.a. speculative load hardening, see |
| // https://llvm.org/docs/SpeculativeLoadHardening.html), the following |
| // implementation choices are made for AArch64: |
| // - Many of the optimizations described at |
| // https://llvm.org/docs/SpeculativeLoadHardening.html to harden fewer |
| // loads haven't been implemented yet - but for some of them there are |
| // FIXMEs in the code. |
| // - loads that load into general purpose (X or W) registers get hardened by |
| // masking the loaded data. For loads that load into other registers, the |
| // address loaded from gets hardened. It is expected that hardening the |
| // loaded data may be more efficient; but masking data in registers other |
| // than X or W is not easy and may result in being slower than just |
| // hardening the X address register loaded from. |
| // - On AArch64, CSDB instructions are inserted between the masking of the |
| // register and its first use, to ensure there's no non-control-flow |
| // speculation that might undermine the hardening mechanism. |
| // |
| // Future extensions/improvements could be: |
| // - Implement this functionality using full speculation barriers, akin to the |
| // x86-slh-lfence option. This may be more useful for the intrinsics-based |
| // approach than for the SLH approach to masking. |
| // Note that this pass already inserts the full speculation barriers if the |
| // function for some niche reason makes use of X16/W16. |
| // - no indirect branch misprediction gets protected/instrumented; but this |
| // could be done for some indirect branches, such as switch jump tables. |
| //===----------------------------------------------------------------------===// |
| |
| #include "AArch64InstrInfo.h" |
| #include "AArch64Subtarget.h" |
| #include "Utils/AArch64BaseInfo.h" |
| #include "llvm/ADT/BitVector.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/CodeGen/MachineBasicBlock.h" |
| #include "llvm/CodeGen/MachineFunction.h" |
| #include "llvm/CodeGen/MachineFunctionPass.h" |
| #include "llvm/CodeGen/MachineInstr.h" |
| #include "llvm/CodeGen/MachineInstrBuilder.h" |
| #include "llvm/CodeGen/MachineOperand.h" |
| #include "llvm/CodeGen/MachineRegisterInfo.h" |
| #include "llvm/CodeGen/RegisterScavenging.h" |
| #include "llvm/IR/DebugLoc.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Support/CodeGen.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Target/TargetMachine.h" |
| #include <cassert> |
| |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "aarch64-speculation-hardening" |
| |
| #define AARCH64_SPECULATION_HARDENING_NAME "AArch64 speculation hardening pass" |
| |
| static cl::opt<bool> HardenLoads("aarch64-slh-loads", cl::Hidden, |
| cl::desc("Sanitize loads from memory."), |
| cl::init(true)); |
| |
| namespace { |
| |
| class AArch64SpeculationHardening : public MachineFunctionPass { |
| public: |
| const TargetInstrInfo *TII; |
| const TargetRegisterInfo *TRI; |
| |
| static char ID; |
| |
| AArch64SpeculationHardening() : MachineFunctionPass(ID) { |
| initializeAArch64SpeculationHardeningPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool runOnMachineFunction(MachineFunction &Fn) override; |
| |
| StringRef getPassName() const override { |
| return AARCH64_SPECULATION_HARDENING_NAME; |
| } |
| |
| private: |
| unsigned MisspeculatingTaintReg; |
| unsigned MisspeculatingTaintReg32Bit; |
| bool UseControlFlowSpeculationBarrier; |
| BitVector RegsNeedingCSDBBeforeUse; |
| BitVector RegsAlreadyMasked; |
| |
| bool functionUsesHardeningRegister(MachineFunction &MF) const; |
| bool instrumentControlFlow(MachineBasicBlock &MBB, |
| bool &UsesFullSpeculationBarrier); |
| bool endsWithCondControlFlow(MachineBasicBlock &MBB, MachineBasicBlock *&TBB, |
| MachineBasicBlock *&FBB, |
| AArch64CC::CondCode &CondCode) const; |
| void insertTrackingCode(MachineBasicBlock &SplitEdgeBB, |
| AArch64CC::CondCode &CondCode, DebugLoc DL) const; |
| void insertSPToRegTaintPropagation(MachineBasicBlock &MBB, |
| MachineBasicBlock::iterator MBBI) const; |
| void insertRegToSPTaintPropagation(MachineBasicBlock &MBB, |
| MachineBasicBlock::iterator MBBI, |
| unsigned TmpReg) const; |
| void insertFullSpeculationBarrier(MachineBasicBlock &MBB, |
| MachineBasicBlock::iterator MBBI, |
| DebugLoc DL) const; |
| |
| bool slhLoads(MachineBasicBlock &MBB); |
| bool makeGPRSpeculationSafe(MachineBasicBlock &MBB, |
| MachineBasicBlock::iterator MBBI, |
| MachineInstr &MI, unsigned Reg); |
| bool lowerSpeculationSafeValuePseudos(MachineBasicBlock &MBB, |
| bool UsesFullSpeculationBarrier); |
| bool expandSpeculationSafeValue(MachineBasicBlock &MBB, |
| MachineBasicBlock::iterator MBBI, |
| bool UsesFullSpeculationBarrier); |
| bool insertCSDB(MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI, |
| DebugLoc DL); |
| }; |
| |
| } // end anonymous namespace |
| |
| char AArch64SpeculationHardening::ID = 0; |
| |
| INITIALIZE_PASS(AArch64SpeculationHardening, "aarch64-speculation-hardening", |
| AARCH64_SPECULATION_HARDENING_NAME, false, false) |
| |
| bool AArch64SpeculationHardening::endsWithCondControlFlow( |
| MachineBasicBlock &MBB, MachineBasicBlock *&TBB, MachineBasicBlock *&FBB, |
| AArch64CC::CondCode &CondCode) const { |
| SmallVector<MachineOperand, 1> analyzeBranchCondCode; |
| if (TII->analyzeBranch(MBB, TBB, FBB, analyzeBranchCondCode, false)) |
| return false; |
| |
| // Ignore if the BB ends in an unconditional branch/fall-through. |
| if (analyzeBranchCondCode.empty()) |
| return false; |
| |
| // If the BB ends with a single conditional branch, FBB will be set to |
| // nullptr (see API docs for TII->analyzeBranch). For the rest of the |
| // analysis we want the FBB block to be set always. |
| assert(TBB != nullptr); |
| if (FBB == nullptr) |
| FBB = MBB.getFallThrough(); |
| |
| // If both the true and the false condition jump to the same basic block, |
| // there isn't need for any protection - whether the branch is speculated |
| // correctly or not, we end up executing the architecturally correct code. |
| if (TBB == FBB) |
| return false; |
| |
| assert(MBB.succ_size() == 2); |
| // translate analyzeBranchCondCode to CondCode. |
| assert(analyzeBranchCondCode.size() == 1 && "unknown Cond array format"); |
| CondCode = AArch64CC::CondCode(analyzeBranchCondCode[0].getImm()); |
| return true; |
| } |
| |
| void AArch64SpeculationHardening::insertFullSpeculationBarrier( |
| MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI, |
| DebugLoc DL) const { |
| // A full control flow speculation barrier consists of (DSB SYS + ISB) |
| BuildMI(MBB, MBBI, DL, TII->get(AArch64::DSB)).addImm(0xf); |
| BuildMI(MBB, MBBI, DL, TII->get(AArch64::ISB)).addImm(0xf); |
| } |
| |
| void AArch64SpeculationHardening::insertTrackingCode( |
| MachineBasicBlock &SplitEdgeBB, AArch64CC::CondCode &CondCode, |
| DebugLoc DL) const { |
| if (UseControlFlowSpeculationBarrier) { |
| insertFullSpeculationBarrier(SplitEdgeBB, SplitEdgeBB.begin(), DL); |
| } else { |
| BuildMI(SplitEdgeBB, SplitEdgeBB.begin(), DL, TII->get(AArch64::CSELXr)) |
| .addDef(MisspeculatingTaintReg) |
| .addUse(MisspeculatingTaintReg) |
| .addUse(AArch64::XZR) |
| .addImm(CondCode); |
| SplitEdgeBB.addLiveIn(AArch64::NZCV); |
| } |
| } |
| |
| bool AArch64SpeculationHardening::instrumentControlFlow( |
| MachineBasicBlock &MBB, bool &UsesFullSpeculationBarrier) { |
| LLVM_DEBUG(dbgs() << "Instrument control flow tracking on MBB: " << MBB); |
| |
| bool Modified = false; |
| MachineBasicBlock *TBB = nullptr; |
| MachineBasicBlock *FBB = nullptr; |
| AArch64CC::CondCode CondCode; |
| |
| if (!endsWithCondControlFlow(MBB, TBB, FBB, CondCode)) { |
| LLVM_DEBUG(dbgs() << "... doesn't end with CondControlFlow\n"); |
| } else { |
| // Now insert: |
| // "CSEL MisSpeculatingR, MisSpeculatingR, XZR, cond" on the True edge and |
| // "CSEL MisSpeculatingR, MisSpeculatingR, XZR, Invertcond" on the False |
| // edge. |
| AArch64CC::CondCode InvCondCode = AArch64CC::getInvertedCondCode(CondCode); |
| |
| MachineBasicBlock *SplitEdgeTBB = MBB.SplitCriticalEdge(TBB, *this); |
| MachineBasicBlock *SplitEdgeFBB = MBB.SplitCriticalEdge(FBB, *this); |
| |
| assert(SplitEdgeTBB != nullptr); |
| assert(SplitEdgeFBB != nullptr); |
| |
| DebugLoc DL; |
| if (MBB.instr_end() != MBB.instr_begin()) |
| DL = (--MBB.instr_end())->getDebugLoc(); |
| |
| insertTrackingCode(*SplitEdgeTBB, CondCode, DL); |
| insertTrackingCode(*SplitEdgeFBB, InvCondCode, DL); |
| |
| LLVM_DEBUG(dbgs() << "SplitEdgeTBB: " << *SplitEdgeTBB << "\n"); |
| LLVM_DEBUG(dbgs() << "SplitEdgeFBB: " << *SplitEdgeFBB << "\n"); |
| Modified = true; |
| } |
| |
| // Perform correct code generation around function calls and before returns. |
| // The below variables record the return/terminator instructions and the call |
| // instructions respectively; including which register is available as a |
| // temporary register just before the recorded instructions. |
| SmallVector<std::pair<MachineInstr *, unsigned>, 4> ReturnInstructions; |
| SmallVector<std::pair<MachineInstr *, unsigned>, 4> CallInstructions; |
| // if a temporary register is not available for at least one of the |
| // instructions for which we need to transfer taint to the stack pointer, we |
| // need to insert a full speculation barrier. |
| // TmpRegisterNotAvailableEverywhere tracks that condition. |
| bool TmpRegisterNotAvailableEverywhere = false; |
| |
| RegScavenger RS; |
| RS.enterBasicBlock(MBB); |
| |
| for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); I++) { |
| MachineInstr &MI = *I; |
| if (!MI.isReturn() && !MI.isCall()) |
| continue; |
| |
| // The RegScavenger represents registers available *after* the MI |
| // instruction pointed to by RS.getCurrentPosition(). |
| // We need to have a register that is available *before* the MI is executed. |
| if (I != MBB.begin()) |
| RS.forward(std::prev(I)); |
| // FIXME: The below just finds *a* unused register. Maybe code could be |
| // optimized more if this looks for the register that isn't used for the |
| // longest time around this place, to enable more scheduling freedom. Not |
| // sure if that would actually result in a big performance difference |
| // though. Maybe RegisterScavenger::findSurvivorBackwards has some logic |
| // already to do this - but it's unclear if that could easily be used here. |
| unsigned TmpReg = RS.FindUnusedReg(&AArch64::GPR64commonRegClass); |
| LLVM_DEBUG(dbgs() << "RS finds " |
| << ((TmpReg == 0) ? "no register " : "register "); |
| if (TmpReg != 0) dbgs() << printReg(TmpReg, TRI) << " "; |
| dbgs() << "to be available at MI " << MI); |
| if (TmpReg == 0) |
| TmpRegisterNotAvailableEverywhere = true; |
| if (MI.isReturn()) |
| ReturnInstructions.push_back({&MI, TmpReg}); |
| else if (MI.isCall()) |
| CallInstructions.push_back({&MI, TmpReg}); |
| } |
| |
| if (TmpRegisterNotAvailableEverywhere) { |
| // When a temporary register is not available everywhere in this basic |
| // basic block where a propagate-taint-to-sp operation is needed, just |
| // emit a full speculation barrier at the start of this basic block, which |
| // renders the taint/speculation tracking in this basic block unnecessary. |
| insertFullSpeculationBarrier(MBB, MBB.begin(), |
| (MBB.begin())->getDebugLoc()); |
| UsesFullSpeculationBarrier = true; |
| Modified = true; |
| } else { |
| for (auto MI_Reg : ReturnInstructions) { |
| assert(MI_Reg.second != 0); |
| LLVM_DEBUG( |
| dbgs() |
| << " About to insert Reg to SP taint propagation with temp register " |
| << printReg(MI_Reg.second, TRI) |
| << " on instruction: " << *MI_Reg.first); |
| insertRegToSPTaintPropagation(MBB, MI_Reg.first, MI_Reg.second); |
| Modified = true; |
| } |
| |
| for (auto MI_Reg : CallInstructions) { |
| assert(MI_Reg.second != 0); |
| LLVM_DEBUG(dbgs() << " About to insert Reg to SP and back taint " |
| "propagation with temp register " |
| << printReg(MI_Reg.second, TRI) |
| << " around instruction: " << *MI_Reg.first); |
| // Just after the call: |
| insertSPToRegTaintPropagation( |
| MBB, std::next((MachineBasicBlock::iterator)MI_Reg.first)); |
| // Just before the call: |
| insertRegToSPTaintPropagation(MBB, MI_Reg.first, MI_Reg.second); |
| Modified = true; |
| } |
| } |
| return Modified; |
| } |
| |
| void AArch64SpeculationHardening::insertSPToRegTaintPropagation( |
| MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI) const { |
| // If full control flow speculation barriers are used, emit a control flow |
| // barrier to block potential miss-speculation in flight coming in to this |
| // function. |
| if (UseControlFlowSpeculationBarrier) { |
| insertFullSpeculationBarrier(MBB, MBBI, DebugLoc()); |
| return; |
| } |
| |
| // CMP SP, #0 === SUBS xzr, SP, #0 |
| BuildMI(MBB, MBBI, DebugLoc(), TII->get(AArch64::SUBSXri)) |
| .addDef(AArch64::XZR) |
| .addUse(AArch64::SP) |
| .addImm(0) |
| .addImm(0); // no shift |
| // CSETM x16, NE === CSINV x16, xzr, xzr, EQ |
| BuildMI(MBB, MBBI, DebugLoc(), TII->get(AArch64::CSINVXr)) |
| .addDef(MisspeculatingTaintReg) |
| .addUse(AArch64::XZR) |
| .addUse(AArch64::XZR) |
| .addImm(AArch64CC::EQ); |
| } |
| |
| void AArch64SpeculationHardening::insertRegToSPTaintPropagation( |
| MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI, |
| unsigned TmpReg) const { |
| // If full control flow speculation barriers are used, there will not be |
| // miss-speculation when returning from this function, and therefore, also |
| // no need to encode potential miss-speculation into the stack pointer. |
| if (UseControlFlowSpeculationBarrier) |
| return; |
| |
| // mov Xtmp, SP === ADD Xtmp, SP, #0 |
| BuildMI(MBB, MBBI, DebugLoc(), TII->get(AArch64::ADDXri)) |
| .addDef(TmpReg) |
| .addUse(AArch64::SP) |
| .addImm(0) |
| .addImm(0); // no shift |
| // and Xtmp, Xtmp, TaintReg === AND Xtmp, Xtmp, TaintReg, #0 |
| BuildMI(MBB, MBBI, DebugLoc(), TII->get(AArch64::ANDXrs)) |
| .addDef(TmpReg, RegState::Renamable) |
| .addUse(TmpReg, RegState::Kill | RegState::Renamable) |
| .addUse(MisspeculatingTaintReg, RegState::Kill) |
| .addImm(0); |
| // mov SP, Xtmp === ADD SP, Xtmp, #0 |
| BuildMI(MBB, MBBI, DebugLoc(), TII->get(AArch64::ADDXri)) |
| .addDef(AArch64::SP) |
| .addUse(TmpReg, RegState::Kill) |
| .addImm(0) |
| .addImm(0); // no shift |
| } |
| |
| bool AArch64SpeculationHardening::functionUsesHardeningRegister( |
| MachineFunction &MF) const { |
| for (MachineBasicBlock &MBB : MF) { |
| for (MachineInstr &MI : MBB) { |
| // treat function calls specially, as the hardening register does not |
| // need to remain live across function calls. |
| if (MI.isCall()) |
| continue; |
| if (MI.readsRegister(MisspeculatingTaintReg, TRI) || |
| MI.modifiesRegister(MisspeculatingTaintReg, TRI)) |
| return true; |
| } |
| } |
| return false; |
| } |
| |
| // Make GPR register Reg speculation-safe by putting it through the |
| // SpeculationSafeValue pseudo instruction, if we can't prove that |
| // the value in the register has already been hardened. |
| bool AArch64SpeculationHardening::makeGPRSpeculationSafe( |
| MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI, MachineInstr &MI, |
| unsigned Reg) { |
| assert(AArch64::GPR32allRegClass.contains(Reg) || |
| AArch64::GPR64allRegClass.contains(Reg)); |
| |
| // Loads cannot directly load a value into the SP (nor WSP). |
| // Therefore, if Reg is SP or WSP, it is because the instruction loads from |
| // the stack through the stack pointer. |
| // |
| // Since the stack pointer is never dynamically controllable, don't harden it. |
| if (Reg == AArch64::SP || Reg == AArch64::WSP) |
| return false; |
| |
| // Do not harden the register again if already hardened before. |
| if (RegsAlreadyMasked[Reg]) |
| return false; |
| |
| const bool Is64Bit = AArch64::GPR64allRegClass.contains(Reg); |
| LLVM_DEBUG(dbgs() << "About to harden register : " << Reg << "\n"); |
| BuildMI(MBB, MBBI, MI.getDebugLoc(), |
| TII->get(Is64Bit ? AArch64::SpeculationSafeValueX |
| : AArch64::SpeculationSafeValueW)) |
| .addDef(Reg) |
| .addUse(Reg); |
| RegsAlreadyMasked.set(Reg); |
| return true; |
| } |
| |
| bool AArch64SpeculationHardening::slhLoads(MachineBasicBlock &MBB) { |
| bool Modified = false; |
| |
| LLVM_DEBUG(dbgs() << "slhLoads running on MBB: " << MBB); |
| |
| RegsAlreadyMasked.reset(); |
| |
| MachineBasicBlock::iterator MBBI = MBB.begin(), E = MBB.end(); |
| MachineBasicBlock::iterator NextMBBI; |
| for (; MBBI != E; MBBI = NextMBBI) { |
| MachineInstr &MI = *MBBI; |
| NextMBBI = std::next(MBBI); |
| // Only harden loaded values or addresses used in loads. |
| if (!MI.mayLoad()) |
| continue; |
| |
| LLVM_DEBUG(dbgs() << "About to harden: " << MI); |
| |
| // For general purpose register loads, harden the registers loaded into. |
| // For other loads, harden the address loaded from. |
| // Masking the loaded value is expected to result in less performance |
| // overhead, as the load can still execute speculatively in comparison to |
| // when the address loaded from gets masked. However, masking is only |
| // easy to do efficiently on GPR registers, so for loads into non-GPR |
| // registers (e.g. floating point loads), mask the address loaded from. |
| bool AllDefsAreGPR = llvm::all_of(MI.defs(), [&](MachineOperand &Op) { |
| return Op.isReg() && (AArch64::GPR32allRegClass.contains(Op.getReg()) || |
| AArch64::GPR64allRegClass.contains(Op.getReg())); |
| }); |
| // FIXME: it might be a worthwhile optimization to not mask loaded |
| // values if all the registers involved in address calculation are already |
| // hardened, leading to this load not able to execute on a miss-speculated |
| // path. |
| bool HardenLoadedData = AllDefsAreGPR; |
| bool HardenAddressLoadedFrom = !HardenLoadedData; |
| |
| // First remove registers from AlreadyMaskedRegisters if their value is |
| // updated by this instruction - it makes them contain a new value that is |
| // not guaranteed to already have been masked. |
| for (MachineOperand Op : MI.defs()) |
| for (MCRegAliasIterator AI(Op.getReg(), TRI, true); AI.isValid(); ++AI) |
| RegsAlreadyMasked.reset(*AI); |
| |
| // FIXME: loads from the stack with an immediate offset from the stack |
| // pointer probably shouldn't be hardened, which could result in a |
| // significant optimization. See section "Don’t check loads from |
| // compile-time constant stack offsets", in |
| // https://llvm.org/docs/SpeculativeLoadHardening.html |
| |
| if (HardenLoadedData) |
| for (auto Def : MI.defs()) { |
| if (Def.isDead()) |
| // Do not mask a register that is not used further. |
| continue; |
| // FIXME: For pre/post-increment addressing modes, the base register |
| // used in address calculation is also defined by this instruction. |
| // It might be a worthwhile optimization to not harden that |
| // base register increment/decrement when the increment/decrement is |
| // an immediate. |
| Modified |= makeGPRSpeculationSafe(MBB, NextMBBI, MI, Def.getReg()); |
| } |
| |
| if (HardenAddressLoadedFrom) |
| for (auto Use : MI.uses()) { |
| if (!Use.isReg()) |
| continue; |
| Register Reg = Use.getReg(); |
| // Some loads of floating point data have implicit defs/uses on a |
| // super register of that floating point data. Some examples: |
| // $s0 = LDRSui $sp, 22, implicit-def $q0 |
| // $q0 = LD1i64 $q0, 1, renamable $x0 |
| // We need to filter out these uses for non-GPR register which occur |
| // because the load partially fills a non-GPR register with the loaded |
| // data. Just skipping all non-GPR registers is safe (for now) as all |
| // AArch64 load instructions only use GPR registers to perform the |
| // address calculation. FIXME: However that might change once we can |
| // produce SVE gather instructions. |
| if (!(AArch64::GPR32allRegClass.contains(Reg) || |
| AArch64::GPR64allRegClass.contains(Reg))) |
| continue; |
| Modified |= makeGPRSpeculationSafe(MBB, MBBI, MI, Reg); |
| } |
| } |
| return Modified; |
| } |
| |
| /// \brief If MBBI references a pseudo instruction that should be expanded |
| /// here, do the expansion and return true. Otherwise return false. |
| bool AArch64SpeculationHardening::expandSpeculationSafeValue( |
| MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI, |
| bool UsesFullSpeculationBarrier) { |
| MachineInstr &MI = *MBBI; |
| unsigned Opcode = MI.getOpcode(); |
| bool Is64Bit = true; |
| |
| switch (Opcode) { |
| default: |
| break; |
| case AArch64::SpeculationSafeValueW: |
| Is64Bit = false; |
| LLVM_FALLTHROUGH; |
| case AArch64::SpeculationSafeValueX: |
| // Just remove the SpeculationSafe pseudo's if control flow |
| // miss-speculation isn't happening because we're already inserting barriers |
| // to guarantee that. |
| if (!UseControlFlowSpeculationBarrier && !UsesFullSpeculationBarrier) { |
| Register DstReg = MI.getOperand(0).getReg(); |
| Register SrcReg = MI.getOperand(1).getReg(); |
| // Mark this register and all its aliasing registers as needing to be |
| // value speculation hardened before its next use, by using a CSDB |
| // barrier instruction. |
| for (MachineOperand Op : MI.defs()) |
| for (MCRegAliasIterator AI(Op.getReg(), TRI, true); AI.isValid(); ++AI) |
| RegsNeedingCSDBBeforeUse.set(*AI); |
| |
| // Mask off with taint state. |
| BuildMI(MBB, MBBI, MI.getDebugLoc(), |
| Is64Bit ? TII->get(AArch64::ANDXrs) : TII->get(AArch64::ANDWrs)) |
| .addDef(DstReg) |
| .addUse(SrcReg, RegState::Kill) |
| .addUse(Is64Bit ? MisspeculatingTaintReg |
| : MisspeculatingTaintReg32Bit) |
| .addImm(0); |
| } |
| MI.eraseFromParent(); |
| return true; |
| } |
| return false; |
| } |
| |
| bool AArch64SpeculationHardening::insertCSDB(MachineBasicBlock &MBB, |
| MachineBasicBlock::iterator MBBI, |
| DebugLoc DL) { |
| assert(!UseControlFlowSpeculationBarrier && "No need to insert CSDBs when " |
| "control flow miss-speculation " |
| "is already blocked"); |
| // insert data value speculation barrier (CSDB) |
| BuildMI(MBB, MBBI, DL, TII->get(AArch64::HINT)).addImm(0x14); |
| RegsNeedingCSDBBeforeUse.reset(); |
| return true; |
| } |
| |
| bool AArch64SpeculationHardening::lowerSpeculationSafeValuePseudos( |
| MachineBasicBlock &MBB, bool UsesFullSpeculationBarrier) { |
| bool Modified = false; |
| |
| RegsNeedingCSDBBeforeUse.reset(); |
| |
| // The following loop iterates over all instructions in the basic block, |
| // and performs 2 operations: |
| // 1. Insert a CSDB at this location if needed. |
| // 2. Expand the SpeculationSafeValuePseudo if the current instruction is |
| // one. |
| // |
| // The insertion of the CSDB is done as late as possible (i.e. just before |
| // the use of a masked register), in the hope that that will reduce the |
| // total number of CSDBs in a block when there are multiple masked registers |
| // in the block. |
| MachineBasicBlock::iterator MBBI = MBB.begin(), E = MBB.end(); |
| DebugLoc DL; |
| while (MBBI != E) { |
| MachineInstr &MI = *MBBI; |
| DL = MI.getDebugLoc(); |
| MachineBasicBlock::iterator NMBBI = std::next(MBBI); |
| |
| // First check if a CSDB needs to be inserted due to earlier registers |
| // that were masked and that are used by the next instruction. |
| // Also emit the barrier on any potential control flow changes. |
| bool NeedToEmitBarrier = false; |
| if (RegsNeedingCSDBBeforeUse.any() && (MI.isCall() || MI.isTerminator())) |
| NeedToEmitBarrier = true; |
| if (!NeedToEmitBarrier) |
| for (MachineOperand Op : MI.uses()) |
| if (Op.isReg() && RegsNeedingCSDBBeforeUse[Op.getReg()]) { |
| NeedToEmitBarrier = true; |
| break; |
| } |
| |
| if (NeedToEmitBarrier && !UsesFullSpeculationBarrier) |
| Modified |= insertCSDB(MBB, MBBI, DL); |
| |
| Modified |= |
| expandSpeculationSafeValue(MBB, MBBI, UsesFullSpeculationBarrier); |
| |
| MBBI = NMBBI; |
| } |
| |
| if (RegsNeedingCSDBBeforeUse.any() && !UsesFullSpeculationBarrier) |
| Modified |= insertCSDB(MBB, MBBI, DL); |
| |
| return Modified; |
| } |
| |
| bool AArch64SpeculationHardening::runOnMachineFunction(MachineFunction &MF) { |
| if (!MF.getFunction().hasFnAttribute(Attribute::SpeculativeLoadHardening)) |
| return false; |
| |
| MisspeculatingTaintReg = AArch64::X16; |
| MisspeculatingTaintReg32Bit = AArch64::W16; |
| TII = MF.getSubtarget().getInstrInfo(); |
| TRI = MF.getSubtarget().getRegisterInfo(); |
| RegsNeedingCSDBBeforeUse.resize(TRI->getNumRegs()); |
| RegsAlreadyMasked.resize(TRI->getNumRegs()); |
| UseControlFlowSpeculationBarrier = functionUsesHardeningRegister(MF); |
| |
| bool Modified = false; |
| |
| // Step 1: Enable automatic insertion of SpeculationSafeValue. |
| if (HardenLoads) { |
| LLVM_DEBUG( |
| dbgs() << "***** AArch64SpeculationHardening - automatic insertion of " |
| "SpeculationSafeValue intrinsics *****\n"); |
| for (auto &MBB : MF) |
| Modified |= slhLoads(MBB); |
| } |
| |
| // 2. Add instrumentation code to function entry and exits. |
| LLVM_DEBUG( |
| dbgs() |
| << "***** AArch64SpeculationHardening - track control flow *****\n"); |
| |
| SmallVector<MachineBasicBlock *, 2> EntryBlocks; |
| EntryBlocks.push_back(&MF.front()); |
| for (const LandingPadInfo &LPI : MF.getLandingPads()) |
| EntryBlocks.push_back(LPI.LandingPadBlock); |
| for (auto Entry : EntryBlocks) |
| insertSPToRegTaintPropagation( |
| *Entry, Entry->SkipPHIsLabelsAndDebug(Entry->begin())); |
| |
| // 3. Add instrumentation code to every basic block. |
| for (auto &MBB : MF) { |
| bool UsesFullSpeculationBarrier = false; |
| Modified |= instrumentControlFlow(MBB, UsesFullSpeculationBarrier); |
| Modified |= |
| lowerSpeculationSafeValuePseudos(MBB, UsesFullSpeculationBarrier); |
| } |
| |
| return Modified; |
| } |
| |
| /// \brief Returns an instance of the pseudo instruction expansion pass. |
| FunctionPass *llvm::createAArch64SpeculationHardeningPass() { |
| return new AArch64SpeculationHardening(); |
| } |
