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//===- 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();
}