1//==-- X86LoadValueInjectionLoadHardening.cpp - LVI load hardening 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///
9/// Description: This pass finds Load Value Injection (LVI) gadgets consisting
10/// of a load from memory (i.e., SOURCE), and any operation that may transmit
11/// the value loaded from memory over a covert channel, or use the value loaded
12/// from memory to determine a branch/call target (i.e., SINK). After finding
13/// all such gadgets in a given function, the pass minimally inserts LFENCE
14/// instructions in such a manner that the following property is satisfied: for
15/// all SOURCE+SINK pairs, all paths in the CFG from SOURCE to SINK contain at
16/// least one LFENCE instruction. The algorithm that implements this minimal
17/// insertion is influenced by an academic paper that minimally inserts memory
18/// fences for high-performance concurrent programs:
19/// http://www.cs.ucr.edu/~lesani/companion/oopsla15/OOPSLA15.pdf
20/// The algorithm implemented in this pass is as follows:
21/// 1. Build a condensed CFG (i.e., a GadgetGraph) consisting only of the
22/// following components:
23/// - SOURCE instructions (also includes function arguments)
24/// - SINK instructions
25/// - Basic block entry points
26/// - Basic block terminators
27/// - LFENCE instructions
28/// 2. Analyze the GadgetGraph to determine which SOURCE+SINK pairs (i.e.,
29/// gadgets) are already mitigated by existing LFENCEs. If all gadgets have been
30/// mitigated, go to step 6.
31/// 3. Use a heuristic or plugin to approximate minimal LFENCE insertion.
32/// 4. Insert one LFENCE along each CFG edge that was cut in step 3.
33/// 5. Go to step 2.
34/// 6. If any LFENCEs were inserted, return `true` from runOnMachineFunction()
35/// to tell LLVM that the function was modified.
36///
37//===----------------------------------------------------------------------===//
38
39#include "ImmutableGraph.h"
40#include "X86.h"
41#include "X86Subtarget.h"
42#include "X86TargetMachine.h"
43#include "llvm/ADT/DenseMap.h"
44#include "llvm/ADT/STLExtras.h"
45#include "llvm/ADT/SmallSet.h"
46#include "llvm/ADT/Statistic.h"
47#include "llvm/ADT/StringRef.h"
48#include "llvm/CodeGen/MachineBasicBlock.h"
49#include "llvm/CodeGen/MachineDominanceFrontier.h"
50#include "llvm/CodeGen/MachineDominators.h"
51#include "llvm/CodeGen/MachineFunction.h"
52#include "llvm/CodeGen/MachineFunctionPass.h"
53#include "llvm/CodeGen/MachineInstr.h"
54#include "llvm/CodeGen/MachineInstrBuilder.h"
55#include "llvm/CodeGen/MachineLoopInfo.h"
56#include "llvm/CodeGen/RDFGraph.h"
57#include "llvm/CodeGen/RDFLiveness.h"
58#include "llvm/InitializePasses.h"
59#include "llvm/Support/CommandLine.h"
60#include "llvm/Support/DOTGraphTraits.h"
61#include "llvm/Support/Debug.h"
62#include "llvm/Support/DynamicLibrary.h"
63#include "llvm/Support/GraphWriter.h"
64#include "llvm/Support/raw_ostream.h"
65
66using namespace llvm;
67
68#define PASS_KEY "x86-lvi-load"
69#define DEBUG_TYPE PASS_KEY
70
71STATISTIC(NumFences, "Number of LFENCEs inserted for LVI mitigation");
72STATISTIC(NumFunctionsConsidered, "Number of functions analyzed");
73STATISTIC(NumFunctionsMitigated, "Number of functions for which mitigations "
74 "were deployed");
75STATISTIC(NumGadgets, "Number of LVI gadgets detected during analysis");
76
77static cl::opt<std::string> OptimizePluginPath(
78 PASS_KEY "-opt-plugin",
79 cl::desc("Specify a plugin to optimize LFENCE insertion"), cl::Hidden);
80
81static cl::opt<bool> NoConditionalBranches(
82 PASS_KEY "-no-cbranch",
83 cl::desc("Don't treat conditional branches as disclosure gadgets. This "
84 "may improve performance, at the cost of security."),
85 cl::init(Val: false), cl::Hidden);
86
87static cl::opt<bool> EmitDot(
88 PASS_KEY "-dot",
89 cl::desc(
90 "For each function, emit a dot graph depicting potential LVI gadgets"),
91 cl::init(Val: false), cl::Hidden);
92
93static cl::opt<bool> EmitDotOnly(
94 PASS_KEY "-dot-only",
95 cl::desc("For each function, emit a dot graph depicting potential LVI "
96 "gadgets, and do not insert any fences"),
97 cl::init(Val: false), cl::Hidden);
98
99static cl::opt<bool> EmitDotVerify(
100 PASS_KEY "-dot-verify",
101 cl::desc("For each function, emit a dot graph to stdout depicting "
102 "potential LVI gadgets, used for testing purposes only"),
103 cl::init(Val: false), cl::Hidden);
104
105static llvm::sys::DynamicLibrary OptimizeDL;
106typedef int (*OptimizeCutT)(unsigned int *Nodes, unsigned int NodesSize,
107 unsigned int *Edges, int *EdgeValues,
108 int *CutEdges /* out */, unsigned int EdgesSize);
109static OptimizeCutT OptimizeCut = nullptr;
110
111namespace {
112
113struct MachineGadgetGraph : ImmutableGraph<MachineInstr *, int> {
114 static constexpr int GadgetEdgeSentinel = -1;
115 static constexpr MachineInstr *const ArgNodeSentinel = nullptr;
116
117 using GraphT = ImmutableGraph<MachineInstr *, int>;
118 using Node = GraphT::Node;
119 using Edge = GraphT::Edge;
120 using size_type = GraphT::size_type;
121 MachineGadgetGraph(std::unique_ptr<Node[]> Nodes,
122 std::unique_ptr<Edge[]> Edges, size_type NodesSize,
123 size_type EdgesSize, int NumFences = 0, int NumGadgets = 0)
124 : GraphT(std::move(Nodes), std::move(Edges), NodesSize, EdgesSize),
125 NumFences(NumFences), NumGadgets(NumGadgets) {}
126 static inline bool isCFGEdge(const Edge &E) {
127 return E.getValue() != GadgetEdgeSentinel;
128 }
129 static inline bool isGadgetEdge(const Edge &E) {
130 return E.getValue() == GadgetEdgeSentinel;
131 }
132 int NumFences;
133 int NumGadgets;
134};
135
136class X86LoadValueInjectionLoadHardeningPass : public MachineFunctionPass {
137public:
138 X86LoadValueInjectionLoadHardeningPass() : MachineFunctionPass(ID) {}
139
140 StringRef getPassName() const override {
141 return "X86 Load Value Injection (LVI) Load Hardening";
142 }
143 void getAnalysisUsage(AnalysisUsage &AU) const override;
144 bool runOnMachineFunction(MachineFunction &MF) override;
145
146 static char ID;
147
148private:
149 using GraphBuilder = ImmutableGraphBuilder<MachineGadgetGraph>;
150 using Edge = MachineGadgetGraph::Edge;
151 using Node = MachineGadgetGraph::Node;
152 using EdgeSet = MachineGadgetGraph::EdgeSet;
153 using NodeSet = MachineGadgetGraph::NodeSet;
154
155 const X86Subtarget *STI = nullptr;
156 const TargetInstrInfo *TII = nullptr;
157 const TargetRegisterInfo *TRI = nullptr;
158
159 std::unique_ptr<MachineGadgetGraph>
160 getGadgetGraph(MachineFunction &MF, const MachineLoopInfo &MLI,
161 const MachineDominatorTree &MDT,
162 const MachineDominanceFrontier &MDF) const;
163 int hardenLoadsWithPlugin(MachineFunction &MF,
164 std::unique_ptr<MachineGadgetGraph> Graph) const;
165 int hardenLoadsWithHeuristic(MachineFunction &MF,
166 std::unique_ptr<MachineGadgetGraph> Graph) const;
167 int elimMitigatedEdgesAndNodes(MachineGadgetGraph &G,
168 EdgeSet &ElimEdges /* in, out */,
169 NodeSet &ElimNodes /* in, out */) const;
170 std::unique_ptr<MachineGadgetGraph>
171 trimMitigatedEdges(std::unique_ptr<MachineGadgetGraph> Graph) const;
172 int insertFences(MachineFunction &MF, MachineGadgetGraph &G,
173 EdgeSet &CutEdges /* in, out */) const;
174 bool instrUsesRegToAccessMemory(const MachineInstr &I, Register Reg) const;
175 bool instrUsesRegToBranch(const MachineInstr &I, Register Reg) const;
176 inline bool isFence(const MachineInstr *MI) const {
177 return MI && (MI->getOpcode() == X86::LFENCE ||
178 (STI->useLVIControlFlowIntegrity() && MI->isCall()));
179 }
180};
181
182} // end anonymous namespace
183
184namespace llvm {
185
186template <>
187struct GraphTraits<MachineGadgetGraph *>
188 : GraphTraits<ImmutableGraph<MachineInstr *, int> *> {};
189
190template <>
191struct DOTGraphTraits<MachineGadgetGraph *> : DefaultDOTGraphTraits {
192 using GraphType = MachineGadgetGraph;
193 using Traits = llvm::GraphTraits<GraphType *>;
194 using NodeRef = Traits::NodeRef;
195 using EdgeRef = Traits::EdgeRef;
196 using ChildIteratorType = Traits::ChildIteratorType;
197 using ChildEdgeIteratorType = Traits::ChildEdgeIteratorType;
198
199 DOTGraphTraits(bool IsSimple = false) : DefaultDOTGraphTraits(IsSimple) {}
200
201 std::string getNodeLabel(NodeRef Node, GraphType *) {
202 if (Node->getValue() == MachineGadgetGraph::ArgNodeSentinel)
203 return "ARGS";
204
205 std::string Str;
206 raw_string_ostream OS(Str);
207 OS << *Node->getValue();
208 return OS.str();
209 }
210
211 static std::string getNodeAttributes(NodeRef Node, GraphType *) {
212 MachineInstr *MI = Node->getValue();
213 if (MI == MachineGadgetGraph::ArgNodeSentinel)
214 return "color = blue";
215 if (MI->getOpcode() == X86::LFENCE)
216 return "color = green";
217 return "";
218 }
219
220 static std::string getEdgeAttributes(NodeRef, ChildIteratorType E,
221 GraphType *) {
222 int EdgeVal = (*E.getCurrent()).getValue();
223 return EdgeVal >= 0 ? "label = " + std::to_string(val: EdgeVal)
224 : "color = red, style = \"dashed\"";
225 }
226};
227
228} // end namespace llvm
229
230char X86LoadValueInjectionLoadHardeningPass::ID = 0;
231
232void X86LoadValueInjectionLoadHardeningPass::getAnalysisUsage(
233 AnalysisUsage &AU) const {
234 MachineFunctionPass::getAnalysisUsage(AU);
235 AU.addRequired<MachineLoopInfoWrapperPass>();
236 AU.addRequired<MachineDominatorTreeWrapperPass>();
237 AU.addRequired<MachineDominanceFrontierWrapperPass>();
238 AU.setPreservesCFG();
239}
240
241static void writeGadgetGraph(raw_ostream &OS, MachineFunction &MF,
242 MachineGadgetGraph *G) {
243 WriteGraph(O&: OS, G, /*ShortNames*/ false,
244 Title: "Speculative gadgets for \"" + MF.getName() + "\" function");
245}
246
247bool X86LoadValueInjectionLoadHardeningPass::runOnMachineFunction(
248 MachineFunction &MF) {
249 LLVM_DEBUG(dbgs() << "***** " << getPassName() << " : " << MF.getName()
250 << " *****\n");
251 STI = &MF.getSubtarget<X86Subtarget>();
252 if (!STI->useLVILoadHardening())
253 return false;
254
255 // FIXME: support 32-bit
256 if (!STI->is64Bit())
257 report_fatal_error(reason: "LVI load hardening is only supported on 64-bit", gen_crash_diag: false);
258
259 // Don't skip functions with the "optnone" attr but participate in opt-bisect.
260 const Function &F = MF.getFunction();
261 if (!F.hasOptNone() && skipFunction(F))
262 return false;
263
264 ++NumFunctionsConsidered;
265 TII = STI->getInstrInfo();
266 TRI = STI->getRegisterInfo();
267 LLVM_DEBUG(dbgs() << "Building gadget graph...\n");
268 const auto &MLI = getAnalysis<MachineLoopInfoWrapperPass>().getLI();
269 const auto &MDT = getAnalysis<MachineDominatorTreeWrapperPass>().getDomTree();
270 const auto &MDF = getAnalysis<MachineDominanceFrontierWrapperPass>().getMDF();
271 std::unique_ptr<MachineGadgetGraph> Graph = getGadgetGraph(MF, MLI, MDT, MDF);
272 LLVM_DEBUG(dbgs() << "Building gadget graph... Done\n");
273 if (Graph == nullptr)
274 return false; // didn't find any gadgets
275
276 if (EmitDotVerify) {
277 writeGadgetGraph(OS&: outs(), MF, G: Graph.get());
278 return false;
279 }
280
281 if (EmitDot || EmitDotOnly) {
282 LLVM_DEBUG(dbgs() << "Emitting gadget graph...\n");
283 std::error_code FileError;
284 std::string FileName = "lvi.";
285 FileName += MF.getName();
286 FileName += ".dot";
287 raw_fd_ostream FileOut(FileName, FileError);
288 if (FileError)
289 errs() << FileError.message();
290 writeGadgetGraph(OS&: FileOut, MF, G: Graph.get());
291 FileOut.close();
292 LLVM_DEBUG(dbgs() << "Emitting gadget graph... Done\n");
293 if (EmitDotOnly)
294 return false;
295 }
296
297 int FencesInserted;
298 if (!OptimizePluginPath.empty()) {
299 if (!OptimizeDL.isValid()) {
300 std::string ErrorMsg;
301 OptimizeDL = llvm::sys::DynamicLibrary::getPermanentLibrary(
302 filename: OptimizePluginPath.c_str(), errMsg: &ErrorMsg);
303 if (!ErrorMsg.empty())
304 report_fatal_error(reason: Twine("Failed to load opt plugin: \"") + ErrorMsg +
305 "\"");
306 OptimizeCut = (OptimizeCutT)OptimizeDL.getAddressOfSymbol(symbolName: "optimize_cut");
307 if (!OptimizeCut)
308 report_fatal_error(reason: "Invalid optimization plugin");
309 }
310 FencesInserted = hardenLoadsWithPlugin(MF, Graph: std::move(Graph));
311 } else { // Use the default greedy heuristic
312 FencesInserted = hardenLoadsWithHeuristic(MF, Graph: std::move(Graph));
313 }
314
315 if (FencesInserted > 0)
316 ++NumFunctionsMitigated;
317 NumFences += FencesInserted;
318 return (FencesInserted > 0);
319}
320
321std::unique_ptr<MachineGadgetGraph>
322X86LoadValueInjectionLoadHardeningPass::getGadgetGraph(
323 MachineFunction &MF, const MachineLoopInfo &MLI,
324 const MachineDominatorTree &MDT,
325 const MachineDominanceFrontier &MDF) const {
326 using namespace rdf;
327
328 // Build the Register Dataflow Graph using the RDF framework
329 DataFlowGraph DFG{MF, *TII, *TRI, MDT, MDF};
330 DFG.build();
331 Liveness L{MF.getRegInfo(), DFG};
332 L.computePhiInfo();
333
334 GraphBuilder Builder;
335 using GraphIter = GraphBuilder::BuilderNodeRef;
336 DenseMap<MachineInstr *, GraphIter> NodeMap;
337 int FenceCount = 0, GadgetCount = 0;
338 auto MaybeAddNode = [&NodeMap, &Builder](MachineInstr *MI) {
339 auto [Ref, Inserted] = NodeMap.try_emplace(Key: MI);
340 if (Inserted) {
341 auto I = Builder.addVertex(V: MI);
342 Ref->second = I;
343 return std::pair<GraphIter, bool>{I, true};
344 }
345 return std::pair<GraphIter, bool>{Ref->getSecond(), false};
346 };
347
348 // The `Transmitters` map memoizes transmitters found for each def. If a def
349 // has not yet been analyzed, then it will not appear in the map. If a def
350 // has been analyzed and was determined not to have any transmitters, then
351 // its list of transmitters will be empty.
352 DenseMap<NodeId, std::vector<NodeId>> Transmitters;
353
354 // Analyze all machine instructions to find gadgets and LFENCEs, adding
355 // each interesting value to `Nodes`
356 auto AnalyzeDef = [&](NodeAddr<DefNode *> SourceDef) {
357 SmallSet<NodeId, 8> UsesVisited, DefsVisited;
358 std::function<void(NodeAddr<DefNode *>)> AnalyzeDefUseChain =
359 [&](NodeAddr<DefNode *> Def) {
360 if (Transmitters.contains(Val: Def.Id))
361 return; // Already analyzed `Def`
362
363 // Use RDF to find all the uses of `Def`
364 rdf::NodeSet Uses;
365 RegisterRef DefReg = Def.Addr->getRegRef(G: DFG);
366 for (auto UseID : L.getAllReachedUses(RefRR: DefReg, DefA: Def)) {
367 auto Use = DFG.addr<UseNode *>(N: UseID);
368 if (Use.Addr->getFlags() & NodeAttrs::PhiRef) { // phi node
369 NodeAddr<PhiNode *> Phi = Use.Addr->getOwner(G: DFG);
370 for (const auto& I : L.getRealUses(P: Phi.Id)) {
371 if (DFG.getPRI().alias(RA: RegisterRef(I.first), RB: DefReg)) {
372 for (const auto &UA : I.second)
373 Uses.emplace(args: UA.first);
374 }
375 }
376 } else { // not a phi node
377 Uses.emplace(args&: UseID);
378 }
379 }
380
381 // For each use of `Def`, we want to know whether:
382 // (1) The use can leak the Def'ed value,
383 // (2) The use can further propagate the Def'ed value to more defs
384 for (auto UseID : Uses) {
385 if (!UsesVisited.insert(V: UseID).second)
386 continue; // Already visited this use of `Def`
387
388 auto Use = DFG.addr<UseNode *>(N: UseID);
389 assert(!(Use.Addr->getFlags() & NodeAttrs::PhiRef));
390 MachineOperand &UseMO = Use.Addr->getOp();
391 MachineInstr &UseMI = *UseMO.getParent();
392 assert(UseMO.isReg());
393
394 // We naively assume that an instruction propagates any loaded
395 // uses to all defs unless the instruction is a call, in which
396 // case all arguments will be treated as gadget sources during
397 // analysis of the callee function.
398 if (UseMI.isCall())
399 continue;
400
401 // Check whether this use can transmit (leak) its value.
402 if (instrUsesRegToAccessMemory(I: UseMI, Reg: UseMO.getReg()) ||
403 (!NoConditionalBranches &&
404 instrUsesRegToBranch(I: UseMI, Reg: UseMO.getReg()))) {
405 Transmitters[Def.Id].push_back(x: Use.Addr->getOwner(G: DFG).Id);
406 if (UseMI.mayLoad())
407 continue; // Found a transmitting load -- no need to continue
408 // traversing its defs (i.e., this load will become
409 // a new gadget source anyways).
410 }
411
412 // Check whether the use propagates to more defs.
413 NodeAddr<InstrNode *> Owner{Use.Addr->getOwner(G: DFG)};
414 for (const auto &ChildDef :
415 Owner.Addr->members_if(P: DataFlowGraph::IsDef, G: DFG)) {
416 if (!DefsVisited.insert(V: ChildDef.Id).second)
417 continue; // Already visited this def
418 if (Def.Addr->getAttrs() & NodeAttrs::Dead)
419 continue;
420 if (Def.Id == ChildDef.Id)
421 continue; // `Def` uses itself (e.g., increment loop counter)
422
423 AnalyzeDefUseChain(ChildDef);
424
425 // `Def` inherits all of its child defs' transmitters.
426 for (auto TransmitterId : Transmitters[ChildDef.Id])
427 Transmitters[Def.Id].push_back(x: TransmitterId);
428 }
429 }
430
431 // Note that this statement adds `Def.Id` to the map if no
432 // transmitters were found for `Def`.
433 auto &DefTransmitters = Transmitters[Def.Id];
434
435 // Remove duplicate transmitters
436 llvm::sort(C&: DefTransmitters);
437 DefTransmitters.erase(first: llvm::unique(R&: DefTransmitters),
438 last: DefTransmitters.end());
439 };
440
441 // Find all of the transmitters
442 AnalyzeDefUseChain(SourceDef);
443 auto &SourceDefTransmitters = Transmitters[SourceDef.Id];
444 if (SourceDefTransmitters.empty())
445 return; // No transmitters for `SourceDef`
446
447 MachineInstr *Source = SourceDef.Addr->getFlags() & NodeAttrs::PhiRef
448 ? MachineGadgetGraph::ArgNodeSentinel
449 : SourceDef.Addr->getOp().getParent();
450 auto GadgetSource = MaybeAddNode(Source);
451 // Each transmitter is a sink for `SourceDef`.
452 for (auto TransmitterId : SourceDefTransmitters) {
453 MachineInstr *Sink = DFG.addr<StmtNode *>(N: TransmitterId).Addr->getCode();
454 auto GadgetSink = MaybeAddNode(Sink);
455 // Add the gadget edge to the graph.
456 Builder.addEdge(E: MachineGadgetGraph::GadgetEdgeSentinel,
457 From: GadgetSource.first, To: GadgetSink.first);
458 ++GadgetCount;
459 }
460 };
461
462 LLVM_DEBUG(dbgs() << "Analyzing def-use chains to find gadgets\n");
463 // Analyze function arguments
464 NodeAddr<BlockNode *> EntryBlock = DFG.getFunc().Addr->getEntryBlock(G: DFG);
465 for (NodeAddr<PhiNode *> ArgPhi :
466 EntryBlock.Addr->members_if(P: DataFlowGraph::IsPhi, G: DFG)) {
467 NodeList Defs = ArgPhi.Addr->members_if(P: DataFlowGraph::IsDef, G: DFG);
468 llvm::for_each(Range&: Defs, F: AnalyzeDef);
469 }
470 // Analyze every instruction in MF
471 for (NodeAddr<BlockNode *> BA : DFG.getFunc().Addr->members(G: DFG)) {
472 for (NodeAddr<StmtNode *> SA :
473 BA.Addr->members_if(P: DataFlowGraph::IsCode<NodeAttrs::Stmt>, G: DFG)) {
474 MachineInstr *MI = SA.Addr->getCode();
475 if (isFence(MI)) {
476 MaybeAddNode(MI);
477 ++FenceCount;
478 } else if (MI->mayLoad()) {
479 NodeList Defs = SA.Addr->members_if(P: DataFlowGraph::IsDef, G: DFG);
480 llvm::for_each(Range&: Defs, F: AnalyzeDef);
481 }
482 }
483 }
484 LLVM_DEBUG(dbgs() << "Found " << FenceCount << " fences\n");
485 LLVM_DEBUG(dbgs() << "Found " << GadgetCount << " gadgets\n");
486 if (GadgetCount == 0)
487 return nullptr;
488 NumGadgets += GadgetCount;
489
490 // Traverse CFG to build the rest of the graph
491 SmallPtrSet<MachineBasicBlock *, 8> BlocksVisited;
492 std::function<void(MachineBasicBlock *, GraphIter, unsigned)> TraverseCFG =
493 [&](MachineBasicBlock *MBB, GraphIter GI, unsigned ParentDepth) {
494 unsigned LoopDepth = MLI.getLoopDepth(BB: MBB);
495 if (!MBB->empty()) {
496 // Always add the first instruction in each block
497 auto NI = MBB->begin();
498 auto BeginBB = MaybeAddNode(&*NI);
499 Builder.addEdge(E: ParentDepth, From: GI, To: BeginBB.first);
500 if (!BlocksVisited.insert(Ptr: MBB).second)
501 return;
502
503 // Add any instructions within the block that are gadget components
504 GI = BeginBB.first;
505 while (++NI != MBB->end()) {
506 auto Ref = NodeMap.find(Val: &*NI);
507 if (Ref != NodeMap.end()) {
508 Builder.addEdge(E: LoopDepth, From: GI, To: Ref->getSecond());
509 GI = Ref->getSecond();
510 }
511 }
512
513 // Always add the terminator instruction, if one exists
514 auto T = MBB->getFirstTerminator();
515 if (T != MBB->end()) {
516 auto EndBB = MaybeAddNode(&*T);
517 if (EndBB.second)
518 Builder.addEdge(E: LoopDepth, From: GI, To: EndBB.first);
519 GI = EndBB.first;
520 }
521 }
522 for (MachineBasicBlock *Succ : MBB->successors())
523 TraverseCFG(Succ, GI, LoopDepth);
524 };
525 // ArgNodeSentinel is a pseudo-instruction that represents MF args in the
526 // GadgetGraph
527 GraphIter ArgNode = MaybeAddNode(MachineGadgetGraph::ArgNodeSentinel).first;
528 TraverseCFG(&MF.front(), ArgNode, 0);
529 std::unique_ptr<MachineGadgetGraph> G{Builder.get(Args&: FenceCount, Args&: GadgetCount)};
530 LLVM_DEBUG(dbgs() << "Found " << G->nodes_size() << " nodes\n");
531 return G;
532}
533
534// Returns the number of remaining gadget edges that could not be eliminated
535int X86LoadValueInjectionLoadHardeningPass::elimMitigatedEdgesAndNodes(
536 MachineGadgetGraph &G, EdgeSet &ElimEdges /* in, out */,
537 NodeSet &ElimNodes /* in, out */) const {
538 if (G.NumFences > 0) {
539 // Eliminate fences and CFG edges that ingress and egress the fence, as
540 // they are trivially mitigated.
541 for (const Edge &E : G.edges()) {
542 const Node *Dest = E.getDest();
543 if (isFence(MI: Dest->getValue())) {
544 ElimNodes.insert(N: *Dest);
545 ElimEdges.insert(E);
546 for (const Edge &DE : Dest->edges())
547 ElimEdges.insert(E: DE);
548 }
549 }
550 }
551
552 // Find and eliminate gadget edges that have been mitigated.
553 int RemainingGadgets = 0;
554 NodeSet ReachableNodes{G};
555 for (const Node &RootN : G.nodes()) {
556 if (llvm::none_of(Range: RootN.edges(), P: MachineGadgetGraph::isGadgetEdge))
557 continue; // skip this node if it isn't a gadget source
558
559 // Find all of the nodes that are CFG-reachable from RootN using DFS
560 ReachableNodes.clear();
561 std::function<void(const Node *, bool)> FindReachableNodes =
562 [&](const Node *N, bool FirstNode) {
563 if (!FirstNode)
564 ReachableNodes.insert(N: *N);
565 for (const Edge &E : N->edges()) {
566 const Node *Dest = E.getDest();
567 if (MachineGadgetGraph::isCFGEdge(E) && !ElimEdges.contains(E) &&
568 !ReachableNodes.contains(N: *Dest))
569 FindReachableNodes(Dest, false);
570 }
571 };
572 FindReachableNodes(&RootN, true);
573
574 // Any gadget whose sink is unreachable has been mitigated
575 for (const Edge &E : RootN.edges()) {
576 if (MachineGadgetGraph::isGadgetEdge(E)) {
577 if (ReachableNodes.contains(N: *E.getDest())) {
578 // This gadget's sink is reachable
579 ++RemainingGadgets;
580 } else { // This gadget's sink is unreachable, and therefore mitigated
581 ElimEdges.insert(E);
582 }
583 }
584 }
585 }
586 return RemainingGadgets;
587}
588
589std::unique_ptr<MachineGadgetGraph>
590X86LoadValueInjectionLoadHardeningPass::trimMitigatedEdges(
591 std::unique_ptr<MachineGadgetGraph> Graph) const {
592 NodeSet ElimNodes{*Graph};
593 EdgeSet ElimEdges{*Graph};
594 int RemainingGadgets =
595 elimMitigatedEdgesAndNodes(G&: *Graph, ElimEdges, ElimNodes);
596 if (ElimEdges.empty() && ElimNodes.empty()) {
597 Graph->NumFences = 0;
598 Graph->NumGadgets = RemainingGadgets;
599 } else {
600 Graph = GraphBuilder::trim(G: *Graph, TrimNodes: ElimNodes, TrimEdges: ElimEdges, Args: 0 /* NumFences */,
601 Args&: RemainingGadgets);
602 }
603 return Graph;
604}
605
606int X86LoadValueInjectionLoadHardeningPass::hardenLoadsWithPlugin(
607 MachineFunction &MF, std::unique_ptr<MachineGadgetGraph> Graph) const {
608 int FencesInserted = 0;
609
610 do {
611 LLVM_DEBUG(dbgs() << "Eliminating mitigated paths...\n");
612 Graph = trimMitigatedEdges(Graph: std::move(Graph));
613 LLVM_DEBUG(dbgs() << "Eliminating mitigated paths... Done\n");
614 if (Graph->NumGadgets == 0)
615 break;
616
617 LLVM_DEBUG(dbgs() << "Cutting edges...\n");
618 EdgeSet CutEdges{*Graph};
619 auto Nodes = std::make_unique<unsigned int[]>(num: Graph->nodes_size() +
620 1 /* terminator node */);
621 auto Edges = std::make_unique<unsigned int[]>(num: Graph->edges_size());
622 auto EdgeCuts = std::make_unique<int[]>(num: Graph->edges_size());
623 auto EdgeValues = std::make_unique<int[]>(num: Graph->edges_size());
624 for (const Node &N : Graph->nodes()) {
625 Nodes[Graph->getNodeIndex(N)] = Graph->getEdgeIndex(E: *N.edges_begin());
626 }
627 Nodes[Graph->nodes_size()] = Graph->edges_size(); // terminator node
628 for (const Edge &E : Graph->edges()) {
629 Edges[Graph->getEdgeIndex(E)] = Graph->getNodeIndex(N: *E.getDest());
630 EdgeValues[Graph->getEdgeIndex(E)] = E.getValue();
631 }
632 OptimizeCut(Nodes.get(), Graph->nodes_size(), Edges.get(), EdgeValues.get(),
633 EdgeCuts.get(), Graph->edges_size());
634 for (int I = 0; I < Graph->edges_size(); ++I)
635 if (EdgeCuts[I])
636 CutEdges.set(I);
637 LLVM_DEBUG(dbgs() << "Cutting edges... Done\n");
638 LLVM_DEBUG(dbgs() << "Cut " << CutEdges.count() << " edges\n");
639
640 LLVM_DEBUG(dbgs() << "Inserting LFENCEs...\n");
641 FencesInserted += insertFences(MF, G&: *Graph, CutEdges);
642 LLVM_DEBUG(dbgs() << "Inserting LFENCEs... Done\n");
643 LLVM_DEBUG(dbgs() << "Inserted " << FencesInserted << " fences\n");
644
645 Graph = GraphBuilder::trim(G: *Graph, TrimNodes: NodeSet{*Graph}, TrimEdges: CutEdges);
646 } while (true);
647
648 return FencesInserted;
649}
650
651int X86LoadValueInjectionLoadHardeningPass::hardenLoadsWithHeuristic(
652 MachineFunction &MF, std::unique_ptr<MachineGadgetGraph> Graph) const {
653 // If `MF` does not have any fences, then no gadgets would have been
654 // mitigated at this point.
655 if (Graph->NumFences > 0) {
656 LLVM_DEBUG(dbgs() << "Eliminating mitigated paths...\n");
657 Graph = trimMitigatedEdges(Graph: std::move(Graph));
658 LLVM_DEBUG(dbgs() << "Eliminating mitigated paths... Done\n");
659 }
660
661 if (Graph->NumGadgets == 0)
662 return 0;
663
664 LLVM_DEBUG(dbgs() << "Cutting edges...\n");
665 EdgeSet CutEdges{*Graph};
666
667 // Begin by collecting all ingress CFG edges for each node
668 DenseMap<const Node *, SmallVector<const Edge *, 2>> IngressEdgeMap;
669 for (const Edge &E : Graph->edges())
670 if (MachineGadgetGraph::isCFGEdge(E))
671 IngressEdgeMap[E.getDest()].push_back(Elt: &E);
672
673 // For each gadget edge, make cuts that guarantee the gadget will be
674 // mitigated. A computationally efficient way to achieve this is to either:
675 // (a) cut all egress CFG edges from the gadget source, or
676 // (b) cut all ingress CFG edges to the gadget sink.
677 //
678 // Moreover, the algorithm tries not to make a cut into a loop by preferring
679 // to make a (b)-type cut if the gadget source resides at a greater loop depth
680 // than the gadget sink, or an (a)-type cut otherwise.
681 for (const Node &N : Graph->nodes()) {
682 for (const Edge &E : N.edges()) {
683 if (!MachineGadgetGraph::isGadgetEdge(E))
684 continue;
685
686 SmallVector<const Edge *, 2> EgressEdges;
687 SmallVector<const Edge *, 2> &IngressEdges = IngressEdgeMap[E.getDest()];
688 for (const Edge &EgressEdge : N.edges())
689 if (MachineGadgetGraph::isCFGEdge(E: EgressEdge))
690 EgressEdges.push_back(Elt: &EgressEdge);
691
692 int EgressCutCost = 0, IngressCutCost = 0;
693 for (const Edge *EgressEdge : EgressEdges)
694 if (!CutEdges.contains(E: *EgressEdge))
695 EgressCutCost += EgressEdge->getValue();
696 for (const Edge *IngressEdge : IngressEdges)
697 if (!CutEdges.contains(E: *IngressEdge))
698 IngressCutCost += IngressEdge->getValue();
699
700 auto &EdgesToCut =
701 IngressCutCost < EgressCutCost ? IngressEdges : EgressEdges;
702 for (const Edge *E : EdgesToCut)
703 CutEdges.insert(E: *E);
704 }
705 }
706 LLVM_DEBUG(dbgs() << "Cutting edges... Done\n");
707 LLVM_DEBUG(dbgs() << "Cut " << CutEdges.count() << " edges\n");
708
709 LLVM_DEBUG(dbgs() << "Inserting LFENCEs...\n");
710 int FencesInserted = insertFences(MF, G&: *Graph, CutEdges);
711 LLVM_DEBUG(dbgs() << "Inserting LFENCEs... Done\n");
712 LLVM_DEBUG(dbgs() << "Inserted " << FencesInserted << " fences\n");
713
714 return FencesInserted;
715}
716
717int X86LoadValueInjectionLoadHardeningPass::insertFences(
718 MachineFunction &MF, MachineGadgetGraph &G,
719 EdgeSet &CutEdges /* in, out */) const {
720 int FencesInserted = 0;
721 for (const Node &N : G.nodes()) {
722 for (const Edge &E : N.edges()) {
723 if (CutEdges.contains(E)) {
724 MachineInstr *MI = N.getValue(), *Prev;
725 MachineBasicBlock *MBB; // Insert an LFENCE in this MBB
726 MachineBasicBlock::iterator InsertionPt; // ...at this point
727 if (MI == MachineGadgetGraph::ArgNodeSentinel) {
728 // insert LFENCE at beginning of entry block
729 MBB = &MF.front();
730 InsertionPt = MBB->begin();
731 Prev = nullptr;
732 } else if (MI->isBranch()) { // insert the LFENCE before the branch
733 MBB = MI->getParent();
734 InsertionPt = MI;
735 Prev = MI->getPrevNode();
736 // Remove all egress CFG edges from this branch because the inserted
737 // LFENCE prevents gadgets from crossing the branch.
738 for (const Edge &E : N.edges()) {
739 if (MachineGadgetGraph::isCFGEdge(E))
740 CutEdges.insert(E);
741 }
742 } else { // insert the LFENCE after the instruction
743 MBB = MI->getParent();
744 InsertionPt = MI->getNextNode() ? MI->getNextNode() : MBB->end();
745 Prev = InsertionPt == MBB->end()
746 ? (MBB->empty() ? nullptr : &MBB->back())
747 : InsertionPt->getPrevNode();
748 }
749 // Ensure this insertion is not redundant (two LFENCEs in sequence).
750 if ((InsertionPt == MBB->end() || !isFence(MI: &*InsertionPt)) &&
751 (!Prev || !isFence(MI: Prev))) {
752 BuildMI(BB&: *MBB, I: InsertionPt, MIMD: DebugLoc(), MCID: TII->get(Opcode: X86::LFENCE));
753 ++FencesInserted;
754 }
755 }
756 }
757 }
758 return FencesInserted;
759}
760
761bool X86LoadValueInjectionLoadHardeningPass::instrUsesRegToAccessMemory(
762 const MachineInstr &MI, Register Reg) const {
763 if (!MI.mayLoadOrStore() || MI.getOpcode() == X86::MFENCE ||
764 MI.getOpcode() == X86::SFENCE || MI.getOpcode() == X86::LFENCE)
765 return false;
766
767 const int MemRefBeginIdx = X86::getFirstAddrOperandIdx(MI);
768 if (MemRefBeginIdx < 0) {
769 LLVM_DEBUG(dbgs() << "Warning: unable to obtain memory operand for loading "
770 "instruction:\n";
771 MI.print(dbgs()); dbgs() << '\n';);
772 return false;
773 }
774
775 const MachineOperand &BaseMO =
776 MI.getOperand(i: MemRefBeginIdx + X86::AddrBaseReg);
777 const MachineOperand &IndexMO =
778 MI.getOperand(i: MemRefBeginIdx + X86::AddrIndexReg);
779 return (BaseMO.isReg() && BaseMO.getReg().isValid() &&
780 TRI->regsOverlap(RegA: BaseMO.getReg(), RegB: Reg)) ||
781 (IndexMO.isReg() && IndexMO.getReg().isValid() &&
782 TRI->regsOverlap(RegA: IndexMO.getReg(), RegB: Reg));
783}
784
785bool X86LoadValueInjectionLoadHardeningPass::instrUsesRegToBranch(
786 const MachineInstr &MI, Register Reg) const {
787 if (!MI.isConditionalBranch())
788 return false;
789 for (const MachineOperand &Use : MI.uses())
790 if (Use.isReg() && Use.getReg() == Reg)
791 return true;
792 return false;
793}
794
795INITIALIZE_PASS_BEGIN(X86LoadValueInjectionLoadHardeningPass, PASS_KEY,
796 "X86 LVI load hardening", false, false)
797INITIALIZE_PASS_DEPENDENCY(MachineLoopInfoWrapperPass)
798INITIALIZE_PASS_DEPENDENCY(MachineDominatorTreeWrapperPass)
799INITIALIZE_PASS_DEPENDENCY(MachineDominanceFrontierWrapperPass)
800INITIALIZE_PASS_END(X86LoadValueInjectionLoadHardeningPass, PASS_KEY,
801 "X86 LVI load hardening", false, false)
802
803FunctionPass *llvm::createX86LoadValueInjectionLoadHardeningPass() {
804 return new X86LoadValueInjectionLoadHardeningPass();
805}
806