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 = typename GraphT::Node;
119 using Edge = typename GraphT::Edge;
120 using size_type = typename 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 = typename Traits::NodeRef;
195 using EdgeRef = typename Traits::EdgeRef;
196 using ChildIteratorType = typename Traits::ChildIteratorType;
197 using ChildEdgeIteratorType = typename 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
230constexpr MachineInstr *MachineGadgetGraph::ArgNodeSentinel;
231constexpr int MachineGadgetGraph::GadgetEdgeSentinel;
232
233char X86LoadValueInjectionLoadHardeningPass::ID = 0;
234
235void X86LoadValueInjectionLoadHardeningPass::getAnalysisUsage(
236 AnalysisUsage &AU) const {
237 MachineFunctionPass::getAnalysisUsage(AU);
238 AU.addRequired<MachineLoopInfoWrapperPass>();
239 AU.addRequired<MachineDominatorTreeWrapperPass>();
240 AU.addRequired<MachineDominanceFrontier>();
241 AU.setPreservesCFG();
242}
243
244static void writeGadgetGraph(raw_ostream &OS, MachineFunction &MF,
245 MachineGadgetGraph *G) {
246 WriteGraph(O&: OS, G, /*ShortNames*/ false,
247 Title: "Speculative gadgets for \"" + MF.getName() + "\" function");
248}
249
250bool X86LoadValueInjectionLoadHardeningPass::runOnMachineFunction(
251 MachineFunction &MF) {
252 LLVM_DEBUG(dbgs() << "***** " << getPassName() << " : " << MF.getName()
253 << " *****\n");
254 STI = &MF.getSubtarget<X86Subtarget>();
255 if (!STI->useLVILoadHardening())
256 return false;
257
258 // FIXME: support 32-bit
259 if (!STI->is64Bit())
260 report_fatal_error(reason: "LVI load hardening is only supported on 64-bit", gen_crash_diag: false);
261
262 // Don't skip functions with the "optnone" attr but participate in opt-bisect.
263 const Function &F = MF.getFunction();
264 if (!F.hasOptNone() && skipFunction(F))
265 return false;
266
267 ++NumFunctionsConsidered;
268 TII = STI->getInstrInfo();
269 TRI = STI->getRegisterInfo();
270 LLVM_DEBUG(dbgs() << "Building gadget graph...\n");
271 const auto &MLI = getAnalysis<MachineLoopInfoWrapperPass>().getLI();
272 const auto &MDT = getAnalysis<MachineDominatorTreeWrapperPass>().getDomTree();
273 const auto &MDF = getAnalysis<MachineDominanceFrontier>();
274 std::unique_ptr<MachineGadgetGraph> Graph = getGadgetGraph(MF, MLI, MDT, MDF);
275 LLVM_DEBUG(dbgs() << "Building gadget graph... Done\n");
276 if (Graph == nullptr)
277 return false; // didn't find any gadgets
278
279 if (EmitDotVerify) {
280 writeGadgetGraph(OS&: outs(), MF, G: Graph.get());
281 return false;
282 }
283
284 if (EmitDot || EmitDotOnly) {
285 LLVM_DEBUG(dbgs() << "Emitting gadget graph...\n");
286 std::error_code FileError;
287 std::string FileName = "lvi.";
288 FileName += MF.getName();
289 FileName += ".dot";
290 raw_fd_ostream FileOut(FileName, FileError);
291 if (FileError)
292 errs() << FileError.message();
293 writeGadgetGraph(OS&: FileOut, MF, G: Graph.get());
294 FileOut.close();
295 LLVM_DEBUG(dbgs() << "Emitting gadget graph... Done\n");
296 if (EmitDotOnly)
297 return false;
298 }
299
300 int FencesInserted;
301 if (!OptimizePluginPath.empty()) {
302 if (!OptimizeDL.isValid()) {
303 std::string ErrorMsg;
304 OptimizeDL = llvm::sys::DynamicLibrary::getPermanentLibrary(
305 filename: OptimizePluginPath.c_str(), errMsg: &ErrorMsg);
306 if (!ErrorMsg.empty())
307 report_fatal_error(reason: Twine("Failed to load opt plugin: \"") + ErrorMsg +
308 "\"");
309 OptimizeCut = (OptimizeCutT)OptimizeDL.getAddressOfSymbol(symbolName: "optimize_cut");
310 if (!OptimizeCut)
311 report_fatal_error(reason: "Invalid optimization plugin");
312 }
313 FencesInserted = hardenLoadsWithPlugin(MF, Graph: std::move(Graph));
314 } else { // Use the default greedy heuristic
315 FencesInserted = hardenLoadsWithHeuristic(MF, Graph: std::move(Graph));
316 }
317
318 if (FencesInserted > 0)
319 ++NumFunctionsMitigated;
320 NumFences += FencesInserted;
321 return (FencesInserted > 0);
322}
323
324std::unique_ptr<MachineGadgetGraph>
325X86LoadValueInjectionLoadHardeningPass::getGadgetGraph(
326 MachineFunction &MF, const MachineLoopInfo &MLI,
327 const MachineDominatorTree &MDT,
328 const MachineDominanceFrontier &MDF) const {
329 using namespace rdf;
330
331 // Build the Register Dataflow Graph using the RDF framework
332 DataFlowGraph DFG{MF, *TII, *TRI, MDT, MDF};
333 DFG.build();
334 Liveness L{MF.getRegInfo(), DFG};
335 L.computePhiInfo();
336
337 GraphBuilder Builder;
338 using GraphIter = typename GraphBuilder::BuilderNodeRef;
339 DenseMap<MachineInstr *, GraphIter> NodeMap;
340 int FenceCount = 0, GadgetCount = 0;
341 auto MaybeAddNode = [&NodeMap, &Builder](MachineInstr *MI) {
342 auto [Ref, Inserted] = NodeMap.try_emplace(Key: MI);
343 if (Inserted) {
344 auto I = Builder.addVertex(V: MI);
345 Ref->second = I;
346 return std::pair<GraphIter, bool>{I, true};
347 }
348 return std::pair<GraphIter, bool>{Ref->getSecond(), false};
349 };
350
351 // The `Transmitters` map memoizes transmitters found for each def. If a def
352 // has not yet been analyzed, then it will not appear in the map. If a def
353 // has been analyzed and was determined not to have any transmitters, then
354 // its list of transmitters will be empty.
355 DenseMap<NodeId, std::vector<NodeId>> Transmitters;
356
357 // Analyze all machine instructions to find gadgets and LFENCEs, adding
358 // each interesting value to `Nodes`
359 auto AnalyzeDef = [&](NodeAddr<DefNode *> SourceDef) {
360 SmallSet<NodeId, 8> UsesVisited, DefsVisited;
361 std::function<void(NodeAddr<DefNode *>)> AnalyzeDefUseChain =
362 [&](NodeAddr<DefNode *> Def) {
363 if (Transmitters.contains(Val: Def.Id))
364 return; // Already analyzed `Def`
365
366 // Use RDF to find all the uses of `Def`
367 rdf::NodeSet Uses;
368 RegisterRef DefReg = Def.Addr->getRegRef(G: DFG);
369 for (auto UseID : L.getAllReachedUses(RefRR: DefReg, DefA: Def)) {
370 auto Use = DFG.addr<UseNode *>(N: UseID);
371 if (Use.Addr->getFlags() & NodeAttrs::PhiRef) { // phi node
372 NodeAddr<PhiNode *> Phi = Use.Addr->getOwner(G: DFG);
373 for (const auto& I : L.getRealUses(P: Phi.Id)) {
374 if (DFG.getPRI().alias(RA: RegisterRef(I.first), RB: DefReg)) {
375 for (const auto &UA : I.second)
376 Uses.emplace(args: UA.first);
377 }
378 }
379 } else { // not a phi node
380 Uses.emplace(args&: UseID);
381 }
382 }
383
384 // For each use of `Def`, we want to know whether:
385 // (1) The use can leak the Def'ed value,
386 // (2) The use can further propagate the Def'ed value to more defs
387 for (auto UseID : Uses) {
388 if (!UsesVisited.insert(V: UseID).second)
389 continue; // Already visited this use of `Def`
390
391 auto Use = DFG.addr<UseNode *>(N: UseID);
392 assert(!(Use.Addr->getFlags() & NodeAttrs::PhiRef));
393 MachineOperand &UseMO = Use.Addr->getOp();
394 MachineInstr &UseMI = *UseMO.getParent();
395 assert(UseMO.isReg());
396
397 // We naively assume that an instruction propagates any loaded
398 // uses to all defs unless the instruction is a call, in which
399 // case all arguments will be treated as gadget sources during
400 // analysis of the callee function.
401 if (UseMI.isCall())
402 continue;
403
404 // Check whether this use can transmit (leak) its value.
405 if (instrUsesRegToAccessMemory(I: UseMI, Reg: UseMO.getReg()) ||
406 (!NoConditionalBranches &&
407 instrUsesRegToBranch(I: UseMI, Reg: UseMO.getReg()))) {
408 Transmitters[Def.Id].push_back(x: Use.Addr->getOwner(G: DFG).Id);
409 if (UseMI.mayLoad())
410 continue; // Found a transmitting load -- no need to continue
411 // traversing its defs (i.e., this load will become
412 // a new gadget source anyways).
413 }
414
415 // Check whether the use propagates to more defs.
416 NodeAddr<InstrNode *> Owner{Use.Addr->getOwner(G: DFG)};
417 for (const auto &ChildDef :
418 Owner.Addr->members_if(P: DataFlowGraph::IsDef, G: DFG)) {
419 if (!DefsVisited.insert(V: ChildDef.Id).second)
420 continue; // Already visited this def
421 if (Def.Addr->getAttrs() & NodeAttrs::Dead)
422 continue;
423 if (Def.Id == ChildDef.Id)
424 continue; // `Def` uses itself (e.g., increment loop counter)
425
426 AnalyzeDefUseChain(ChildDef);
427
428 // `Def` inherits all of its child defs' transmitters.
429 for (auto TransmitterId : Transmitters[ChildDef.Id])
430 Transmitters[Def.Id].push_back(x: TransmitterId);
431 }
432 }
433
434 // Note that this statement adds `Def.Id` to the map if no
435 // transmitters were found for `Def`.
436 auto &DefTransmitters = Transmitters[Def.Id];
437
438 // Remove duplicate transmitters
439 llvm::sort(C&: DefTransmitters);
440 DefTransmitters.erase(first: llvm::unique(R&: DefTransmitters),
441 last: DefTransmitters.end());
442 };
443
444 // Find all of the transmitters
445 AnalyzeDefUseChain(SourceDef);
446 auto &SourceDefTransmitters = Transmitters[SourceDef.Id];
447 if (SourceDefTransmitters.empty())
448 return; // No transmitters for `SourceDef`
449
450 MachineInstr *Source = SourceDef.Addr->getFlags() & NodeAttrs::PhiRef
451 ? MachineGadgetGraph::ArgNodeSentinel
452 : SourceDef.Addr->getOp().getParent();
453 auto GadgetSource = MaybeAddNode(Source);
454 // Each transmitter is a sink for `SourceDef`.
455 for (auto TransmitterId : SourceDefTransmitters) {
456 MachineInstr *Sink = DFG.addr<StmtNode *>(N: TransmitterId).Addr->getCode();
457 auto GadgetSink = MaybeAddNode(Sink);
458 // Add the gadget edge to the graph.
459 Builder.addEdge(E: MachineGadgetGraph::GadgetEdgeSentinel,
460 From: GadgetSource.first, To: GadgetSink.first);
461 ++GadgetCount;
462 }
463 };
464
465 LLVM_DEBUG(dbgs() << "Analyzing def-use chains to find gadgets\n");
466 // Analyze function arguments
467 NodeAddr<BlockNode *> EntryBlock = DFG.getFunc().Addr->getEntryBlock(G: DFG);
468 for (NodeAddr<PhiNode *> ArgPhi :
469 EntryBlock.Addr->members_if(P: DataFlowGraph::IsPhi, G: DFG)) {
470 NodeList Defs = ArgPhi.Addr->members_if(P: DataFlowGraph::IsDef, G: DFG);
471 llvm::for_each(Range&: Defs, F: AnalyzeDef);
472 }
473 // Analyze every instruction in MF
474 for (NodeAddr<BlockNode *> BA : DFG.getFunc().Addr->members(G: DFG)) {
475 for (NodeAddr<StmtNode *> SA :
476 BA.Addr->members_if(P: DataFlowGraph::IsCode<NodeAttrs::Stmt>, G: DFG)) {
477 MachineInstr *MI = SA.Addr->getCode();
478 if (isFence(MI)) {
479 MaybeAddNode(MI);
480 ++FenceCount;
481 } else if (MI->mayLoad()) {
482 NodeList Defs = SA.Addr->members_if(P: DataFlowGraph::IsDef, G: DFG);
483 llvm::for_each(Range&: Defs, F: AnalyzeDef);
484 }
485 }
486 }
487 LLVM_DEBUG(dbgs() << "Found " << FenceCount << " fences\n");
488 LLVM_DEBUG(dbgs() << "Found " << GadgetCount << " gadgets\n");
489 if (GadgetCount == 0)
490 return nullptr;
491 NumGadgets += GadgetCount;
492
493 // Traverse CFG to build the rest of the graph
494 SmallSet<MachineBasicBlock *, 8> BlocksVisited;
495 std::function<void(MachineBasicBlock *, GraphIter, unsigned)> TraverseCFG =
496 [&](MachineBasicBlock *MBB, GraphIter GI, unsigned ParentDepth) {
497 unsigned LoopDepth = MLI.getLoopDepth(BB: MBB);
498 if (!MBB->empty()) {
499 // Always add the first instruction in each block
500 auto NI = MBB->begin();
501 auto BeginBB = MaybeAddNode(&*NI);
502 Builder.addEdge(E: ParentDepth, From: GI, To: BeginBB.first);
503 if (!BlocksVisited.insert(Ptr: MBB).second)
504 return;
505
506 // Add any instructions within the block that are gadget components
507 GI = BeginBB.first;
508 while (++NI != MBB->end()) {
509 auto Ref = NodeMap.find(Val: &*NI);
510 if (Ref != NodeMap.end()) {
511 Builder.addEdge(E: LoopDepth, From: GI, To: Ref->getSecond());
512 GI = Ref->getSecond();
513 }
514 }
515
516 // Always add the terminator instruction, if one exists
517 auto T = MBB->getFirstTerminator();
518 if (T != MBB->end()) {
519 auto EndBB = MaybeAddNode(&*T);
520 if (EndBB.second)
521 Builder.addEdge(E: LoopDepth, From: GI, To: EndBB.first);
522 GI = EndBB.first;
523 }
524 }
525 for (MachineBasicBlock *Succ : MBB->successors())
526 TraverseCFG(Succ, GI, LoopDepth);
527 };
528 // ArgNodeSentinel is a pseudo-instruction that represents MF args in the
529 // GadgetGraph
530 GraphIter ArgNode = MaybeAddNode(MachineGadgetGraph::ArgNodeSentinel).first;
531 TraverseCFG(&MF.front(), ArgNode, 0);
532 std::unique_ptr<MachineGadgetGraph> G{Builder.get(Args&: FenceCount, Args&: GadgetCount)};
533 LLVM_DEBUG(dbgs() << "Found " << G->nodes_size() << " nodes\n");
534 return G;
535}
536
537// Returns the number of remaining gadget edges that could not be eliminated
538int X86LoadValueInjectionLoadHardeningPass::elimMitigatedEdgesAndNodes(
539 MachineGadgetGraph &G, EdgeSet &ElimEdges /* in, out */,
540 NodeSet &ElimNodes /* in, out */) const {
541 if (G.NumFences > 0) {
542 // Eliminate fences and CFG edges that ingress and egress the fence, as
543 // they are trivially mitigated.
544 for (const Edge &E : G.edges()) {
545 const Node *Dest = E.getDest();
546 if (isFence(MI: Dest->getValue())) {
547 ElimNodes.insert(N: *Dest);
548 ElimEdges.insert(E);
549 for (const Edge &DE : Dest->edges())
550 ElimEdges.insert(E: DE);
551 }
552 }
553 }
554
555 // Find and eliminate gadget edges that have been mitigated.
556 int RemainingGadgets = 0;
557 NodeSet ReachableNodes{G};
558 for (const Node &RootN : G.nodes()) {
559 if (llvm::none_of(Range: RootN.edges(), P: MachineGadgetGraph::isGadgetEdge))
560 continue; // skip this node if it isn't a gadget source
561
562 // Find all of the nodes that are CFG-reachable from RootN using DFS
563 ReachableNodes.clear();
564 std::function<void(const Node *, bool)> FindReachableNodes =
565 [&](const Node *N, bool FirstNode) {
566 if (!FirstNode)
567 ReachableNodes.insert(N: *N);
568 for (const Edge &E : N->edges()) {
569 const Node *Dest = E.getDest();
570 if (MachineGadgetGraph::isCFGEdge(E) && !ElimEdges.contains(E) &&
571 !ReachableNodes.contains(N: *Dest))
572 FindReachableNodes(Dest, false);
573 }
574 };
575 FindReachableNodes(&RootN, true);
576
577 // Any gadget whose sink is unreachable has been mitigated
578 for (const Edge &E : RootN.edges()) {
579 if (MachineGadgetGraph::isGadgetEdge(E)) {
580 if (ReachableNodes.contains(N: *E.getDest())) {
581 // This gadget's sink is reachable
582 ++RemainingGadgets;
583 } else { // This gadget's sink is unreachable, and therefore mitigated
584 ElimEdges.insert(E);
585 }
586 }
587 }
588 }
589 return RemainingGadgets;
590}
591
592std::unique_ptr<MachineGadgetGraph>
593X86LoadValueInjectionLoadHardeningPass::trimMitigatedEdges(
594 std::unique_ptr<MachineGadgetGraph> Graph) const {
595 NodeSet ElimNodes{*Graph};
596 EdgeSet ElimEdges{*Graph};
597 int RemainingGadgets =
598 elimMitigatedEdgesAndNodes(G&: *Graph, ElimEdges, ElimNodes);
599 if (ElimEdges.empty() && ElimNodes.empty()) {
600 Graph->NumFences = 0;
601 Graph->NumGadgets = RemainingGadgets;
602 } else {
603 Graph = GraphBuilder::trim(G: *Graph, TrimNodes: ElimNodes, TrimEdges: ElimEdges, Args: 0 /* NumFences */,
604 Args&: RemainingGadgets);
605 }
606 return Graph;
607}
608
609int X86LoadValueInjectionLoadHardeningPass::hardenLoadsWithPlugin(
610 MachineFunction &MF, std::unique_ptr<MachineGadgetGraph> Graph) const {
611 int FencesInserted = 0;
612
613 do {
614 LLVM_DEBUG(dbgs() << "Eliminating mitigated paths...\n");
615 Graph = trimMitigatedEdges(Graph: std::move(Graph));
616 LLVM_DEBUG(dbgs() << "Eliminating mitigated paths... Done\n");
617 if (Graph->NumGadgets == 0)
618 break;
619
620 LLVM_DEBUG(dbgs() << "Cutting edges...\n");
621 EdgeSet CutEdges{*Graph};
622 auto Nodes = std::make_unique<unsigned int[]>(num: Graph->nodes_size() +
623 1 /* terminator node */);
624 auto Edges = std::make_unique<unsigned int[]>(num: Graph->edges_size());
625 auto EdgeCuts = std::make_unique<int[]>(num: Graph->edges_size());
626 auto EdgeValues = std::make_unique<int[]>(num: Graph->edges_size());
627 for (const Node &N : Graph->nodes()) {
628 Nodes[Graph->getNodeIndex(N)] = Graph->getEdgeIndex(E: *N.edges_begin());
629 }
630 Nodes[Graph->nodes_size()] = Graph->edges_size(); // terminator node
631 for (const Edge &E : Graph->edges()) {
632 Edges[Graph->getEdgeIndex(E)] = Graph->getNodeIndex(N: *E.getDest());
633 EdgeValues[Graph->getEdgeIndex(E)] = E.getValue();
634 }
635 OptimizeCut(Nodes.get(), Graph->nodes_size(), Edges.get(), EdgeValues.get(),
636 EdgeCuts.get(), Graph->edges_size());
637 for (int I = 0; I < Graph->edges_size(); ++I)
638 if (EdgeCuts[I])
639 CutEdges.set(I);
640 LLVM_DEBUG(dbgs() << "Cutting edges... Done\n");
641 LLVM_DEBUG(dbgs() << "Cut " << CutEdges.count() << " edges\n");
642
643 LLVM_DEBUG(dbgs() << "Inserting LFENCEs...\n");
644 FencesInserted += insertFences(MF, G&: *Graph, CutEdges);
645 LLVM_DEBUG(dbgs() << "Inserting LFENCEs... Done\n");
646 LLVM_DEBUG(dbgs() << "Inserted " << FencesInserted << " fences\n");
647
648 Graph = GraphBuilder::trim(G: *Graph, TrimNodes: NodeSet{*Graph}, TrimEdges: CutEdges);
649 } while (true);
650
651 return FencesInserted;
652}
653
654int X86LoadValueInjectionLoadHardeningPass::hardenLoadsWithHeuristic(
655 MachineFunction &MF, std::unique_ptr<MachineGadgetGraph> Graph) const {
656 // If `MF` does not have any fences, then no gadgets would have been
657 // mitigated at this point.
658 if (Graph->NumFences > 0) {
659 LLVM_DEBUG(dbgs() << "Eliminating mitigated paths...\n");
660 Graph = trimMitigatedEdges(Graph: std::move(Graph));
661 LLVM_DEBUG(dbgs() << "Eliminating mitigated paths... Done\n");
662 }
663
664 if (Graph->NumGadgets == 0)
665 return 0;
666
667 LLVM_DEBUG(dbgs() << "Cutting edges...\n");
668 EdgeSet CutEdges{*Graph};
669
670 // Begin by collecting all ingress CFG edges for each node
671 DenseMap<const Node *, SmallVector<const Edge *, 2>> IngressEdgeMap;
672 for (const Edge &E : Graph->edges())
673 if (MachineGadgetGraph::isCFGEdge(E))
674 IngressEdgeMap[E.getDest()].push_back(Elt: &E);
675
676 // For each gadget edge, make cuts that guarantee the gadget will be
677 // mitigated. A computationally efficient way to achieve this is to either:
678 // (a) cut all egress CFG edges from the gadget source, or
679 // (b) cut all ingress CFG edges to the gadget sink.
680 //
681 // Moreover, the algorithm tries not to make a cut into a loop by preferring
682 // to make a (b)-type cut if the gadget source resides at a greater loop depth
683 // than the gadget sink, or an (a)-type cut otherwise.
684 for (const Node &N : Graph->nodes()) {
685 for (const Edge &E : N.edges()) {
686 if (!MachineGadgetGraph::isGadgetEdge(E))
687 continue;
688
689 SmallVector<const Edge *, 2> EgressEdges;
690 SmallVector<const Edge *, 2> &IngressEdges = IngressEdgeMap[E.getDest()];
691 for (const Edge &EgressEdge : N.edges())
692 if (MachineGadgetGraph::isCFGEdge(E: EgressEdge))
693 EgressEdges.push_back(Elt: &EgressEdge);
694
695 int EgressCutCost = 0, IngressCutCost = 0;
696 for (const Edge *EgressEdge : EgressEdges)
697 if (!CutEdges.contains(E: *EgressEdge))
698 EgressCutCost += EgressEdge->getValue();
699 for (const Edge *IngressEdge : IngressEdges)
700 if (!CutEdges.contains(E: *IngressEdge))
701 IngressCutCost += IngressEdge->getValue();
702
703 auto &EdgesToCut =
704 IngressCutCost < EgressCutCost ? IngressEdges : EgressEdges;
705 for (const Edge *E : EdgesToCut)
706 CutEdges.insert(E: *E);
707 }
708 }
709 LLVM_DEBUG(dbgs() << "Cutting edges... Done\n");
710 LLVM_DEBUG(dbgs() << "Cut " << CutEdges.count() << " edges\n");
711
712 LLVM_DEBUG(dbgs() << "Inserting LFENCEs...\n");
713 int FencesInserted = insertFences(MF, G&: *Graph, CutEdges);
714 LLVM_DEBUG(dbgs() << "Inserting LFENCEs... Done\n");
715 LLVM_DEBUG(dbgs() << "Inserted " << FencesInserted << " fences\n");
716
717 return FencesInserted;
718}
719
720int X86LoadValueInjectionLoadHardeningPass::insertFences(
721 MachineFunction &MF, MachineGadgetGraph &G,
722 EdgeSet &CutEdges /* in, out */) const {
723 int FencesInserted = 0;
724 for (const Node &N : G.nodes()) {
725 for (const Edge &E : N.edges()) {
726 if (CutEdges.contains(E)) {
727 MachineInstr *MI = N.getValue(), *Prev;
728 MachineBasicBlock *MBB; // Insert an LFENCE in this MBB
729 MachineBasicBlock::iterator InsertionPt; // ...at this point
730 if (MI == MachineGadgetGraph::ArgNodeSentinel) {
731 // insert LFENCE at beginning of entry block
732 MBB = &MF.front();
733 InsertionPt = MBB->begin();
734 Prev = nullptr;
735 } else if (MI->isBranch()) { // insert the LFENCE before the branch
736 MBB = MI->getParent();
737 InsertionPt = MI;
738 Prev = MI->getPrevNode();
739 // Remove all egress CFG edges from this branch because the inserted
740 // LFENCE prevents gadgets from crossing the branch.
741 for (const Edge &E : N.edges()) {
742 if (MachineGadgetGraph::isCFGEdge(E))
743 CutEdges.insert(E);
744 }
745 } else { // insert the LFENCE after the instruction
746 MBB = MI->getParent();
747 InsertionPt = MI->getNextNode() ? MI->getNextNode() : MBB->end();
748 Prev = InsertionPt == MBB->end()
749 ? (MBB->empty() ? nullptr : &MBB->back())
750 : InsertionPt->getPrevNode();
751 }
752 // Ensure this insertion is not redundant (two LFENCEs in sequence).
753 if ((InsertionPt == MBB->end() || !isFence(MI: &*InsertionPt)) &&
754 (!Prev || !isFence(MI: Prev))) {
755 BuildMI(BB&: *MBB, I: InsertionPt, MIMD: DebugLoc(), MCID: TII->get(Opcode: X86::LFENCE));
756 ++FencesInserted;
757 }
758 }
759 }
760 }
761 return FencesInserted;
762}
763
764bool X86LoadValueInjectionLoadHardeningPass::instrUsesRegToAccessMemory(
765 const MachineInstr &MI, Register Reg) const {
766 if (!MI.mayLoadOrStore() || MI.getOpcode() == X86::MFENCE ||
767 MI.getOpcode() == X86::SFENCE || MI.getOpcode() == X86::LFENCE)
768 return false;
769
770 const int MemRefBeginIdx = X86::getFirstAddrOperandIdx(MI);
771 if (MemRefBeginIdx < 0) {
772 LLVM_DEBUG(dbgs() << "Warning: unable to obtain memory operand for loading "
773 "instruction:\n";
774 MI.print(dbgs()); dbgs() << '\n';);
775 return false;
776 }
777
778 const MachineOperand &BaseMO =
779 MI.getOperand(i: MemRefBeginIdx + X86::AddrBaseReg);
780 const MachineOperand &IndexMO =
781 MI.getOperand(i: MemRefBeginIdx + X86::AddrIndexReg);
782 return (BaseMO.isReg() && BaseMO.getReg().isValid() &&
783 TRI->regsOverlap(RegA: BaseMO.getReg(), RegB: Reg)) ||
784 (IndexMO.isReg() && IndexMO.getReg().isValid() &&
785 TRI->regsOverlap(RegA: IndexMO.getReg(), RegB: Reg));
786}
787
788bool X86LoadValueInjectionLoadHardeningPass::instrUsesRegToBranch(
789 const MachineInstr &MI, Register Reg) const {
790 if (!MI.isConditionalBranch())
791 return false;
792 for (const MachineOperand &Use : MI.uses())
793 if (Use.isReg() && Use.getReg() == Reg)
794 return true;
795 return false;
796}
797
798INITIALIZE_PASS_BEGIN(X86LoadValueInjectionLoadHardeningPass, PASS_KEY,
799 "X86 LVI load hardening", false, false)
800INITIALIZE_PASS_DEPENDENCY(MachineLoopInfoWrapperPass)
801INITIALIZE_PASS_DEPENDENCY(MachineDominatorTreeWrapperPass)
802INITIALIZE_PASS_DEPENDENCY(MachineDominanceFrontier)
803INITIALIZE_PASS_END(X86LoadValueInjectionLoadHardeningPass, PASS_KEY,
804 "X86 LVI load hardening", false, false)
805
806FunctionPass *llvm::createX86LoadValueInjectionLoadHardeningPass() {
807 return new X86LoadValueInjectionLoadHardeningPass();
808}
809