1 //===-- CodeGen/AsmPrinter/WinCFGuard.cpp - Control Flow Guard Impl ------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file contains support for writing the metadata for Windows Control Flow
10 // Guard, including address-taken functions and valid longjmp targets.
11 //
12 //===----------------------------------------------------------------------===//
13
14 #include "WinCFGuard.h"
15 #include "llvm/CodeGen/AsmPrinter.h"
16 #include "llvm/CodeGen/MachineFunction.h"
17 #include "llvm/CodeGen/MachineModuleInfo.h"
18 #include "llvm/CodeGen/MachineOperand.h"
19 #include "llvm/IR/Constants.h"
20 #include "llvm/IR/Instructions.h"
21 #include "llvm/IR/Metadata.h"
22 #include "llvm/MC/MCAsmInfo.h"
23 #include "llvm/MC/MCObjectFileInfo.h"
24 #include "llvm/MC/MCStreamer.h"
25
26 #include <vector>
27
28 using namespace llvm;
29
WinCFGuard(AsmPrinter * A)30 WinCFGuard::WinCFGuard(AsmPrinter *A) : AsmPrinterHandler(), Asm(A) {}
31
~WinCFGuard()32 WinCFGuard::~WinCFGuard() {}
33
endFunction(const MachineFunction * MF)34 void WinCFGuard::endFunction(const MachineFunction *MF) {
35
36 // Skip functions without any longjmp targets.
37 if (MF->getLongjmpTargets().empty())
38 return;
39
40 // Copy the function's longjmp targets to a module-level list.
41 llvm::append_range(LongjmpTargets, MF->getLongjmpTargets());
42 }
43
44 /// Returns true if this function's address is escaped in a way that might make
45 /// it an indirect call target. Function::hasAddressTaken gives different
46 /// results when a function is called directly with a function prototype
47 /// mismatch, which requires a cast.
isPossibleIndirectCallTarget(const Function * F)48 static bool isPossibleIndirectCallTarget(const Function *F) {
49 SmallVector<const Value *, 4> Users{F};
50 while (!Users.empty()) {
51 const Value *FnOrCast = Users.pop_back_val();
52 for (const Use &U : FnOrCast->uses()) {
53 const User *FnUser = U.getUser();
54 if (isa<BlockAddress>(FnUser))
55 continue;
56 if (const auto *Call = dyn_cast<CallBase>(FnUser)) {
57 if (!Call->isCallee(&U))
58 return true;
59 } else if (isa<Instruction>(FnUser)) {
60 // Consider any other instruction to be an escape. This has some weird
61 // consequences like no-op intrinsics being an escape or a store *to* a
62 // function address being an escape.
63 return true;
64 } else if (const auto *C = dyn_cast<Constant>(FnUser)) {
65 // If this is a constant pointer cast of the function, don't consider
66 // this escape. Analyze the uses of the cast as well. This ensures that
67 // direct calls with mismatched prototypes don't end up in the CFG
68 // table. Consider other constants, such as vtable initializers, to
69 // escape the function.
70 if (C->stripPointerCasts() == F)
71 Users.push_back(FnUser);
72 else
73 return true;
74 }
75 }
76 }
77 return false;
78 }
79
lookupImpSymbol(const MCSymbol * Sym)80 MCSymbol *WinCFGuard::lookupImpSymbol(const MCSymbol *Sym) {
81 if (Sym->getName().startswith("__imp_"))
82 return nullptr;
83 return Asm->OutContext.lookupSymbol(Twine("__imp_") + Sym->getName());
84 }
85
endModule()86 void WinCFGuard::endModule() {
87 const Module *M = Asm->MMI->getModule();
88 std::vector<const MCSymbol *> GFIDsEntries;
89 std::vector<const MCSymbol *> GIATsEntries;
90 for (const Function &F : *M) {
91 if (isPossibleIndirectCallTarget(&F)) {
92 // If F is a dllimport and has an "__imp_" symbol already defined, add the
93 // "__imp_" symbol to the .giats section.
94 if (F.hasDLLImportStorageClass()) {
95 if (MCSymbol *impSym = lookupImpSymbol(Asm->getSymbol(&F))) {
96 GIATsEntries.push_back(impSym);
97 }
98 }
99 // Add the function's symbol to the .gfids section.
100 // Note: For dllimport functions, MSVC sometimes does not add this symbol
101 // to the .gfids section, but only adds the corresponding "__imp_" symbol
102 // to the .giats section. Here we always add the symbol to the .gfids
103 // section, since this does not introduce security risks.
104 GFIDsEntries.push_back(Asm->getSymbol(&F));
105 }
106 }
107
108 if (GFIDsEntries.empty() && GIATsEntries.empty() && LongjmpTargets.empty())
109 return;
110
111 // Emit the symbol index of each GFIDs entry to form the .gfids section.
112 auto &OS = *Asm->OutStreamer;
113 OS.SwitchSection(Asm->OutContext.getObjectFileInfo()->getGFIDsSection());
114 for (const MCSymbol *S : GFIDsEntries)
115 OS.EmitCOFFSymbolIndex(S);
116
117 // Emit the symbol index of each GIATs entry to form the .giats section.
118 OS.SwitchSection(Asm->OutContext.getObjectFileInfo()->getGIATsSection());
119 for (const MCSymbol *S : GIATsEntries) {
120 OS.EmitCOFFSymbolIndex(S);
121 }
122
123 // Emit the symbol index of each longjmp target to form the .gljmp section.
124 OS.SwitchSection(Asm->OutContext.getObjectFileInfo()->getGLJMPSection());
125 for (const MCSymbol *S : LongjmpTargets) {
126 OS.EmitCOFFSymbolIndex(S);
127 }
128 }
129