aboutsummaryrefslogtreecommitdiffstats
path: root/lib/CodeGen/StackProtector.cpp
blob: e8ee82213a4a4169cc1e46c6bbd8b22499de1658 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
//===-- StackProtector.cpp - Stack Protector Insertion --------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass inserts stack protectors into functions which need them. A variable
// with a random value in it is stored onto the stack before the local variables
// are allocated. Upon exiting the block, the stored value is checked. If it's
// changed, then there was some sort of violation and the program aborts.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "stack-protector"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Attributes.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Intrinsics.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetLowering.h"
using namespace llvm;

// SSPBufferSize - The lower bound for a buffer to be considered for stack
// smashing protection.
static cl::opt<unsigned>
SSPBufferSize("stack-protector-buffer-size", cl::init(8),
              cl::desc("Lower bound for a buffer to be considered for "
                       "stack protection"));

namespace {
  class StackProtector : public FunctionPass {
    /// TLI - Keep a pointer of a TargetLowering to consult for determining
    /// target type sizes.
    const TargetLowering *TLI;

    Function *F;
    Module *M;

    /// InsertStackProtectors - Insert code into the prologue and epilogue of
    /// the function.
    ///
    ///  - The prologue code loads and stores the stack guard onto the stack.
    ///  - The epilogue checks the value stored in the prologue against the
    ///    original value. It calls __stack_chk_fail if they differ.
    bool InsertStackProtectors();

    /// CreateFailBB - Create a basic block to jump to when the stack protector
    /// check fails.
    BasicBlock *CreateFailBB();

    /// RequiresStackProtector - Check whether or not this function needs a
    /// stack protector based upon the stack protector level.
    bool RequiresStackProtector() const;
  public:
    static char ID;             // Pass identification, replacement for typeid.
    StackProtector() : FunctionPass(&ID), TLI(0) {}
    StackProtector(const TargetLowering *tli)
      : FunctionPass(&ID), TLI(tli) {}

    virtual bool runOnFunction(Function &Fn);
  };
} // end anonymous namespace

char StackProtector::ID = 0;
static RegisterPass<StackProtector>
X("stack-protector", "Insert stack protectors");

FunctionPass *llvm::createStackProtectorPass(const TargetLowering *tli) {
  return new StackProtector(tli);
}

bool StackProtector::runOnFunction(Function &Fn) {
  F = &Fn;
  M = F->getParent();

  if (!RequiresStackProtector()) return false;
  
  return InsertStackProtectors();
}

/// RequiresStackProtector - Check whether or not this function needs a stack
/// protector based upon the stack protector level. The heuristic we use is to
/// add a guard variable to functions that call alloca, and functions with
/// buffers larger than SSPBufferSize bytes.
bool StackProtector::RequiresStackProtector() const {
  if (F->hasFnAttr(Attribute::StackProtectReq))
    return true;

  if (!F->hasFnAttr(Attribute::StackProtect))
    return false;

  const TargetData *TD = TLI->getTargetData();

  for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
    BasicBlock *BB = I;

    for (BasicBlock::iterator
           II = BB->begin(), IE = BB->end(); II != IE; ++II)
      if (AllocaInst *AI = dyn_cast<AllocaInst>(II)) {
        if (AI->isArrayAllocation())
          // This is a call to alloca with a variable size. Emit stack
          // protectors.
          return true;

        if (const ArrayType *AT = dyn_cast<ArrayType>(AI->getAllocatedType())) {
          // We apparently only care about character arrays.
          if (AT->getElementType() != Type::getInt8Ty(AT->getContext()))
            continue;

          // If an array has more than SSPBufferSize bytes of allocated space,
          // then we emit stack protectors.
          if (SSPBufferSize <= TD->getTypeAllocSize(AT))
            return true;
        }
      }
  }

  return false;
}

/// InsertStackProtectors - Insert code into the prologue and epilogue of the
/// function.
///
///  - The prologue code loads and stores the stack guard onto the stack.
///  - The epilogue checks the value stored in the prologue against the original
///    value. It calls __stack_chk_fail if they differ.
bool StackProtector::InsertStackProtectors() {
  BasicBlock *FailBB = 0;       // The basic block to jump to if check fails.
  AllocaInst *AI = 0;           // Place on stack that stores the stack guard.
  Constant *StackGuardVar = 0;  // The stack guard variable.

  for (Function::iterator I = F->begin(), E = F->end(); I != E; ) {
    BasicBlock *BB = I++;

    ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator());
    if (!RI) continue;

    if (!FailBB) {
      // Insert code into the entry block that stores the __stack_chk_guard
      // variable onto the stack:
      //
      //   entry:
      //     StackGuardSlot = alloca i8*
      //     StackGuard = load __stack_chk_guard
      //     call void @llvm.stackprotect.create(StackGuard, StackGuardSlot)
      // 
      PointerType *PtrTy = PointerType::getUnqual(
          Type::getInt8Ty(RI->getContext()));
      StackGuardVar = M->getOrInsertGlobal("__stack_chk_guard", PtrTy);

      BasicBlock &Entry = F->getEntryBlock();
      Instruction *InsPt = &Entry.front();

      AI = new AllocaInst(PtrTy, "StackGuardSlot", InsPt);
      LoadInst *LI = new LoadInst(StackGuardVar, "StackGuard", false, InsPt);

      Value *Args[] = { LI, AI };
      CallInst::
        Create(Intrinsic::getDeclaration(M, Intrinsic::stackprotector),
               &Args[0], array_endof(Args), "", InsPt);

      // Create the basic block to jump to when the guard check fails.
      FailBB = CreateFailBB();
    }

    // For each block with a return instruction, convert this:
    //
    //   return:
    //     ...
    //     ret ...
    //
    // into this:
    //
    //   return:
    //     ...
    //     %1 = load __stack_chk_guard
    //     %2 = load StackGuardSlot
    //     %3 = cmp i1 %1, %2
    //     br i1 %3, label %SP_return, label %CallStackCheckFailBlk
    //
    //   SP_return:
    //     ret ...
    //
    //   CallStackCheckFailBlk:
    //     call void @__stack_chk_fail()
    //     unreachable

    // Split the basic block before the return instruction.
    BasicBlock *NewBB = BB->splitBasicBlock(RI, "SP_return");

    // Remove default branch instruction to the new BB.
    BB->getTerminator()->eraseFromParent();

    // Move the newly created basic block to the point right after the old basic
    // block so that it's in the "fall through" position.
    NewBB->moveAfter(BB);

    // Generate the stack protector instructions in the old basic block.
    LoadInst *LI1 = new LoadInst(StackGuardVar, "", false, BB);
    LoadInst *LI2 = new LoadInst(AI, "", true, BB);
    ICmpInst *Cmp = new ICmpInst(*BB, CmpInst::ICMP_EQ, LI1, LI2, "");
    BranchInst::Create(NewBB, FailBB, Cmp, BB);
  }

  // Return if we didn't modify any basic blocks. I.e., there are no return
  // statements in the function.
  if (!FailBB) return false;

  return true;
}

/// CreateFailBB - Create a basic block to jump to when the stack protector
/// check fails.
BasicBlock *StackProtector::CreateFailBB() {
  BasicBlock *FailBB = BasicBlock::Create(F->getContext(),
                                          "CallStackCheckFailBlk", F);
  Constant *StackChkFail =
    M->getOrInsertFunction("__stack_chk_fail",
                           Type::getVoidTy(F->getContext()), NULL);
  CallInst::Create(StackChkFail, "", FailBB);
  new UnreachableInst(F->getContext(), FailBB);
  return FailBB;
}