Add size check before calling stack_.at(dict_pos) in unpickler.cpp (pytorch#94300)

Hi!

I've been fuzzing different pytorch modules, and found a crash inside one of them.

Specifically, I'm talking about a module for unpickling and a function called `Unpickler::readInstruction()`. Running this function with provided crash file results in a crash, which occurs while calling `auto dict = stack_.at(dict_pos).toGenericDict();` [unpickler.cpp:561](https://github.com/pytorch/pytorch/blob/0e94fbc0c8ab1572c88159c1a4c397b6eb824c01/torch/csrc/jit/serialization/unpickler.cpp#L561). The crash occurs, because the index `dict_pos` is out of bounds (which itself happens because the stack size is 0).

Besides this pull-request, there is another one related to unpickler hardening: pytorch#84343

All tests were performed on this pytorch version: [abc54f9](https://github.com/pytorch/pytorch/tree/abc54f93145830b502400faa92bec86e05422fbd)

### How to reproduce

1. To reproduce the crash, use provided docker: [Dockerfile](https://github.com/ispras/oss-sydr-fuzz/tree/master/projects/pytorch)

2. Build the container: `docker build -t oss-sydr-fuzz-pytorch-reproduce .`

3. Copy crash file to the current directory:

    - [crash-042dff5e121580425d9d34d0f293918f3c9fbf1e.zip](https://github.com/pytorch/pytorch/files/10674361/crash-042dff5e121580425d9d34d0f293918f3c9fbf1e.zip)

4. Run the container: ``docker run --privileged --network host -v `pwd`:/homedir --rm -it oss-sydr-fuzz-pytorch-reproduce /bin/bash``

5. And execute the binary: `/message_deserialize_sydr /homedir/crash-042dff5e121580425d9d34d0f293918f3c9fbf1e`

After execution completes you will see this error message:

```txt
terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 18446744073709551613) >= this->size() (which is 0)
```

And this stacktrace:

```asan
erminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 18446744073709551613) >= this->size() (which is 0)
==39== ERROR: libFuzzer: deadly signal
    #0 0x5d0df1 in __sanitizer_print_stack_trace /llvm-project/compiler-rt/lib/asan/asan_stack.cpp:87:3
    #1 0x545727 in fuzzer::PrintStackTrace() /llvm-project/compiler-rt/lib/fuzzer/FuzzerUtil.cpp:210:5
    #2 0x52b933 in fuzzer::Fuzzer::CrashCallback() /llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:233:3
    #3 0x7f9118e0341f  (/lib/x86_64-linux-gnu/libpthread.so.0+0x1441f)
    #4 0x7f9118c2300a in raise (/lib/x86_64-linux-gnu/libc.so.6+0x4300a)
    #5 0x7f9118c02858 in abort (/lib/x86_64-linux-gnu/libc.so.6+0x22858)
    pytorch#6 0x7f9119040910  (/lib/x86_64-linux-gnu/libstdc++.so.6+0x9e910)
    pytorch#7 0x7f911904c38b  (/lib/x86_64-linux-gnu/libstdc++.so.6+0xaa38b)
    pytorch#8 0x7f911904c3f6 in std::terminate() (/lib/x86_64-linux-gnu/libstdc++.so.6+0xaa3f6)
    pytorch#9 0x7f911904c6a8 in __cxa_throw (/lib/x86_64-linux-gnu/libstdc++.so.6+0xaa6a8)
    pytorch#10 0x7f91190433aa  (/lib/x86_64-linux-gnu/libstdc++.so.6+0xa13aa)
    pytorch#11 0x63acdf in std::vector<c10::IValue, std::allocator<c10::IValue> >::_M_range_check(unsigned long) const /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/stl_vector.h:1073:4
    pytorch#12 0xce8f93e in std::vector<c10::IValue, std::allocator<c10::IValue> >::at(unsigned long) /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/bits/stl_vector.h:1094:2
    pytorch#13 0xce8f93e in torch::jit::Unpickler::readInstruction() /pytorch_fuzz/torch/csrc/jit/serialization/unpickler.cpp:546:26
    pytorch#14 0xce8d527 in torch::jit::Unpickler::run() /pytorch_fuzz/torch/csrc/jit/serialization/unpickler.cpp:235:27
    pytorch#15 0xce8d1c2 in torch::jit::Unpickler::parse_ivalue() /pytorch_fuzz/torch/csrc/jit/serialization/unpickler.cpp:192:3
    pytorch#16 0xcdf0792 in torch::jit::unpickle(std::function<unsigned long (char*, unsigned long)>, std::function<c10::StrongTypePtr (c10::QualifiedName const&)>, c10::ArrayRef<at::Tensor>, c10::Type::SingletonOrSharedTypePtr<c10::Type> (*)(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)) /pytorch_fuzz/torch/csrc/jit/serialization/pickle.cpp:127:20
    pytorch#17 0xcdf104d in torch::jit::unpickle(char const*, unsigned long, std::function<c10::StrongTypePtr (c10::QualifiedName const&)>, c10::ArrayRef<at::Tensor>, c10::Type::SingletonOrSharedTypePtr<c10::Type> (*)(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)) /pytorch_fuzz/torch/csrc/jit/serialization/pickle.cpp:137:10
    pytorch#18 0xe0532db in torch::distributed::rpc::ScriptRemoteCall::fromMessage(torch::distributed::rpc::Message const&) /pytorch_fuzz/torch/csrc/distributed/rpc/script_remote_call.cpp:74:16
    pytorch#19 0xe0ffa10 in torch::distributed::rpc::deserializeRequest(torch::distributed::rpc::Message const&) /pytorch_fuzz/torch/csrc/distributed/rpc/utils.cpp:108:14
    pytorch#20 0x602a41 in LLVMFuzzerTestOneInput /message_deserialize_fuzz.cc:192:27
    pytorch#21 0x52ce61 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    pytorch#22 0x516d7c in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:324:6
    pytorch#23 0x51cacb in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:860:9
    pytorch#24 0x546062 in main /llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    pytorch#25 0x7f9118c04082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082)
    pytorch#26 0x51169d in _start (/message_deserialize_fuzz+0x51169d)

NOTE: libFuzzer has rudimentary signal handlers.
      Combine libFuzzer with AddressSanitizer or similar for better crash reports.
SUMMARY: libFuzzer: deadly signal
```
Pull Request resolved: pytorch#94300
Approved by: https://github.com/malfet, https://github.com/apach301

Author: m4drat

torch/csrc/jit/serialization/unpickler.cpp

@@ -597,10 +597,20 @@ PickleOpCode Unpickler::readInstruction() {
       // | Dict         | -> (stack_size - 3)
       // | Key          | -> (stack_size - 2)
       // | Value        | -> (stack_size - 1)
+      TORCH_CHECK(
+          stack_.size() >= 3,
+          "Parsing error: stack doesn't have enough elements");
+
       auto stack_size = stack_.size();
       auto dict_pos = stack_size - 3;
       auto key_pos = stack_size - 2;
       auto val_pos = stack_size - 1;
+
+      TORCH_CHECK(
+          (dict_pos < stack_size) && (key_pos < stack_size) &&
+              (val_pos < stack_size),
+          "Parsing error: attempted out-of-bounds access while processing SETITEM opcode");
+
       auto dict = stack_.at(dict_pos).toGenericDict();
       dict.insert_or_assign(stack_.at(key_pos), stack_.at(val_pos));
       stack_.erase(stack_.begin() + (key_pos), stack_.end());