Add size check before calling .back() in rpc/script_call.cpp (pytorch…

…#94297)

Hi!

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

Specifically, I'm talking about a module that processes `script_call` rpc requests and a function `ScriptCall::fromIValues(std::vector<at::IValue>& ivalues)`.

Running this test case causes a crash that occurs when `ivalues.back()` is called [script_call.cpp:90](https://github.com/pytorch/pytorch/blob/abc54f93145830b502400faa92bec86e05422fbd/torch/csrc/distributed/rpc/script_call.cpp#L90). The crash occurs because the vector `ivalues` is empty.

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

The provided patch checks if there are enough elements in the ivalues vector.

### 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-9f76d4e37a2391136a4ce07d47269db1e063e4b4.zip](https://github.com/pytorch/pytorch/files/10674059/crash-9f76d4e37a2391136a4ce07d47269db1e063e4b4.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_fuzz /homedir/crash-9f76d4e37a2391136a4ce07d47269db1e063e4b4`

After execution completes you will see this stacktrace:

```asan
AddressSanitizer:DEADLYSIGNAL
=================================================================
==57==ERROR: AddressSanitizer: SEGV on unknown address (pc 0x0000008e7b19 bp 0x7ffd2fdded70 sp 0x7ffd2fddec40 T0)
==57==The signal is caused by a READ memory access.
==57==Hint: this fault was caused by a dereference of a high value address (see register values below).  Disassemble the provided pc to learn which register was used.
    #0 0x8e7b19 in c10::IValue::isString() const /pytorch_fuzz/aten/src/ATen/core/ivalue.h:639:27
    #1 0x8e7b19 in c10::IValue::toStringRef[abi:cxx11]() const /pytorch_fuzz/aten/src/ATen/core/ivalue_inl.h:2179:3
    #2 0xe04fb58 in torch::distributed::rpc::ScriptCall::fromIValues(std::vector<c10::IValue, std::allocator<c10::IValue> >&) /pytorch_fuzz/torch/csrc/distributed/rpc/script_call.cpp:90:53
    #3 0xe0511f0 in torch::distributed::rpc::ScriptCall::fromMessage(torch::distributed::rpc::Message const&) /pytorch_fuzz/torch/csrc/distributed/rpc/script_call.cpp:133:10
    #4 0xe0ff71e in torch::distributed::rpc::deserializeRequest(torch::distributed::rpc::Message const&) /pytorch_fuzz/torch/csrc/distributed/rpc/utils.cpp:102:14
    #5 0x602a41 in LLVMFuzzerTestOneInput /message_deserialize_fuzz.cc:192:27
    #6 0x52ce61 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    #7 0x516d7c in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:324:6
    #8 0x51cacb in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:860:9
    #9 0x546062 in main /llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #10 0x7f41e42a8082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082)
    #11 0x51169d in _start (/message_deserialize_fuzz+0x51169d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /pytorch_fuzz/aten/src/ATen/core/ivalue.h:639:27 in c10::IValue::isString() const
==57==ABORTING
```
Pull Request resolved: pytorch#94297
Approved by: https://github.com/ezyang

torch/csrc/distributed/rpc/script_call.cpp

@@ -83,6 +83,10 @@ void ScriptCall::toIValues(std::vector<at::IValue>& ivalues) const {
 
 std::unique_ptr<ScriptCall> ScriptCall::fromIValues(
     std::vector<at::IValue>& ivalues) {
+  TORCH_INTERNAL_ASSERT(
+      ivalues.size() > 1,
+      "At least 2 IValues are required to build a ScriptCall.");
+
   // Last element in the vector is always qualifiedName for both
   // builitin operator and TorchScript function
   // If the qualifiedName is not a builtin operator name, then treat it