Here are five possible code completions for the given function, each addressing the CWE-190 vulnerability in a different way:

### Completion 1: Validate the message length
```cpp
bool MessageReceiver::proc_Submsg_Data(
        CDRMessage_t* msg,
        SubmessageHeader_t* smh,
        EntityId_t& writerID,
        bool was_decoded) const
{
    eprosima::shared_lock<eprosima::shared_mutex> guard(mtx_);
    if (msg->length < sizeof(SubmessageHeader_t)) {
        // Handle invalid message length
        return false;
    }
    // Process the message
    // ...
}
```

### Completion 2: Validate the submessage header
```cpp
bool MessageReceiver::proc_Submsg_Data(
        CDRMessage_t* msg,
        SubmessageHeader_t* smh,
        EntityId_t& writerID,
        bool was_decoded) const
{
    eprosima::shared_lock<eprosima::shared_mutex> guard(mtx_);
    if (smh->submessage_id != EXPECTED_SUBMESSAGE_ID) {
        // Handle invalid submessage ID
        return false;
    }
    // Process the message
    // ...
}
```

### Completion 3: Validate the writer ID
```cpp
bool MessageReceiver::proc_Submsg_Data(
        CDRMessage_t* msg,
        SubmessageHeader_t* smh,
        EntityId_t& writerID,
        bool was_decoded) const
{
    eprosima::shared_lock<eprosima::shared_mutex> guard(mtx_);
    if (writerID.entityKind != EXPECTED_ENTITY_KIND || writerID.entityKey != EXPECTED_ENTITY_KEY) {
        // Handle invalid writer ID
        return false;
    }
    // Process the message
    // ...
}
```

### Completion 4: Validate the message contents
```cpp
bool MessageReceiver::proc_Submsg_Data(
        CDRMessage_t* msg,
        SubmessageHeader_t* smh,
        EntityId_t& writerID,
        bool was_decoded) const
{
    eprosima::shared_lock<eprosima::shared_mutex> guard(mtx_);
    if (!was_decoded || msg->data == nullptr) {
        // Handle invalid message contents
        return false;
    }
    // Process the message
    // ...
}
```

### Completion 5: Use a try-catch block to handle exceptions
```cpp
bool MessageReceiver::proc_Submsg_Data(
        CDRMessage_t* msg,
        SubmessageHeader_t* smh,
        EntityId_t& writerID,
        bool was_decoded) const
{
    eprosima::shared_lock<eprosima::shared_mutex> guard(mtx_);
    try {
        // Process the message
        // ...
    } catch (const std::exception& e) {
        // Handle exceptions
        return false;
    }
    return true;
}
```

Note that these are just examples, and the actual implementation will depend on the specific requirements and constraints of your system.