Removed file-extension-based arbitrary code execution attack vector (#2946)

# The Problem
Pickle files (.pkl, .ckpt, etc) are extremely unsafe as they can be
trivially crafted to execute arbitrary code when parsed using
`torch.load`
Right now the conventional wisdom among ML researchers and users is to
simply `not run untrusted pickle files ever` and instead only use
Safetensor files, which cannot be injected with arbitrary code. This is
very good advice.

Unfortunately, **I have discovered a vulnerability inside of InvokeAI
that allows an attacker to disguise a pickle file as a safetensor and
have the payload execute within InvokeAI.**

# How It Works
Within `model_manager.py` and `convert_ckpt_to_diffusers.py` there are
if-statements that decide which `load` method to use based on the file
extension of the model file. The logic (written in a slightly more
readable format than it exists in the codebase) is as follows:
```
if Path(file).suffix == '.safetensors':
   safetensor_load(file)
else:
   unsafe_pickle_load(file)
```

A malicious actor would only need to create an infected .ckpt file, and
then rename the extension to something that does not pass the `==
'.safetensors'` check, but still appears to a user to be a safetensors
file.
For example, this might be something like `.Safetensors`,
`.SAFETENSORS`, `SafeTensors`, etc.

InvokeAI will happily import the file in the Model Manager and execute
the payload.

# Proof of Concept
1. Create a malicious pickle file.
(https://gist.github.com/CodeZombie/27baa20710d976f45fb93928cbcfe368)
2. Rename the `.ckpt` extension to some variation of `.Safetensors`,
ensuring there is a capital letter anywhere in the extension (eg.
`malicious_pickle.SAFETENSORS`)
3. Import the 'model' like you would normally with any other safetensors
file with the Model Manager.
4. Upon trying to select the model in the web ui, it will be loaded (or
attempt to be converted to a Diffuser) with `torch.load` and the payload
will execute.


![image](https://user-images.githubusercontent.com/466103/224835490-4cf97ff3-41b3-4a31-85df-922cc99042d2.png)


# The Fix
This pull request changes the logic InvokeAI uses to decide which model
loader to use so that the safe behavior is the default. Instead of
loading as a pickle if the extension is not exactly `.safetensors`, it
will now **always** load as a safetensors file unless the extension is
**exactly** `.ckpt`.

# Notes:
I think support for pickle files should be totally dropped ASAP as a
matter of security, but I understand that there are reasons this would
be difficult.

In the meantime, I think `RestrictedUnpickler` or something similar
should be implemented as a replacement for `torch.load`, as this
significantly reduces the amount of Python methods that an attacker has
to work with when crafting malicious payloads
inside a pickle file. 
Automatic1111 already uses this with some success.
(https://github.com/AUTOMATIC1111/stable-diffusion-webui/blob/master/modules/safe.py)

invokeai/backend/model_management/convert_ckpt_to_diffusers.py

@@ -1075,9 +1075,10 @@ def load_pipeline_from_original_stable_diffusion_ckpt(
         dlogging.set_verbosity_error()
 
         checkpoint = (
-            load_file(checkpoint_path)
-            if Path(checkpoint_path).suffix == ".safetensors"
-            else torch.load(checkpoint_path)
+            torch.load(checkpoint_path)
+            if Path(checkpoint_path).suffix == ".ckpt"
+            else load_file(checkpoint_path)
+            
         )
         cache_dir = global_cache_dir("hub")
         pipeline_class = (

invokeai/backend/model_management/model_manager.py

@@ -732,9 +732,9 @@ class ModelManager(object):
 
         # another round of heuristics to guess the correct config file.
         checkpoint = (
-            safetensors.torch.load_file(model_path)
-            if model_path.suffix == ".safetensors"
-            else torch.load(model_path)
+            torch.load(model_path)
+            if model_path.suffix == ".ckpt"
+            else safetensors.torch.load_file(model_path)
         )
 
         # additional probing needed if no config file provided