CVE-2024-30251: DoS in aiohttp

[Sam Bull <9m199i () sambull org>]

Aiohttp is an HTTP client and server-side web framework in Python. This issue only affects
users of the server-side web framework. We've not seen any evidence of this being
exploited in the wild yet, and fixes were already included in the 3.9.4 and 3.9.5
releases.

https://github.com/aio-libs/aiohttp/security/advisories/GHSA-5m98-qgg9-wh84

### Summary
An attacker can send a specially crafted POST (multipart/form-data) request. When the
aiohttp server processes it, the server will enter an infinite loop and be unable to
process any further requests.

### Impact
An attacker can stop the application from serving requests after sending a single request.

-------

For anyone needing to patch older versions of aiohttp, the minimum diff needed to resolve
the issue is (located in `_read_chunk_from_length()`):

   diff --git a/aiohttp/multipart.py b/aiohttp/multipart.py
   index 227be605c..71fc2654a 100644
   --- a/aiohttp/multipart.py
   +++ b/aiohttp/multipart.py
   @@ -338,6 +338,8 @@ class BodyPartReader:
            assert self._length is not None, "Content-Length required for chunked read"
            chunk_size = min(size, self._length - self._read_bytes)
            chunk = await self._content.read(chunk_size)
   +        if self._content.at_eof():
   +            self._at_eof = True
            return chunk
    
        async def _read_chunk_from_stream(self, size: int) -> bytes:

This does however introduce some very minor issues with handling form data. So, if
possible, it would be recommended to also backport the changes in:
https://github.com/aio-libs/aiohttp/commit/cebe526b9c34dc3a3da9140409db63014bc4cf19
https://github.com/aio-libs/aiohttp/commit/7eecdff163ccf029fbb1ddc9de4169d4aaeb6597
https://github.com/aio-libs/aiohttp/commit/f21c6f2ca512a026ce7f0f6c6311f62d6a638866

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