[gnutls-devel] GnuTLS | ktls fallback to userspace (#1420)

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toidiu created an issue: https://gitlab.com/gnutls/gnutls/-/issues/1420



This was originally added as a comment: https://gitlab.com/gnutls/gnutls/-/merge_requests/1625#note_1145092003

Looking at the ktls support for `key_update` code and the feature claims that it supports fallback to userspace TLS if a key_update is received (and kernel patch is not applied). However, having done local testing with ktls in other TLS libraries, I dont think its possible to fallback to userspace and undo the TCP_ULP once it has been enabled and crypto_info has been set.

The current code is setting [`session->internals.ktls_enabled = 0;`](https://gitlab.com/gnutls/gnutls/-/blob/master/lib/tls13/key_update.c#L43) but the socket is still ktls enabled and has the previous encryption keys so from what I can tell there will be double encryption == garbage being sent on the wire.

```
ktls-enabled
plaintext -> ktls -> ciphertext

ktls-disabled
plaintext -> gnutls_encrypt -> ktls -> garbage
```

I might also be a good idea to add a test for the fallback scenario to verify behavior.

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