fix(ktls): Validate record type in s2n_ktls_recvmsg - #6030
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Reject unexpected TLS ContentType values received via kTLS ancillary data. Only alert (21), handshake (22), and application_data (23) are valid post-handshake record types. All other values now fail with S2N_ERR_BAD_MESSAGE per RFC 8446 Section 5. Add an exhaustive unit test that injects all 256 possible byte values as the record type and verifies the accept/reject behavior.
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This kind feels like the wrong place to handle this. All of the record types eventually bubble up through s2n_read_full_record, so I'd think that any validation we're doing should probably just be done there?
Also it might be easier to pause this until #6017, since that actually does some similar assertion cleanup?
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Goal
Validate the TLS ContentType received via kTLS ancillary data in
s2n_ktls_recvmsg.Why
RFC 8446 Section 5: "If a TLS implementation receives an unexpected record type, it MUST terminate the connection with an 'unexpected_message' alert."
How
Range check after extracting the record type from the control message. Only values 21–23 (alert, handshake, application_data) are accepted. All others fail with
S2N_ERR_BAD_MESSAGE.Testing
Exhaustive test iterating all 256 byte values as the record type. Values {21, 22, 23} succeed; all others are rejected.
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