CVE-2026-64389: ksmbd: validate NTLMv2 response before updating session key

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: validate NTLMv2 response before updating session key

ksmbdauthntlmv2() derives the NTLMv2 session key into sess->sesskey before it verifies the NTLMv2 response. ksmbddecodentlmsspauthblob() then continues into KEYXCH even when ksmbdauthntlmv2() failed.

With SMB3 multichannel binding, the failed authentication operates on an existing session and the session setup error path does not expire binding sessions. A client can send a binding session setup with a bad NT proof and KEYXCH and still modify sess->sesskey before STATUSLOGONFAILURE is returned.

Relevant path:

smb2sesssetup() -> conn->binding = true -> ntlmauthenticate() -> sessionuser() -> ksmbddecodentlmsspauthblob() -> ksmbdauthntlmv2() -> calcntlmv2hash() -> hmacmd5usingrawkey(..., sess->sesskey) -> cryptomemneq() returns mismatch -> KEYXCH arc4crypt(..., sess->sesskey, ...) -> outerr without expiring the binding session

Derive the base session key into a local buffer and copy it to sess->sesskey only after the proof matches. Return immediately on authentication failure so KEYXCH is only processed after successful authentication.