The good driver consists of P1+C1+X. That's what gets sent to the signing authority. They verify it doesn't do anything bad and return a signature listing SHA

The good driver consists of P1+C1+X. That's what gets sent to the signing authority. They verify it doesn't do anything bad and return a signature listing SHA1(P1+C1+X). But that signature is also valid for the malicious driver P2+C2+X.