Unauthorized instruction execution via invalid MULHU operands

Improper instruction validation in OpenHW Group CVA6 commit 909d85a allows attackers to execute a prohibited MULHU sequence. The RTL fails to raise the required illegal-instruction exception when MULHU's high-result register matches either source register, so the multiplication unit accepts a register combination forbidden by the RISC-V ISA. Triggering the flaw requires code execution on a CVA6-based core and permits only the otherwise-illegal multiplication sequence; no broader data disclosure, privilege gain, or service interruption is documented.