Kernel compromise via shared overflow shadow stack

Race in the RISC-V vmap stack-overflow handler in affected Linux kernels allows local users to corrupt kernel execution during concurrent overflows. The handler switches to one global shadow_stack before calling get_overflow_stack(), so two or more overflowing harts can overwrite the same temporary stack. Local kernel execution and a narrow cross-hart timing window are required; resulting stack corruption can compromise kernel confidentiality, integrity, and availability.