Kernel memory disclosure via uninitialized ARM sigpage

Information disclosure in the ARM sigpage of Linux Kernel 5.4.54 and 5.4.66 allows local users to read residual kernel memory. The ARM get_signal_page function allocates a page and copies signal-return code into it without first initializing the remaining bytes, then maps the whole page into user space. Running an ARM userland process is sufficient to inspect the mapped sigpage; the consequence is limited to disclosure of leftover kernel memory.