System calls require address checks

As a security measure, system calls such as read() or any other system call that modifies memory, must verify that the user-provided addresses lie in user-space. Otherwise, this opens a back door that may be exploited to take control of the OS or to crash the OS.

This applies only to PROTECTED and KERNEL build modes.

Start by tracing the read() system call and other system calls that modify memory, focusing on the PROTECTED and KERNEL build modes. The work is done when user-provided addresses are checked as user-space addresses before memory is modified, with the relevant system-call behavior covered by tests.

Written by the indexing model from the issue text.