Kernel privilege escalation via signed syscall check

Privilege escalation in affected Zephyr RTOS releases allows local users with user-thread code execution to gain kernel privileges. ARM and ARC syscall dispatch code compares the attacker-controlled syscall number to the table limit as a signed integer, so a large unsigned value passes the check and selects an attacker-controlled function pointer. Exploitation requires Zephyr userspace and existing code execution in a user thread; successful use executes code with kernel privileges.