how does an interrupt put CPU into the required privilege level?

Question

I'm not quite understanding one sentence from WIKI about the System Call "The operating system executes at the highest level of privilege, and allows applications to request services via system calls, which are often executed via interrupts; an interrupt automatically puts the CPU into some required privilege level, and then passes control to the kernel, which determines whether the calling program should be granted the requested service."

How physically can an CPU be put into a certain privilege level and what does it mean by passing the control to kernel? Please explain these in the CPU-registers level.

Answer 1

This is an excellent question and privilege levels are one of the most beautiful concepts of Operating Systems.

This forum however is not the right place to ask.

However since you've asked, I'll paint you a general picture. Now you know that the OS does a lot of scheduling of processes. The scheduler must be called at periodic intervals. The CPU maintains a counter which causes a Timer interrupt.

The code which handles the Timer interrupt calls the scheduler. Now during scheduling OS level data structures are modified (process queues, etc.). At this point, if the user program were to be active for some reason, it can mess with those data structures leading to a crash.

This is handled via privilege levels. So, during scheduling, the CPU is said to be in a privilege mode - the kernel mode. The user programs can't access the CPU now.

Here comes the awesome part now. If suppose this switch in privilege level was to be made by the software, if there was a command, it could potentially be exploited by malicious user programs.

For this reason, we can't rely on the software to do the switch. We need hardware support. The hardware is designed so that receiving interrupts sets the "privilege bit register". When the interrupt code is finished (scheduling is done), the return causes the hardware to clear the bit.

The interrupt handling code is located in a protected area in the memory reserved for OS code. User programs can't access this code (If it tries to access that part of the memory, an exception is thrown by the hardware).

Thus sanity is preserved.