Protection and security

Vocabulary

English	Meaning
rings	Hardware privilege levels controlling access to protected processor operations.
kernel mode	A privileged processor mode used to run trusted operating-system code.
system call	An interface through which a program requests a service from the operating-system kernel.
least privilege	Granting only the permissions needed to carry out an authorised task.

Two rings, one door

User mode runs application code; kernel mode runs the OS and may touch devices and page tables. The CPU's mode bit enforces the rings; the crossing points are system calls — the wall's only doors.

{exercise:uni-cs0706-rings}

Practice

User code reaches kernel services only through —

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open("/notes/x") from user mode: the trap switches the mode bit, saves user registers, jumps to a fixed vector; the kernel validates the path and fills a descriptor; the mode bit flips back.
Two crossings per syscall at about 100 ns each: a program doing $10^6$ syscalls pays $0.2$ s — the reason libraries buffer and batch.

{exercise:uni-cs0706-crossing}

Practice

One syscall costs two crossings of about 100 ns; a million syscalls cost —

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Least privilege gives each process only what it needs: page tables do it per process, file permissions do it per user ($9$ bits: read/write/execute for owner/group/other), capabilities do it per object.
A privileged printer daemon asked to print /salary.db uses its own authority, not the user's — the confused deputy. The fix: pass capabilities (open descriptors), not names. Validate every crossing; a smaller privileged base is a smaller target.

{exercise:uni-cs0706-least}

Practice

The confused deputy is fixed by passing —

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