LC

Understand Privilege Levels: Ring 0 vs Ring 3

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Add Sandbox Using seccomp-bpf to Whitelist Allowed Syscalls Implement System Call Fuzzer: Send Invalid Arguments

Language

Compiler

2.0s Limit 256MB

Implementation

Demonstrate privilege level transitions.

Requirements:

Explain Ring 0 (kernel) vs Ring 3 (user)
Show how syscalls change privilege level
Demonstrate protection violation
Explain CPL (Current Privilege Level)
Show segment descriptor and page table bits
Test: accessing kernel memory from userspace fails

Test:

Show kernel entry via syscall
Show protection violation

Why This Matters

Ring 0 versus Ring 3 is the hardware boundary that made the Meltdown vulnerability so severe in 2018: it broke the assumption that Ring 3 code could never read Ring 0 memory, forcing every OS vendor to ship KPTI (kernel page-table isolation) patches with a real performance cost. Hypervisors like KVM add a Ring -1 on top of this exact model, which is why understanding CPL transitions here also explains how virtual machines stay isolated from each other.

Hints

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Validation Suite

main.c

Workspace 1.0