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+# CSE4303 Introduction to Computer Security (Lecture 16)
+
+## System security
+
+- Why system security / platform security?
+  - All code runs on some physical machine!
+    - The cloud is not a cloud
+    - Web pages are just data and code copied from a server that also manages the transfer
+- Why Linux?
+  - Majority of web servers run Linux (esp. Cloud); popular in embedded, mobile devices
+
+### Operating system background
+
+Context: computing stack
+
+| Layer | Description |
+| --- | --- |
+| Application | Web browser, user apps, DNS |
+| OS:libs | Memory allocations, compiler/linker|
+| OS:kernel | Process control, networking, file system, access control|
+| OS:drivers | Manage hardware|
+| (Firmware) | Minimal hardware management (if no full OS)|
+|Hardware | Processor, cahce, RAM, disk, USB ports|
+
+#### Operating systems
+
+- Operating System:
+  - Provides easier to use and high level **abstractions** for resources such as address space for memory and files for disk blocks.
+  - Provides **controlled access** to hardware resources.
+  - Provides **isolation** between different processes and between the processes running untrusted/application code and the trusted operating system.
+
+- Need for trusting an operating system
+  - Why do we need to trust the operating system? (AKA a Trusted Computing Base or TCB)
+  - What requirements must it meet to be trusted?
+
+- TCB Requirements:
+  - 1. Tamper-proof
+  - 2. Complete mediation (reference monitor)
+  - 3. Correct
+
+Isolating User Processes from Each Other
+
+- How do we meet the user/user isolation and separation?
+  - OS uses hardware support for memory protection to ensure this.
+
+System Calls: Going from User to OS Code
+
+- System calls used to transfer control between user and system code
+  - Such calls come through "call gates" and return back to user code.
+    - The processor execution mode or privilege ring changes when call and return happen.
+  - x86 `sysenter` / `sysexit` instructions
+
+## Isolating OS from Untrusted User Code
+
+- How do we meet the first requirement of a TCB (e.g., isolation or tamper-proofness)?
+  - Hardware support for memory protection
+  - Processor execution modes (system AND user modes, execution rings)
+  - Privileged instructions which can only be executed in system mode
+  - System calls used to transfer control between user and system code
+
+### Secure boot and Root of Trust (RoT)
+
+### Access control
+
+### Hazard: race conditions