To enhance resource allocation and security, computers divide the operating system (OS) into user and kernel modes. This separation protects the system’s core functionality and ensures stability by isolating more abstract functions from critical components during operation.
The CPU switches between user and kernel modes depending on the code being executed, with user applications typically restricted to user mode and basic OS components functioning in kernel mode.
User mode provides restricted access to hardware and resources, limiting application privileges to prevent interference with system stability. In contrast, kernel mode allows unrestricted access to system resources and hardware, enabling core OS functions. In kernel mode, all code shares the same memory address space, allowing the CPU to switch between programs and access user and kernel memory directly.
While user applications like word processors operate in user mode to prevent data interference, critical tasks such as process management run in kernel mode. Interrupts triggered by components like hardware or system updates necessitate system calls from user mode to switch to kernel mode and access necessary resources.
Kernel mode operates at the most privileged level in Ring 0, while user mode operates at the least privileged level in Ring 3. When trusted processes, like security software or networking protocols, require kernel-level access, they use system calls to interact with hardware and perform essential tasks.
The 2024 CrowdStrike outage exemplifies the importance of running only trustworthy processes in kernel mode to prevent system crashes. A malfunctioning content validator in the CrowdStrike Falcon sensor allowed problematic data to pass through during an update, causing the software and Windows machines that received it to crash. This incident underscores the critical role of trust and validation in maintaining system stability and security.