(This is an updated version of an older post - the introduction remains the same, the technical details are different) Linux draws a distinction between code running in kernel (kernel space) and applications running in userland (user space). This is enforced at the hardware level - in x86-speak1, kernel space code runs in ring 0 and user space code runs in ring 32. If you’re running in ring 3 and you attempt to touch memory that’s only accessible in ring 0, the hardware will raise a fault.