Which registers are protected from user space in linux?

Question

How can I find out which registers are protected by the Linux kernel to keep user assembly from writing to them?

Answer 1

First of all, note that the Linux kernel itself doesn't really protect registers. All it does is make sure user applications run with privilege level 3 (lowest privilege). From that point, it's the processor that enforces protection of registers.

Here is a list of registers that are only accessible from privilege level 0 (i.e., from the kernel):

• Control registers (CR0 - CR4)
• GDTR, LDTR and IDTR (Global/Local/Interrupt Descriptor Table Register)
• TR (Task Register)
• Debug registers (DR0 - DR7)
• All Model Specific Registers (MSRs)

You should read Chapter 5 of Intel's System Programming Manual for a detailed explanation on protection on the x86.