How to access the control registers cr0,cr2,cr3 from a program? Getting segmentation fault

Question

I have written a program which tries to read from and write to the control registers.

The program compiles fine, but when the inline assembly is about to be executed, it produces a segmentation fault.

Code:

void instructions(int val)
{
    int i;
    int value;
    for(i = 0; i < val; i++)
         __asm__("mov %cr0, %eax");
}

I used GDB and stepped through each assembly line and it is on the mov %cr0,%eax that the segmentation fault is occurring.

Anyone who knows what is wrong?

Answer 1

Quoting from Intel® 64 and IA-32 Architectures Software Developer Manuals 3-650 Vol. 2A on moving to and from control registers:

This instruction can be executed only when the current privilege level is 0.

Which means the instruction can only be executed in kernel mode.

A minimal kernel module, that logs the contents of cr0, cr2 and cr3 could look something like this (32-bit code path untested):

/* hello.c */
#include <linux/module.h>
#include <linux/kernel.h>

int init_module(void)
{
#ifdef __x86_64__
    u64 cr0, cr2, cr3;
    __asm__ __volatile__ (
        "mov %%cr0, %%rax\n\t"
        "mov %%eax, %0\n\t"
        "mov %%cr2, %%rax\n\t"
        "mov %%eax, %1\n\t"
        "mov %%cr3, %%rax\n\t"
        "mov %%eax, %2\n\t"
    : "=m" (cr0), "=m" (cr2), "=m" (cr3)
    : /* no input */
    : "%rax"
    );
#elif defined(__i386__)
    u32 cr0, cr2, cr3;
    __asm__ __volatile__ (
        "mov %%cr0, %%eax\n\t"
        "mov %%eax, %0\n\t"
        "mov %%cr2, %%eax\n\t"
        "mov %%eax, %1\n\t"
        "mov %%cr3, %%eax\n\t"
        "mov %%eax, %2\n\t"
    : "=m" (cr0), "=m" (cr2), "=m" (cr3)
    : /* no input */
    : "%eax"
    );
#endif
    printk(KERN_INFO "cr0 = 0x%8.8X\n", cr0);
    printk(KERN_INFO "cr2 = 0x%8.8X\n", cr2);
    printk(KERN_INFO "cr3 = 0x%8.8X\n", cr3);
    return 0;
}

void cleanup_module(void)
{
}

 

# Makefile

obj-m += hello.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

test: all
    sudo insmod ./hello.ko
    sudo rmmod hello
    dmesg | tail