What is the error of "linux unable to handle kernel paging request at ffffffff00000010"?

Question

I have written some Linux kernel code, which causes runtime error, and reports linux unable to handle kernel paging request at ffffffff00000010.

It is just a code to hook the open system call in Linux kernel programming.

The code is listed below:

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <asm/uaccess.h>
#include <asm/fcntl.h>
#include <asm/unistd.h>
#include <asm/ia32_unistd.h>
#include <asm/msr.h>
unsigned long *sys_table = NULL;
asmlinkage long (*old_open) (const char __user *filename, int flags, umode_t mode);
static void *memmem(const void *haystack, size_t haystack_len,
            const void *needle, size_t needle_len);
#define dbg(format,args...) \
    printk("intercept: function:%s-L%d: "format, __FUNCTION__, __LINE__, ##args);
asmlinkage long new_open(char *filename, int flags, int mode)
{
    printk("call open()\n");
    return old_open (filename, flags, mode);
}
unsigned int clear_and_return_cr0(void)
{
    unsigned long cr0 = 0;
    unsigned long ret;
    asm volatile ("movq %%cr0, %%rax"
              : "=a"(cr0)
              );
    ret = cr0;
    /* clear the 20 bit of CR0, a.k.a WP bit */
    cr0 &= 0xfffffffffffeffff;
    asm volatile ("movq %%rax, %%cr0"
              :
              : "a"(cr0)
              );
    return ret;
}
void setback_cr0(unsigned long val)
{
    asm volatile ("movq %%rax, %%cr0"
              :
              : "a"(val)
              );
}
static unsigned long get_syscall_table_long(void) 
{ 
    #define OFFSET_SYSCALL 200 
    unsigned long syscall_long, retval; 
    char sc_asm[OFFSET_SYSCALL]; 
    rdmsrl(MSR_LSTAR, syscall_long); 
    memcpy(sc_asm, (char *)syscall_long, OFFSET_SYSCALL); 
    retval = (unsigned long) memmem(sc_asm, OFFSET_SYSCALL, "/xff/x14/xc5", 3); 
    if ( retval != 0 ) {
        retval = (unsigned long) ( * (unsigned long *)(retval+3) ); 
    } else { 
        printk("long mode : memmem found nothing, returning NULL:("); 
        retval = 0; 
    }
    #undef OFFSET_SYSCALL 
    return retval; 
}
static void *memmem(const void *haystack, size_t haystack_len, 
            const void *needle, size_t needle_len) 
{
    const char *begin; 
    const char *const last_possible = (const char *) haystack + haystack_len - needle_len;
    if (needle_len == 0){ 
        /* The first occurrence of the empty string is deemed to occur at 
          the beginning of the string. */ 
        return (void *) haystack;
    }
    if (__builtin_expect(haystack_len < needle_len, 0)){ 
        return NULL;
    }
    for (begin = (const char *) haystack; begin <= last_possible; ++begin) 
    { 
        if (begin[0] == ((const char *) needle)[0] 
            && !memcmp((const void *) &begin[1], 
                  (const void *) ((const char *) needle + 1), 
                  needle_len - 1)){
            return (void *) begin; 
        }
    }
    return NULL; 
}
static int init_sys_call_table(void)
{
    printk("init_sys_call_table\n");
    unsigned long orig_cr0 = clear_and_return_cr0();
    printk("orig_cr0 %lu\n",orig_cr0);
    sys_table = (unsigned long *) get_syscall_table_long();
    if (sys_table == 0){
        dbg("sys_table == 0/n");
        return -1;
    }
    sys_table = (unsigned long)sys_table | 0xffffffff00000000;
#define REPLACE(x) old_##x = sys_table[__NR_##x];\
    sys_table[__NR_##x] = new_##x
    REPLACE(open);
    setback_cr0(orig_cr0);
    return 0;
}
static void clean_sys_call_table(void)
{
    unsigned long orig_cr0 = clear_and_return_cr0();
#define RESTORE(x) sys_table[__NR_##x] = old_##x
    RESTORE(open);
    setback_cr0(orig_cr0);
    return ;
}
static int __init init_64mod(void)
{
    init_sys_call_table();
    return 0;
}
static void __exit exit_64mod(void)
{
    clean_sys_call_table();
}
module_init(init_64mod);
module_exit(exit_64mod);
MODULE_AUTHOR("[email protected]");

Answer 1

It means that somewhere in your code you have an invalid pointer that you are trying to access. I cannot debug your code on the fly but I can give you some suggestions:

• try to avoid casting until is strictly necessary
• when you are casting to a pointer, double check that it is what you want to do
• in the error message there is also the stack, take a look at it in order to identify where is the error
• you can simply put some printk("%p", pointer) in your code to check the content of your variables. Alternatively, you can use systemtap or similar tools.