System call interception in linux-kernel module (kernel 3.5)

Question

I need to replace a standard system call (e.g. SYS_mkdir) with my own implementation.

As I read in some sources, including this question on Stackoverflow, the sys_call_table is not exported symbol since kernel version 2.6.

I tried the following code:

    #include <linux/module.h> 
    #include <linux/kernel.h> 
    #include <linux/unistd.h> 
    #include <asm/syscall.h> 

    int (*orig_mkdir)(const char *path); 

    ....

    int init_module(void) 
    { 
            orig_mkdir=sys_call_table[__NR_mkdir]; 
            sys_call_table[__NR_mkdir]=own_mkdir;  
            printk("sys_mkdir replaced\n"); 
            return(0); 
    } 

    ....

Unfortunately I receive compiler error:

 error: assignment of read-only location ‘sys_call_table[83]’

How can I replace the system call?

EDIT: Is there any solution without kernel patching?

Answer 1

this works for me.

See Linux Kernel: System call hooking example and https://bbs.archlinux.org/viewtopic.php?id=139406

asmlinkage long (*ref_sys_open)(const char __user *filename, int flags, umode_t mode);
asmlinkage long new_sys_open(const char __user *filename, int flags, umode_t mode)
{
  return ref_sys_open(filename, flags, mode);
}

static unsigned long **aquire_sys_call_table(void)
{
  unsigned long int offset = PAGE_OFFSET;
  unsigned long **sct;

  while (offset < ULLONG_MAX) {
    sct = (unsigned long **)offset;

    if (sct[__NR_close] == (unsigned long *) sys_close) 
      return sct;

    offset += sizeof(void *);
  }
  print("Getting syscall table failed. :(");
  return NULL;
}


// Crazy copypasted asm stuff. Could use linux function as well...
// but this works and will work in the future they say.
static void disable_page_protection(void) 
{
  unsigned long value;
  asm volatile("mov %%cr0, %0" : "=r" (value));

  if(!(value & 0x00010000))
    return;

  asm volatile("mov %0, %%cr0" : : "r" (value & ~0x00010000));
}

static void enable_page_protection(void) 
{
  unsigned long value;
  asm volatile("mov %%cr0, %0" : "=r" (value));

  if((value & 0x00010000))
    return;

  asm volatile("mov %0, %%cr0" : : "r" (value | 0x00010000));
}


static int __init rootkit_start(void) 
{

  //Hide me

  print("loaded");

  if(!(sys_call_table = aquire_sys_call_table()))
    return -1;

  disable_page_protection(); 
  {
    ref_sys_open = (void *)sys_call_table[__NR_open];
    sys_call_table[__NR_open] = (unsigned long *)new_sys_open;
  }
  enable_page_protection();
  return 0;
}

static void __exit rootkit_end(void) 
{
  print("exiting");

  if(!sys_call_table) {
    return;
  }

  disable_page_protection();
  {
    sys_call_table[__NR_open] = (unsigned long *)ref_sys_open;
  }
  enable_page_protection();
}