Hijack Linux System Calls: Part III. System Call Table

This is the last part of the Hijack Linux System Calls series. By now, we have created a simple loadable kernel module which registers a miscellaneous character device. This means, that we have everything we need in order to patch the system call table. Almost everything, to be honest. We still have to fill the our_ioctl function and add a couple of declarations to our source file. By the end of this article we will be able to intercept any system call in our system should there be a need for that.

System Call Table

System Call table is simply an area in the kernel memory space that contains addresses of system call handlers. Actually, a system call number is an offset into that table. This means that when we call sys_write (to be more precise - when libc calls sys_write) on a 32 bit system and passes number 4 in EAX register before int 0x80, it simply tells the kernel to go to the system call table, get the value at offset 4 from the system call table's address and call the function that address points to. It may be number 1 in RAX in case of a 64 bit system (and syscall instead of int 0x80). System call numbers are defined in arch/x86/include/asm/unistd_32.h and arch/x86/include/asm/unistd_64.h for 32 and 64 bit platforms respectively. In this article, we are going to deal with sys_open system call which is number 5 for 32 bit systems and number 2 for 64 bit systems.

Due to the fact, that modern kernels do not export the sys_call_table symbol any more, we will have to find its location in memory ourselves. There are some "hackish" ways of finding the location of the sys_call_table programmatically, but the problem is that they may work, but may not work as well. Especially the way they are written. Therefore, we are going to use the simplest and the safest way - read its location from /boot/System.map file. For simplicity reasons, we will just use grep and hardcode the address. On my computer, the command grep "sys_call_table" /boot/System.map (you should check the file name on your system, as on mine it is /boot/System.map-2.6.38-11-generic) gives this output "ffffffff816002e0 R sys_call_table". Add global variable unsigned long *sys_call_table = (unsigned long*)0xYour_Address_Of_Sys_call_table.

Preparations

We will start, as usual, by adding new includes to our code. This time, those include files are:

   #include <linux/highmem.h>

   #include <asm/unistd.h>

The first one is needed due to the fact that system call table is located in read only memory area in modern kernels and we will have to modify the protection attributes of the memory page containing the address of the system call that we want to intercept. The second one is self explanatory after the previous paragraph. We are not going to use hardcoded values for system calls, instead, we will use the values defined in unistd.h header.

Now we define two values, which would be used as cmd argument to our_ioctl function. One will tell us to patch the table, another one will tell us to fix it by restoring the original value.

   /* IOCTL commands */

   #define IOCTL_PATCH_TABLE 0x00000001

   #define IOCTL_FIX_table   0x00000004

Add one more global variable int is_set=0 which will be used as flag telling whether the real (0) or custom(1) system call is in use.

It is important to save the address of the original sys_open as we are not going to fully implement our own, instead, our function will log information about the call arguments and then perform the actual (original) call. Therefore, we define a function pointer (for original call) and a function (for custom call):

   /* Pointer to the original sys_open */

   asmlinkage int (*real_open)(const char* __user, int, int);

   

   /* Our replacement */

   asmlinkage int custom_open(const char* __user file_name, int flags, int mode)

   {

      printk("interceptor: open(\"%s\", %X, %X)\n", file_name,

                                                    flags, 

                                                    mode);

      return real_open(file_name, flags, mode);

   }

You have noticed the "asmlinkage" attribute. Well, it is, actually, a define for the attribute. We will not go that deep this time, I will just say that this attribute tells the compiler about how it should pass arguments to the function, given that it is being called from an assembly code. The "__user" macro, signifies that the argument is in user space and the function must perform certain operations to copy it to kernel space when needed. We do not need that, meaning that we may ignore it for now.

Another couple of crucial functions is the set that will allow us modify the memory page protection attributes directly. One may say that his is risky, but, in my opinion, this is less risky then actually patching the system call table as it is, first of all, architecture dependent  and we know that architectures do not change drastically, second - we use kernel functions for that.

   /* Make the page writable */

   int make_rw(unsigned long address)

   {

      unsigned int level;

      pte_t *pte = lookup_address(address, &level);

      if(pte->pte &~ _PAGE_RW)

         pte->pte |= _PAGE_RW;

      return 0;

   }

   /* Make the page write protected */

   int make_ro(unsinged long address)

   {

      unsigned int level;

      pte_t *pte = lookup_address(address, &level);

      pte->pte = pte->pte &~ _PAGE_RW;

      return 0;

   }

pte_t stands for typedef struct { unsigned long pte } pte_t and represents the page table entry Although, it is simply an unsigned long, it is declared as struct in order to avoid type misuse.

pte_t *lookup_address(unsigned long address, unsigned int *level) is provided by the kernel and performs all the dirty work for us and returns a pointer to the page table entry that describes the page containing the address. This function accepts the following arguments:

address - an address in virtual memory;

level - pointer to unsigned integer value which accepts the level of the mapping.

Let's Get to Business

We are almost there. The only thing left is the actual implementation of the our_ioctl function. Add the following lines:

   switch(cmd)

   {

      case IOCTL_PATCH_TABLE:

         make_rw((unsigned long)sys_call_table);

         real_open = (void*)*(sys_call_table + __NR_open);

         *(sys_call_table + __NR_open) = (unsigned long)custom_open;

         make_ro((unsigned long)sys_call_table);

         is_set=1;

         break;

      case IOCTL_FIX_TABLE:

         make_rw((unsigned long)sys_call_table);

         *(sys_call_table + __NR_open) = (unsigned long)real_open;

         make_ro((unsigned long)sys_call_table);

         is_set=0;

         break;

      default:

         printk("Ooops....\n");

         break;

   }

And these lines to the cleanup_module function:

   if(is_set)

   {

      make_rw((unsigned long)sys_call_table);

      *(sys_call_table + __NR_open) = (unsigned long)real_open;

      make_ro((unsigned long)sys_call_table);

   }

Our interceptor module is ready. Well, almost ready as we need to compile it. Do that as usual - make.

Test

Finally, we have our module set and ready to use, but we have to create a "client" application, the code that will "talk" to our module and tell it what to do. Fortunately, this is much simpler then the rest of the work, that we have done here. Create a new source file and enter the following lines:


   #include <stdio.h>
   #include <sys/ioctl.h>
   #include <sys/types.h>
   #include <sys/stat.h>
   #include <fcntl.h>


   /* Define ioctl commands */
   #define IOCTL_PATCH_TABLE 0x00000001
   #define IOCTL_FIX_TABLE   0x00000004


   int main(void)
   {
      int device = open("/dev/interceptor", O_RDWR);
      ioctl(device, IOCTL_PATCH_TABLE);
      sleep(5);
      ioctl(device, IOCTL_FIX_TABLE);
      close(device);
      return 0;
   }


save it as manager.c and compile it with gcc -o manager manager.c. 


Load the module, run ./manager and then unload the module when manager exits. If you issue the dmesg | tail command. If you see lines containing "interceptor: open(blah blah blah)", then you know that those lines were produced by our handler.




Now we are able to intercept system calls in modern kernels despite the fact that sys_call_table is no longer exported. Although, we deal with low level structures, which normally are only used by kernel, this still is a relatively safe method as long as your module is compiled against the running kernel.


Hope this post was helpful. See you at the next one!

Hijack Linux System Calls: Part II. Miscellaneous Character Drivers

We all know what device drivers are - the hands of the operating system that make it possible for the kernel to handle hardware.  We also know that there are two types of devices  - character and block, depending on the way they handle data transmissions, but what does "miscellaneous" device mean? To put it simple - it means what it means. On one hand, this may be a driver that handles simple hardware, on the other hand, it is the way Linux allows us to create virtual devices, as one of the ways to communicate with kernel modules, which is exactly what we need in order to hijack Linux System Calls. 

In this section, we will create a simple virtual character device, which will be used by a user space process to instruct our kernel module whether it should hijack or restore certain system call. This virtual device will be controlled with ioctl function. For simplicity, I decided not to add read/write handlers to this device as it is not really required for what we are about to do. Although, it is a "nice to have" feature.

Miscellaneous devices are represented with the struct miscdevice which is declared in /include/linux/miscdevice.h as

   struct miscdevice

   {

      int minor;

      const char *name;

      const struct file_operations *fops;

      struct list_head list;

      struct device *parent;

      struct device *this_device;

      const char *nodename;

      mode_t mode;

   };

Quite a big one, ha? However, we only should take care of the first three members of the structure:

minor stands for the minor number of the device. It is preferred to set it to

              MISC_DYNAMIC_MINOR (at least in for our module) unless you need some specific

              number to be used.

name  this is the name of our device as it should appear in the /dev filesystem

struct file_operations is a set of pointers to corresponding implementations of IO

              functions and a pointer to the owner module. 

              This structure is too big to be presented here, but you may find it in 

              /include/linux/fs.h

So, first of all, add another include file to your code (which we've written in previous article) with

   #include <linux/miscdevice.h>

Then, we should add custom handlers for functions and global variables we are interested in, namely - open, release, ioctl and variable in_use.

   /* We will set this variable to 1 in our open handler and erset it

       back to zero in release handler*/

   int in_use = 0;

   /* This function will be invoked each time a user process attempts

       to open our device. You should keep in mind that the prototype

      of this function may change along different kernel versions. */

   static int our_open(struct inode *inode, struct file *file)

   {

      /* We would not like to allow multiple processes to open this device */

      if(in_use)

         return -EBUSY;

      in_use++;

      printk("device has been opened\n");

      return 0;

   }

   /* This function, in turn, will be called when a process closes our device */

   static int our_release(struct inode *inode, struct file *file)

   {

      in_use--;

      printk("device has been closed\n");

      return 0;

   }

   /* This function will handle ioctl calls performed on our device */

   static int our_ioctl(struct file *file, unsigned int cmd, unsigned long arg)

   {

      int retval = 0;

      

      /* We will fill this function in the Part III of this series */

      return retval;

   }

Now it's time to create the struct file_operations and struct miscdevice and populate the relevant fields:

   static const struct file_operations our_fops =\

   {

      .owner = THIS_MODULE,

      .open = &our_open,

      .release = &our_release,

      .unlocked_ioctl = (void*)&our_ioctl,

      .compat_ioctl = (void*)&our_ioctl

   }

A reasonable question would be "Why do we set unlocked_ioctl and compat_ioctl with the same value and where the heck is the regular ioctl?". There is nothing special about this. unlocked_ioctl is used on 64 bit platforms and compat_ioctl in 32 bit or in compatibility mode and it is totally normal to make them point at the same location as long as you handler function does not mess the types up. As to ioctl, it is simply not there any more...

   static struct miscdevice our_device = \

   {

      MISC_DYNAMIC_MINOR,

      "interceptor",

      &our_fops

   };

After all this, we should make a small adjustment to our init_module function by inserting the following code:

   int retval = misc_register(&our_device);

You should also change "return 0;" to "return retval;". The code above tells the system to register a miscellaneous device described by the miscdevice structure with the kernel. In case the minor field is assigned MISC_DYNAMIC_MINOR (our case exactly), kernel fills it with random (from our point of view) number, otherwise, the requested minor number is used.

Our cleanup_module function should have this line added:

   misc_deregister(&our_device);

in order to unregister our device and remove it from the system.

Important note: functions exported by kernel return 0 upon success or negative error code in case of failure.

By now we have a working kernel module which registers a miscellaneous device when loaded and unregisters it when unloaded. Build it now with the make command. Load it with insmod and check the content of the /dev file system. You will see that there is a new device called "interceptor" with number 10 as major number and what ever has been assigned as minor number. You may unload it now. 

If you wish, you may try to open and close the /dev/interceptor device from a user process and check the log with dmesg | tail. You will see the lines "device has been opened" and "device has been closed" respectively. You may also try to open the device from two user processes simultaneously, then you will see that only one process may successfully open it.

In the next section we are going to add some code to our module, which would make it possible to actually patch the sys_call_table and replace original calls with custom wrappers.

Hope this post was helpful. See you at the next one!