Linux Kernel: System call hooking example - c

Question

I'm trying to write some simple test code as a demonstration of hooking the system call table.
"sys_call_table" is no longer exported in 2.6, so I'm just grabbing the address from the System.map file, and I can see it is correct (Looking through the memory at the address I found, I can see the pointers to the system calls).
However, when I try to modify this table, the kernel gives an "Oops" with "unable to handle kernel paging request at virtual address c061e4f4" and the machine reboots.
This is CentOS 5.4 running 2.6.18-164.10.1.el5. Is there some sort of protection or do I just have a bug? I know it comes with SELinux, and I've tried putting it in to permissive mode, but it doesn't make a difference
Here's my code:
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/unistd.h>
void **sys_call_table;
asmlinkage int (*original_call) (const char*, int, int);
asmlinkage int our_sys_open(const char* file, int flags, int mode)
{
printk("A file was opened\n");
return original_call(file, flags, mode);
}
int init_module()
{
// sys_call_table address in System.map
sys_call_table = (void*)0xc061e4e0;
original_call = sys_call_table[__NR_open];
// Hook: Crashes here
sys_call_table[__NR_open] = our_sys_open;
}
void cleanup_module()
{
// Restore the original call
sys_call_table[__NR_open] = original_call;
}

Answer 1

I finally found the answer myself.
http://www.linuxforums.org/forum/linux-kernel/133982-cannot-modify-sys_call_table.html
The kernel was changed at some point so that the system call table is read only.
cypherpunk:
Even if it is late but the Solution
may interest others too: In the
entry.S file you will find: Code:
.section .rodata,"a"
#include "syscall_table_32.S"
sys_call_table -> ReadOnly You have to
compile the Kernel new if you want to
"hack" around with sys_call_table...
The link also has an example of changing the memory to be writable.
nasekomoe:
Hi everybody. Thanks for replies. I
solved the problem long ago by
modifying access to memory pages. I
have implemented two functions that do
it for my upper level code:
#include <asm/cacheflush.h>
#ifdef KERN_2_6_24
#include <asm/semaphore.h>
int set_page_rw(long unsigned int _addr)
{
struct page *pg;
pgprot_t prot;
pg = virt_to_page(_addr);
prot.pgprot = VM_READ | VM_WRITE;
return change_page_attr(pg, 1, prot);
}
int set_page_ro(long unsigned int _addr)
{
struct page *pg;
pgprot_t prot;
pg = virt_to_page(_addr);
prot.pgprot = VM_READ;
return change_page_attr(pg, 1, prot);
}
#else
#include <linux/semaphore.h>
int set_page_rw(long unsigned int _addr)
{
return set_memory_rw(_addr, 1);
}
int set_page_ro(long unsigned int _addr)
{
return set_memory_ro(_addr, 1);
}
#endif // KERN_2_6_24
Here's a modified version of the original code that works for me.
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/unistd.h>
#include <asm/semaphore.h>
#include <asm/cacheflush.h>
void **sys_call_table;
asmlinkage int (*original_call) (const char*, int, int);
asmlinkage int our_sys_open(const char* file, int flags, int mode)
{
printk("A file was opened\n");
return original_call(file, flags, mode);
}
int set_page_rw(long unsigned int _addr)
{
struct page *pg;
pgprot_t prot;
pg = virt_to_page(_addr);
prot.pgprot = VM_READ | VM_WRITE;
return change_page_attr(pg, 1, prot);
}
int init_module()
{
// sys_call_table address in System.map
sys_call_table = (void*)0xc061e4e0;
original_call = sys_call_table[__NR_open];
set_page_rw(sys_call_table);
sys_call_table[__NR_open] = our_sys_open;
}
void cleanup_module()
{
// Restore the original call
sys_call_table[__NR_open] = original_call;
}

Answer 2

Thanks Stephen, your research here was helpful to me. I had a few problems, though, as I was trying this on a 2.6.32 kernel, and getting WARNING: at arch/x86/mm/pageattr.c:877 change_page_attr_set_clr+0x343/0x530() (Not tainted) followed by a kernel OOPS about not being able to write to the memory address.
The comment above the mentioned line states:
// People should not be passing in unaligned addresses
The following modified code works:
int set_page_rw(long unsigned int _addr)
{
return set_memory_rw(PAGE_ALIGN(_addr) - PAGE_SIZE, 1);
}
int set_page_ro(long unsigned int _addr)
{
return set_memory_ro(PAGE_ALIGN(_addr) - PAGE_SIZE, 1);
}
Note that this still doesn't actually set the page as read/write in some situations. The static_protections() function, which is called inside of set_memory_rw(), removes the _PAGE_RW flag if:
It's in the BIOS area
The address is inside .rodata
CONFIG_DEBUG_RODATA is set and the kernel is set to read-only
I found this out after debugging why I still got "unable to handle kernel paging request" when trying to modify the address of kernel functions. I was eventually able to solve that problem by finding the page table entry for the address myself and manually setting it to writable. Thankfully, the lookup_address() function is exported in version 2.6.26+. Here is the code I wrote to do that:
void set_addr_rw(unsigned long addr) {
unsigned int level;
pte_t *pte = lookup_address(addr, &level);
if (pte->pte &~ _PAGE_RW) pte->pte |= _PAGE_RW;
}
void set_addr_ro(unsigned long addr) {
unsigned int level;
pte_t *pte = lookup_address(addr, &level);
pte->pte = pte->pte &~_PAGE_RW;
}
Finally, while Mark's answer is technically correct, it'll case problem when ran inside Xen. If you want to disable write-protect, use the read/write cr0 functions. I macro them like this:
#define GPF_DISABLE write_cr0(read_cr0() & (~ 0x10000))
#define GPF_ENABLE write_cr0(read_cr0() | 0x10000)
Hope this helps anyone else who stumbles upon this question.

Answer 3

Note that the following will also work instead of using change_page_attr and cannot be depreciated:
static void disable_page_protection(void) {
unsigned long value;
asm volatile("mov %%cr0,%0" : "=r" (value));
if (value & 0x00010000) {
value &= ~0x00010000;
asm volatile("mov %0,%%cr0": : "r" (value));
}
}
static void enable_page_protection(void) {
unsigned long value;
asm volatile("mov %%cr0,%0" : "=r" (value));
if (!(value & 0x00010000)) {
value |= 0x00010000;
asm volatile("mov %0,%%cr0": : "r" (value));
}
}

Answer 4

If you are dealing with kernel 3.4 and later (it can also work with earlier kernels, I didn't test it) I would recommend a smarter way to acquire the system callы table location.
For example
#include <linux/module.h>
#include <linux/kallsyms.h>
static unsigned long **p_sys_call_table;
/* Aquire system calls table address */
p_sys_call_table = (void *) kallsyms_lookup_name("sys_call_table");
That's it. No addresses, it works fine with every kernel I've tested.
The same way you can use a not exported Kernel function from your module:
static int (*ref_access_remote_vm)(struct mm_struct *mm, unsigned long addr,
void *buf, int len, int write);
ref_access_remote_vm = (void *)kallsyms_lookup_name("access_remote_vm");
Enjoy!

Answer 5

As others have hinted, the whole story is a bit different now on modern kernels. I'll be covering x86-64 here, for syscall hijacking on modern arm64 refer to this other answer of mine. Also NOTE: this is plain and simple syscall hijacking. Non-invasive hooking can be done in a much nicer way using kprobes.
Since Linux v4.17, x86 (both 64 and 32 bit) now uses syscall wrappers that take a struct pt_regs * as the only argument (see commit 1, commit 2). You can see arch/x86/include/asm/syscall.h for the definitions.
Additionally, as others have described already in different answers, the simplest way to modify sys_call_table is to temporarily disable CR0 WP (Write-Protect) bit, which could be done using read_cr0() and write_cr0(). However, since Linux v5.3, [native_]write_cr0 will check sensitive bits that should never change (like WP) and refuse to change them (commit). In order to work around this, we need to write CR0 manually using inline assembly.
Here is a working kernel module (tested on Linux 5.10 and 5.18) that does syscall hijacking on modern Linux x86-64 considering the above caveats and assuming that you already know the address of sys_call_table (if you also want to find that in the module, see Proper way of getting the address of non-exported kernel symbols in a Linux kernel module):
// SPDX-License-Identifier: (GPL-2.0 OR MIT)
/**
* Test syscall table hijacking on x86-64. This module will replace the `read`
* syscall with a simple wrapper which logs every invocation of `read` using
* printk().
*
* Tested on Linux x86-64 v5.10, v5.18.
*
* Usage:
*
* sudo cat /proc/kallsyms | grep sys_call_table # grab address
* sudo insmod syscall_hijack.ko sys_call_table_addr=0x<address_here>
*/
#include <linux/init.h> // module_{init,exit}()
#include <linux/module.h> // THIS_MODULE, MODULE_VERSION, ...
#include <linux/kernel.h> // printk(), pr_*()
#include <asm/special_insns.h> // {read,write}_cr0()
#include <asm/processor-flags.h> // X86_CR0_WP
#include <asm/unistd.h> // __NR_*
#ifdef pr_fmt
#undef pr_fmt
#endif
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
typedef long (*sys_call_ptr_t)(const struct pt_regs *);
static sys_call_ptr_t *real_sys_call_table;
static sys_call_ptr_t original_read;
static unsigned long sys_call_table_addr;
module_param(sys_call_table_addr, ulong, 0);
MODULE_PARM_DESC(sys_call_table_addr, "Address of sys_call_table");
// Since Linux v5.3 [native_]write_cr0 won't change "sensitive" CR0 bits, need
// to re-implement this ourselves.
static void write_cr0_unsafe(unsigned long val)
{
asm volatile("mov %0,%%cr0": "+r" (val) : : "memory");
}
static long myread(const struct pt_regs *regs)
{
pr_info("read(%ld, 0x%lx, %lx)\n", regs->di, regs->si, regs->dx);
return original_read(regs);
}
static int __init modinit(void)
{
unsigned long old_cr0;
real_sys_call_table = (typeof(real_sys_call_table))sys_call_table_addr;
pr_info("init\n");
// Temporarily disable CR0 WP to be able to write to read-only pages
old_cr0 = read_cr0();
write_cr0_unsafe(old_cr0 & ~(X86_CR0_WP));
// Overwrite syscall and save original to be restored later
original_read = real_sys_call_table[__NR_read];
real_sys_call_table[__NR_read] = myread;
// Restore CR0 WP
write_cr0_unsafe(old_cr0);
pr_info("init done\n");
return 0;
}
static void __exit modexit(void)
{
unsigned long old_cr0;
pr_info("exit\n");
old_cr0 = read_cr0();
write_cr0_unsafe(old_cr0 & ~(X86_CR0_WP));
// Restore original syscall
real_sys_call_table[__NR_read] = original_read;
write_cr0_unsafe(old_cr0);
pr_info("goodbye\n");
}
module_init(modinit);
module_exit(modexit);
MODULE_VERSION("0.1");
MODULE_DESCRIPTION("Test syscall table hijacking on x86-64.");
MODULE_AUTHOR("Marco Bonelli");
MODULE_LICENSE("Dual MIT/GPL");