Bypassing ptrace calls with LD_PRELOAD on Linux

#beginners #linux #reversing

Hello,

Here's a quick article on how to bypass calls to ptrace when debugging a Linux executable.

By calling ptrace with the PTRACE_TRACEME option, a process can detect if it's being debugged and execute different instructions. This an effective anti-debugging technique.

For example, take the following C program:

#include <stdio.h>
#include <sys/ptrace.h>

int main() {
    if (ptrace(PTRACE_TRACEME, 0, 0, 0) < 0) {
      printf("I'm being debugged!\n");
    } else {
      printf("Normal flow\n");
    }
    return 0;
}

If we execute the program from above we get Normal flow on our screen but if we debug it with gdb we get Err: I'm being debugged!.

root@kali:~/Downloads# strace ./a.out 
...
munmap(0x7fb7c945e000, 90919)           = 0
ptrace(PTRACE_TRACEME)                  = -1 EPERM (Operation not permitted)
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(0x88, 0), ...}) = 0
brk(NULL)                               = 0x55c2d37e6000
brk(0x55c2d3807000)                     = 0x55c2d3807000
write(1, "Err: I'm being debugged!\n", 25Err: I'm being debugged!
) = 25
exit_group(0)                           = ?
+++ exited with 0 +++

To bypass this, we can use the LD_PRELOAD environment variable. It lets us control the loading path of a shared library, which allows us to stub out library functions such as ptrace.

We can create the following file:

long ptrace(int request, int pid, void *addr, void *data) {
    return 0;
}

And compile it as a shared library with the following command:

gcc -shared ptrace.c -o ptrace.so

Next, we can set the environment variable LD_PRELOAD using the following commands:

in the shell: export LD_PRELOAD=./ptrace.so
in gdb: set environment LD_PRELOAD=./ptrace.so

Demo:

gdb-peda$ file a.out 
Reading symbols from a.out...
(No debugging symbols found in a.out)
gdb-peda$ r
Starting program: /root/Downloads/a.out 
Err: I'm being debugged!
[Inferior 1 (process 1939) exited normally]
Warning: not running
gdb-peda$ set environment LD_PRELOAD=./ptrace.so
gdb-peda$ r
Starting program: /root/Downloads/a.out 
Normal flow
[Inferior 1 (process 1946) exited normally]
Warning: not running
gdb-peda$

Thanks for reading!