CNIT 127 Ch 3: Shellcode
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This document discusses shellcode and system calls. It provides an overview of protection rings, syscalls, assembler, and other topics related to writing shellcode. It then demonstrates how to write simple shellcode to call the exit syscall using nasm and ld. It discusses replacing instructions to remove null bytes. Finally, it introduces how to go beyond simple exit shellcode to spawn an interactive shell using the fork and execve system calls.
CNIT 127 Ch 3: Shellcode
- 1. CNIT 127: Exploit Development Ch 3: Shellcode Updated 2-22-21
- 2. Topics • Protection rings • Syscalls • Shellcode • nasm Assembler • ld GNU Linker • objdump to see contents of object files • strace System Call Tracer • Removing Nulls • Spawning a Shell
- 4. Shellcode • Written in assembler • Translated into hexadecimal opcodes • Intended to inject into a system by exploiting a vulnerability • Typically spawns a root shell, but may do something else
- 5. System Calls (or Syscalls) • Syscalls directly access the kernel, to: – Get input – Produce output – Exit a process – Execute a binary file – And more • They are the interface between protected kernel mode and user mode
- 6. Protection Rings • Although the x86 provides four rings, only rings 0 and 3 are used by Windows or Unix • Ring 3 is user- land • Ring 0 is kernel- land • Links Ch 3a-3c
- 7. Protecting the Kernel • Protected kernel mode – Prevents user applications from compromising the OS • If a user mode program attempts to access kernel memory, this generates an access exception • Syscalls are the interface between user mode and kernel mode
- 8. Libc • C library wrapper • C functions that perform syscalls • Advantages of libc – Allows programs to continue to function normally even if a syscall is changed – Provides useful functions, like malloc – (malloc allocates space on the heap) • See link Ch 3d
- 9. Syscalls use INT 0x80 1. Load syscall number into EAX 2. Put arguments in other registers 3. Execute INT 0x80 4. CPU switches to kernel mode 5. Syscall function executes
- 10. Syscall Number and Arguments • Syscall number is an integer in EAX • Up to six arguments are loaded into – EBX, ECX, EDX, ESI, EDI, and EBP • For more than six arguments, the first argument holds a pointer to a data structure
- 11. Demonstration Using Debian 10 64-Bit
- 12. exit() • The libc exit function does a lot of preparation, carefully covering many possible situations, and then calls SYSCALL to exit
- 13. Disassembling exit • gdb -q e – disassemble main – main calls exit – exit calls __run_exit_handlers – __run_exit_handlers calls _exit – disassemble _exit • int 0x80 – call *$gs:10 – int 0x80
- 14. Four Ways to Do Syscall • Link Ch 3o
- 15. Disassembling _exit • syscall 252 (0xfc), exit_group() (kill all threads) • syscall 1, exit() (kill calling thread) – Link Ch 3e
- 16. Writing Shellcode for the exit() Syscall
- 17. Shellcode Size • Shellcode should be as simple and compact as possible • Because vulnerabilities often only allow a small number of injected bytes – It therefore lacks error-handling, and will crash easily
- 18. sys_exit Syscall • Two arguments: eax=1, ebx is return value (0 in our case) • Link Ch 3m
- 19. Simplest code for exit(0)
- 20. nasm and ld • sudo apt install nasm • nasm creates object file • gcc links it, creating an executable ELF file
- 21. objdump • Shows the contents of object files
- 22. C Code to Test Shellcode • From link Ch 3k • Textbook version explained at link Ch 3i
- 23. Compile and Run • Textbook omits the "-z execstack" option • It's required now or you get a segfault • Next, we'll use "strace" to see all system calls when this program runs • That shows a lot of complex calls, and "exit(0)" at the end
- 24. Using strace • sudo apt install strace
- 26. Getting Rid of Nulls • We have null bytes, which will terminate a string and break the exploit
- 27. Replacing Instructions • This instruction contains nulls – mov ebx,0 • This one doesn't – xor ebx,ebx • This instruction contains nulls, because it moves 32 bits – mov eax,1 • This one doesn't, moving only 8 bits – mov al, 1
- 28. OLD NEW
- 29. objdump of New Exit Shellcode
- 30. Spawning a Shell
- 31. Beyond exit() • The exit() shellcode stops the program, so it's just a DoS attack • Any illegal instruction can make the program crash, so that's of little use • We want shellcode that offers the attacker a shell, so the attacker can type in arbitrary commands
- 32. Five Steps to Shellcode 1. Write high-level code 2. Compile and disassemble 3. Analyze the assembly 4. Clean up assembly, remove nulls 5. Extract commands and create shellcode
- 33. fork() and execve() • Two ways to create a new process in Linux • Replace a running process – Uses execve() • Copy a running process to create a new one – Uses fork() and execve() together
- 34. man execve
- 35. C Program to Use execve() • Static linking preserves our execve syscall
- 36. In gdb, disassemble main • Pushes 3 Arguments • Calls __execve
- 37. disassemble execve • Puts four parameters into edx, ecx, ebx, and eax
- 38. Versions of syscall • Link Ch 3n
- 40. Final Shellcode