Embedded-Hacking/WEEK02/WEEK02-01-S.md

Embedded Systems Reverse Engineering

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Week 2

Hello, World - Debugging and Hacking Basics: Debugging and Hacking a Basic Program for the Pico 2

Non-Credit Practice Exercise 1 Solution: Change the Message

Answers

Attack Summary

The goal is to write a custom message into SRAM at 0x20000000 and redirect r0 to print it instead of the original "hello, world" string, without changing the source code.

GDB Commands

(gdb) target extended-remote :3333
(gdb) monitor reset halt
(gdb) b *0x1000023c                    # Breakpoint before __wrap_puts
(gdb) c                                 # Continue to breakpoint
(gdb) set {char[20]} 0x20000000 = {'Y','o','u','r',' ','N','a','m','e','!','\r','\0'}
(gdb) set $r0 = 0x20000000             # Redirect r0 to injected message
(gdb) c                                 # Resume - serial shows custom message

Verification

(gdb) x/s 0x20000000                   # Should show your injected message
(gdb) x/s 0x100019cc                   # Original string still in Flash

Reflection Answers

1. Why does the string have to live in SRAM instead of flash during runtime? Flash memory is read-only at runtime. The original string at 0x100019cc cannot be modified. SRAM starting at 0x20000000 is read-write, so that is where we must place our replacement string.

2. What would happen if you forgot the null terminator in your injected string? puts() reads characters until it encounters \0. Without it, puts() would continue reading past the intended string, printing garbage characters from adjacent memory until a null byte happens to appear. This could crash the program or leak sensitive data.

3. How does changing r0 alter the behavior of puts() without touching source code? In the ARM calling convention, the first function argument is passed in r0. When bl __wrap_puts executes at 0x1000023c, it reads the string address from r0. By changing r0 from 0x100019cc (original Flash string) to 0x20000000 (our SRAM string), we redirect what puts() prints.