Binary Exploitation Core Dumps

Binary Exploitation: Core Dumps & Crash Analysis

This skill helps you enable core dump generation and analyze crash dumps using GDB for binary exploitation and debugging.

What are Core Files?

Core files are memory snapshots created when a process crashes. They capture:

• The process's memory state
• Register values
• Program counter (instruction pointer)
• Stack contents
• All variables at the time of crash

This is invaluable for understanding why a program crashed and where in the code the crash occurred.

Enabling Core Dump Generation

Quick Enable (Current Session)

ulimit -c unlimited

This allows unlimited core file size for the current shell session. Note: This is not persistent across reboots or new terminal sessions.

Verify Core Dump is Enabled

ulimit -c

If it returns unlimited or a large number, core dumps are enabled. If it returns 0, they're disabled.

Persistent Configuration (System-Wide)

To make core dumps permanent across reboots:

1. Edit /etc/security/limits.conf
2. Add this line:

   * soft core unlimited
   

3. Reboot or re-login for changes to take effect

Core File Location

Core files are typically saved in:

• Current working directory (default on many systems)
• /var/crash/ (systemd-based systems)
• Custom path via /proc/sys/kernel/core_pattern

Check the pattern:

cat /proc/sys/kernel/core_pattern

Analyzing Core Files with GDB

Basic Analysis

gdb /path/to/executable /path/to/core_file

This loads both the executable and the crash snapshot into GDB.

Essential GDB Commands for Crash Analysis

Once in GDB:

# See where the crash occurred
bt
# or
bt full

# Examine the instruction that caused the crash
x/10i $pc
# or
x/10i $rip

# View register values
info registers

# Examine the stack
x/20gx $sp
# or
x/20gx $rsp

# Look at specific memory addresses
x/16wx 0x7fffffffe000

# Examine variables (if debug symbols exist)
print variable_name

# Disassemble the current function
disas

# Set a breakpoint and continue (if you want to step through)
break function_name
continue

Common Crash Scenarios

Segmentation Fault (SIGSEGV)

# Check what address caused the fault
info registers
# Look at rdi, rsi, rdx, rax for potential pointers

# Examine the faulting address
x/4wx $rdi  # or whichever register held the bad pointer

Stack Overflow

# Check stack pointer vs base pointer
info registers
# Look at $rsp and $rbp

# Examine stack contents
x/50gx $rsp

Buffer Overflow Detection

# Look for overwritten return addresses
x/20gx $rsp

# Check if canary was overwritten (if ASLR/stack canary enabled)
info registers
# Look for 0x0000000000000000 in stack (canary failure)

Practical Workflow

Step 1: Enable Core Dumps

ulimit -c unlimited

Step 2: Run the Vulnerable Program

./vulnerable_program
# or with input
./vulnerable_program < input.txt
# or with arguments
./vulnerable_program arg1 arg2

Step 3: Locate the Core File

# Find core files in current directory
ls -la core*

# Or search system-wide
find / -name "core*" -type f 2>/dev/null

Step 4: Analyze with GDB

gdb ./vulnerable_program core

Then run the essential commands above to understand the crash.

Tips for Binary Exploitation

1. Always enable core dumps first - Don't wait until after the crash
2. Keep the executable - You need the original binary to analyze the core
3. Check for debug symbols - Core analysis is much easier with symbols
4. Look at the stack - Most buffer overflows show up in stack memory
5. Compare registers - See if registers contain unexpected values
6. Document findings - Note the crash address, register values, and stack state

Common Issues

"No core file generated"

• Check ulimit -c - it might be 0
• Check /proc/sys/kernel/core_pattern - might be redirecting to a different location
• Check disk space - core dumps need space
• Check permissions - you need write access to the output directory

"GDB can't find symbols"

• Recompile with -g flag: gcc -g program.c -o program
• Or use strip to check if symbols were removed

"Core file is empty or corrupted"

• The process might have crashed before core dump could complete
• Try running with gdb directly instead: gdb ./program

Example: Analyzing a Buffer Overflow

# Enable core dumps
ulimit -c unlimited

# Run the vulnerable program with overflow input
python3 -c "print('A'*1000)" | ./vulnerable_program

# Find and analyze the core
ls -la core*
gdb ./vulnerable_program core

# In GDB:
(gdb) bt
# Shows: #0  0x00007ffff7a23456 in __libc_start_main ()
#        #1  0x0000555555555123 in main ()

(gdb) x/20gx $rsp
# Shows stack contents - look for 'AAAA' patterns

(gdb) info registers
# Check if return address was overwritten

When to Use This Skill

Use this skill when:

• A program crashes and you need to understand why
• You're working on CTF binary exploitation challenges
• You're debugging segmentation faults or other crashes
• You need to analyze buffer overflows or memory corruption
• You're doing reverse engineering and need crash state information
• You want to understand the memory state at the time of a crash