Cybersecurity Notes
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  • Cybersecurity Notes
  • Binary Exploitation
    • Stack
      • Introduction
      • ret2win
      • De Bruijn Sequences
      • Shellcode
      • NOPs
      • 32- vs 64-bit
      • No eXecute
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        • Using RSP
        • ret2reg
          • Using ret2reg
      • One Gadgets and Malloc Hook
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          • pop rsp
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    • Heap
      • Introduction to the Heap
      • Chunks
      • Freeing Chunks and the Bins
        • Operations of the Fastbin
        • Operations of the Other Bins
      • Malloc State
      • malloc_consolidate()
      • Heap Overflow
        • heap0
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        • Double-Free Protections
        • Double-Free Exploit
      • Unlink Exploit
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      • Safe Linking
    • Kernel
      • Introduction
      • Writing a Char Module
        • An Interactive Char Driver
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      • A Basic Kernel Interaction Challenge
      • Compiling, Customising and booting the Kernel
      • Double-Fetch
        • Double-Fetch without Sleep
      • The Ultimate Aim of Kernel Exploitation - Process Credentials
      • Kernel ROP - ret2usr
      • Debugging a Kernel Module
      • SMEP
        • Kernel ROP - Disabling SMEP
        • Kernel ROP - Privilege Escalation in Kernel Space
      • SMAP
      • modprobe_path
      • KASLR
      • KPTI
    • Browser Exploitation
      • *CTF 2019 - oob-v8
        • The Challenge
      • picoCTF 2021 - Kit Engine
      • picoCTF 2021 - Download Horsepower
  • Reverse Engineering
    • Strings in C++
    • C++ Decompilation Tricks
    • Reverse Engineering ARM
  • Blockchain
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  • Hosting a Testnet and Deploying a Contract
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        • Binary Exploitation
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        • Pwn
          • Do I Know You?
          • Naughty
        • Web
          • PHP Master
      • HTB CyberSanta 2021
        • Crypto
          • Common Mistake
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          • Meet Me Halfway
  • Miscellaneous
    • pwntools
      • Introduction
      • Processes and Communication
      • Logging and Context
      • Packing
      • ELF
      • ROP
    • scanf Bypasses
    • Challenges in Containers
    • Using Z3
    • Cross-Compiling for arm32
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  1. Binary Exploitation
  2. Heap
  3. Freeing Chunks and the Bins

Operations of the Other Bins

Last updated 1 year ago

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When a non-fast chunk is freed, it gets put into the Unsorted Bin. When new chunks are requested, glibc looks at all of the bins

  • If the requested size is fastbin size,

    • If there is a chunk in it, return it

  • If the requested chunk is of smallbin size,

    • If there is a chunk in it, return it

  • If the requested chunk is large (of largebin size), with . We will get into the mechanisms of this at a later point, but essentially I lied earlier - fastbins do consolidate, but not on freeing!

  • Finally, we iterate through the chunks in the unsorted bin

    • If it is empty, we service the request through making the heap larger by moving the top chunk back and making space

  • If the requested size is equal to the size of the chunk in the bin, return the chunk

  • If it's smaller, split the chunk in the bin in two and return a portion of the correct size

  • If it's larger,

One thing that is very easy to forget is what happens on allocation and what happens on freeing, as it can be a bit counter-intuitive. For example, the fastbin consolidation is triggered from an allocation!

check the corresponding fastbin
check the corresponding smallbin
we first consolidate the largebins
malloc_consolidate()