Draft of UMassCTF pwn/factory-monitor#93
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May 1, 2026
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Great work! Review the comments and you're good to go!
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| After creating a machine, we can start running it with the `start()` function. This function forks a child process for the machine, fills in the machine‘s PID and changes its state to `STATE_RUNNING`, and sets up bidirectional communication with parent process using the pipes. Once setup is complete, the machine calls its `main_func`, which by default is the `machine_main_demo()` function reproduced below. | ||
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| ``` |
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Can you briefly explain what the code is doing too?
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| ``` | ||
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| There are two key insights from this function. |
| ## Exploit | ||
| The buffer passed to `read_line_fd()` from the machine is stack allocated in `machine_main_demo()`, so we can deterministically write past it to reach the return address saved on the stack and overwrite whither we return. | ||
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| Since our ultimate goal is to get the flag from the `flag.txt` file, we must find a sequence of commands to open the file, read its contents, and print the flag. We can see from the symbol table that the `open()` function is imported at offset `0x38a40` from the start of the executable, and we have the option of `read()` or `read_line_fd()` to read the flag. |
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Maybe drop an image of the symbol table?
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| Since our ultimate goal is to get the flag from the `flag.txt` file, we must find a sequence of commands to open the file, read its contents, and print the flag. We can see from the symbol table that the `open()` function is imported at offset `0x38a40` from the start of the executable, and we have the option of `read()` or `read_line_fd()` to read the flag. | ||
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| Because the program’s stack is not executable (seen through `readelf -l`), we cannot directly execute shellcode to run these functions. Instead, we can use existing instruction sequences from the programmer’s `.text` section, which is executable, to build an ROP chain and execute our own commands. The binary has sequences to pop a value into `rdi` at `0xc028` (`5f 5d c3`) and into `rsi` at `0x15bc7` (`5e 5d c3`). However, there is no gadget which would give us `rdx` (`5a 5d c3`), so we have to use the two-argument `read_line_fd()` rather than the three-argument `read()`. We will also use a `ret` instruction from `0xa382` to align the stack before calling our functions. |
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Explain briefly how read_line_fd is used to give us rdx?
| This script, `exploit.py`, gets the flag: | ||
| ``` | ||
| UMASS{AsLR_L3Ak} | ||
| ``` |
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Great writeup! Only thing I would add is that maybe a diagram or some visual of the ROP chain would be helpful!
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