mikalv / cexigua

Linux based inter-process code injection without ptrace(2)

1. Launch sleep

2. Using /proc/self/maps, find all required gadgets* in loaded libraries, if we can't find a gadget in automatically loaded libraries, find one anywhere in /usr/lib, and remember in which file it was found

3. Execute a longer running sleep, if necessary adding the new libraries from 2 via LD_PRELOAD

4. Re-find the gadgets, with the correct ASLR offset, and add them to our payload, along with any data

5. dd the payload directly over the stack, found in /proc/${PID}/maps, over /proc/${PID}/mem, pre-padded with NOPs

6. Wait for sleep to return from the nanosleep syscall, and our code is executed.

7. The payload I've written opens the file specified on the CLI, creates a memfd, uses sendfile to copy the binary to memory, and then uses fexecve to execute the in memory binary (which uses execve(/proc/self/fd/X) under the hood). Any payload ROP payload is possible, previously I had a payload which would mmap a PROT_EXEC section, copy a standard shellcode file into memory and execute that, which itself mounted a FUSE filesystem. This method was more flexible but also seemingly more brittle, and had large amounts of handwritten ASM.

• We only actually need a NOP, POP {RDI, RSI, RDX, RCX, R8, R9}, SYSCALL and a JMP [SOMETHING] (I've used RAX for parity with syscalls) for syscalls and PLT calls

Files

• memfdcreate.sh - The actual payload
• overwrite.sh - The entrypoint (bash overwrite.sh ./busybox-x86_64)
• payload.sh - Wrapper around memfdcreate.sh to create the payload
• readsyms.sh - ELF Parser
• utils.sh - Utility functions, including ROP generator

Features:

• We can call any PLT (e.g glibc) function or syscall, with arbitrary arguments, including string arguments

• Pure Bash ROPChain generator, including ELF parser to ensure grepped gadgets are within the r-x .text section

Future work

• Cache gadget offsets from the ASLR base on the first run, so the second run is faster

• Interactivity with processes executed via the fexecve method. This can be achieved using FUSE's passthrough example, but this requires libfuse to be available.

Why this works:

Parent processes can write to their children's /proc/${PID}/mem in most distros, due to the default value of /proc/sys/kernel/yama/ptrace_scope (1). The less secure setting (0) allows for any process sharing a UID to write to another processes /proc/${PID}/mem. To be the correct parent, we have to exec dd after we've generated the payload. This means that dd becomes the parent of sleep, but we then are unable to execute something like wait to make the process interactive.

Binary dependencies

• Bash
• dd
• GNU grep (this can be worked around but it's slow)

Bash is the worst thing in the world

Bash does not handle binary data. ELF objects are binary data. This was 'fun'.