Introduction

These are my private notes that I used to keep for myself but I figured that there is some in my opinion cool stuff worth sharing there. I use them all the time myself!

Running non-native binaries with Qemu

Run:

qemu-aarch64-static -g 2345 -L optional_library_path ./a.out

…and after this we can connect with gdb-multiarch through:

pwndbg> target remote 0.0.0.0:2345

However it can be super wonky. Ctrl-c to stop execution doesn’t work, but we can instead send the sigint signal to the qemu process and then it stops execution after a continue. It’s still buggy though, for example when we disconnect and connect GDB again after a continue, then sending sigint instead of stopping the execution, terminates the process. It’s a common problem there’s a 12 years old bug report: https://gdb.sourceware.narkive.com/Z1LCkr20/remote-qemu-ctrl-c-does-not-work . Also plugins like pwndbg and gef might appear buggy cuz qemu might not provide vmmap info to GDB: https://lore.kernel.org/all/20220221030910.3203063-1-dominik.b.czarnota@gmail.com/ .

Copying libc and ld from a Docker Container

docker run --rm -ti -v "$PWD":/host ubuntu bash # to get an ubuntu container with a mounted path to copy things out
ldd /bin/ls # to see where the libc is or whatever (in this case libc is in /lib/x86_64-linux-gnu)
cp /lib/x86_64-linux-gnu/libc.so.6 /host

Papers

Linux kernel exploitation

In case a challenge doesn’t provide helper scripts there are some templates based on https://lkmidas.github.io/posts/20210123-linux-kernel-pwn-part-1/ . For compressing the initramfs directory:

#!/bin/sh

# try to move the exploit automatically to initramfs
gcc -o exploit -static $1
mv ./exploit ./initramfs

# compress the initramfs dir
cd initramfs
find . \
| cpio -ov --format=newc --owner=root \
| gzip -v1 > initramfs.cpio.gz
mv ./initramfs.cpio.gz ../

For decompressing initramfs.cpio.gz:

#!/bin/sh

mkdir initramfs
cd initramfs
cp ../initramfs.cpio.gz .
gunzip ./initramfs.cpio.gz
cpio -idm < ./initramfs.cpio
rm initramfs.cpio

For decompressing the compressed kernel image (usually called vmlinuz) use: this.

For running the kernel vm with an exposed GDB port:

#!/bin/sh

qemu-system-x86_64 \
    -m 128M \
    -cpu kvm64,+smep,+smap \
    -kernel vmlinuz \
    -initrd initramfs.cpio.gz \
    -hdb flag.txt \
    -snapshot \
    -nographic \
    -monitor /dev/null \
    -gdb tcp::1234 \
    -no-reboot \
    -append "console=ttyS0 kaslr pti=on quiet panic=1"

More or less obscure tricks

  • Code execution with a write primitive on last libc
  • There’s the _dl_make_stack_executable function in glibc’s ld. I’ve never seen this used in an actual exploit but seems cool to know about nonetheless.
  • mmap returns addresses placed close to libc (check out house of muney for more info).
  • The read and write syscalls do not complain about being fed an invalid memory address and simply return an error. Might be useful for finding writeable memory without having a leak.
  • Allegedly theres a magic race condition in one one_gadget (the gadget is actually in exec_comm and to find it you have to use one_gadget with -l 1 argument). It can return a shell even though the constraints aren’t met. I saw it a long ago in a ctf being used but when I tried to recreate it recently it didn’t seem to work, so take this with a grain of salt and do your own research. 
    0x10dbca posix_spawn(rsp+0x64, "/bin/sh", [rsp+0x40], 0, rsp+0x70, r9)
constraints:
[rsp+0x70] == NULL
[r9] == NULL || r9 == NULL
[rsp+0x40] == NULL || (s32)[[rsp+0x40]+0x4] <= 0

Important versions of things

  • glibc-2.24 - Added check for FILEs vtable address.
  • glibc-2.26 - Moved the FILEs vtable to a non-writable memory.
  • glibc-2.28 - Added tcache double free check.
  • glibc-2.29 - Moved the FILEs vtable back to a writable area.
  • glibc-2.34 - Removed the ret2csu gadget.
  • glibc-2.34 - malloc hooks removed from the API.
  • Linux 6.2 - commit_creds(prepare_kernel_cred(NULL)); no longer works. Now we need to pass &init_cred as the arg.

Pyjails

Example payloads to reuse:

(__builtins__:=__import__('code'))==(lambda:interact())()

# from https://hackmd.io/@crazyman/H1s0b1Hii
__import__('antigravity',setattr(__import__('os'),'environ',dict(BROWSER='/bin/sh -c "/readflag giveflag" #%s')))

Tricks that might be helpful:

LaTeX

For simple RCE’s I like the hacktricks article. If it’s not enough there’s this very cool paper: Are Text-Only Data Formats Safe? Or, Use This LATEX Class File to Pwn Your Computer. Also reading binary files might be useful.