This POC demonstrates execution of bytes located in supposedly non-executable region of binary, therefore completely bypassing executable-space protection.
The root cause of this can be found here: https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/elf.h#L280
As it turns out, binary files built on either systems lacking NX or IA32 systems with NX, which do NOT contain the PT_GNU_STACK header will be marked with exec-all.
This allows for complete RWX to/from everywhere in the binary.
To achieve this, we use "historical" building tools.
In this case, gcc 3.2.2 running on x86 Slackware9 with Linux 2.4.20
We will end up with a binary file which can be executed on modern Linux systems, in this case Linux 5.16.1
The very same effect MIGHT be achievable with specific linker arguments/scripts, although I have NOT verified this.
The following code will copy assembled bytes of function dummy() to character array harmless_str_buf and execute the destination array as function.
This demonstration serves completely for educational purposes. Under no circumstances can the author of this code be held responsible for any direct or indirect damage caused by misusing any provided code and/or information.
See LICENSE for more details