:(

Sounds super 1337 and I hope it's actually possible somehow.

Parse /proc/<pid>/maps to find the relevant target_addr in your process-under-attack. And then its a matter of:

    $ dd if=shellcode.bin of=/proc/<pid>/mem bs=1 seek=$((target_addr)) ...
See also: DDExec

https://github.com/arget13/DDexec

What legitimate purpose does this feature serve? Why should a process be able to write into the virtual memory of another process?