Linux Polkit pkexec helper PTRACE_TRACEME local root exploit

Description

This module exploits an issue in ptrace_link in kernel/ptrace.c before Linux kernel 5.1.17. This issue can be exploited from a Linux desktop terminal, but not over an SSH session, as it requires execution from within the context of a user with an active Polkit agent. In the Linux kernel before 5.1.17, ptrace_link in kernel/ptrace.c mishandles the recording of the credentials of a process that wants to create a ptrace relationship, which allows local users to obtain root access by leveraging certain scenarios with a parent-child process relationship, where a parent drops privileges and calls execve (potentially allowing control by an attacker). One contributing factor is an object lifetime issue (which can also cause a panic). Another contributing factor is incorrect marking of a ptrace relationship as privileged, which is exploitable through (for example) Polkit's pkexec helper with PTRACE_TRACEME.