FreeBSD -- hwpmc fails to detach PMCs during exec credential transitions

Problem Description:

When a process calls execve(2) to execute a setuid or setgid image, hwpmc(4) is supposed to detach PMCs owned by unprivileged processes. An inverted check meant that this scenario was not handled properly.

Impact:

An unprivileged local user who has attached PMCs to a process can continue monitoring it after the process executes a setuid or setgid binary, contrary to the intended policy.