FreeBSD: VID-bba850fd-770e-11eb-b87a-901b0ef719ab (CVE-2020-25582): FreeBSD -- jail_attach(2) relies on the caller to change the cwd

Problem Description: When a process, such as jexec(8) or killall(1), calls jail_attach(2) to enter a jail, the jailed root can attach to it using ptrace(2) before the current working directory is changed. Impact: A process with superuser privileges running inside a jail could change the root directory outside of the jail, thereby gaining full read and writing access to all files and directories in the system.