In FreeBSD before 11.2-STABLE(r343782), 11.2-RELEASE-p9, 12.0-STABLE(r343781), and 12.0-RELEASE-p3, kernel callee-save registers are not properly sanitized before return from system calls, potentially allowing some kernel data used in the system call to be exposed.
https://security.FreeBSD.org/advisories/FreeBSD-SA-19:01.syscall.asc