Errata patches for exec_subr have been released for OpenBSD 6.1 and 6.0. An additional mitigation is added by placing a gap of 1 MB between the stack and mmap spaces.
Binary updates for the amd64 and i386 platforms are available via the syspatch utility. Source code patches can be found on the respective errata pages: https://www.openbsd.org/errata60.html https://www.openbsd.org/errata61.html As this affects the kernel, a reboot will be needed after patching.