untrusted comment: signature from openbsd 6.1 base secret key
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OpenBSD 6.1 errata 008, May 18, 2017:

Add a gap of 1MB between the stack and mmap spaces.

Apply by doing:
   signify -Vep /etc/signify/openbsd-61-base.pub -x 008_exec_subr.patch.sig \
       -m - | (cd /usr/src && patch -p0)

And then rebuild and install a new kernel:
   KK=`sysctl -n kern.osversion | cut -d# -f1`
   cd /usr/src/sys/arch/`machine`/compile/$KK
   make obj
   make config
   make
   make install

Index: sys/kern/exec_subr.c
===================================================================
RCS file: /cvs/src/sys/kern/exec_subr.c,v
retrieving revision 1.51
diff -u -p -r1.51 exec_subr.c
--- sys/kern/exec_subr.c        29 Aug 2015 05:40:01 -0000      1.51
+++ sys/kern/exec_subr.c        18 May 2017 18:55:42 -0000
@@ -315,6 +315,10 @@ vmcmd_randomize(struct proc *p, struct e
       return (error);
}

+#ifndef MAXSSIZ_GUARD
+#define MAXSSIZ_GUARD  (1024 * 1024)
+#endif
+
/*
 * exec_setup_stack(): Set up the stack segment for an executable.
 *
@@ -336,7 +340,7 @@ exec_setup_stack(struct proc *p, struct
       epp->ep_maxsaddr = USRSTACK;
       epp->ep_minsaddr = USRSTACK + MAXSSIZ;
#else
-       epp->ep_maxsaddr = USRSTACK - MAXSSIZ;
+       epp->ep_maxsaddr = USRSTACK - MAXSSIZ - MAXSSIZ_GUARD;
       epp->ep_minsaddr = USRSTACK;
#endif
       epp->ep_ssize = round_page(p->p_rlimit[RLIMIT_STACK].rlim_cur);