untrusted comment: signature from openbsd 6.2 base secret key
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OpenBSD 6.2 errata 001, October 13th, 2017:

A local user could trigger a kernel panic by using an invalid TCB value.

Apply by doing:
   signify -Vep /etc/signify/openbsd-62-base.pub -x 001_tcb_invalid.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/arch/amd64/include/tcb.h
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/arch/amd64/include/tcb.h,v
retrieving revision 1.5
diff -u -p -r1.5 tcb.h
--- sys/arch/amd64/include/tcb.h        27 Sep 2017 05:43:55 -0000      1.5
+++ sys/arch/amd64/include/tcb.h        10 Oct 2017 18:09:12 -0000
@@ -28,7 +28,7 @@ void  tcb_set(struct proc *_p, void *_new
#define TCB_SET(p, addr)       tcb_set(p, addr)

/* address must be in canonical form; requiring lower-half is okay */
-#define TCB_INVALID(addr)      ((u_long)(addr) > 0x0000ffffffffffff)
+#define TCB_INVALID(addr)      ((u_long)(addr) > 0x00007fffffffffff)

#else /* _KERNEL */