untrusted comment: signature from openbsd 6.1 base secret key
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OpenBSD 6.1 errata 005, May 5, 2017:

Expired pf source tracking entries never got removed, leading to
memory exhaustion.

Apply by doing:
   signify -Vep /etc/signify/openbsd-61-base.pub -x 005_pf_src_tracking.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/net/pf.c
===================================================================
RCS file: /cvs/src/sys/net/pf.c,v
retrieving revision 1.1019
diff -u -p -r1.1019 pf.c
--- sys/net/pf.c        17 Mar 2017 17:19:16 -0000      1.1019
+++ sys/net/pf.c        5 May 2017 17:00:46 -0000
@@ -1256,6 +1256,7 @@ pf_purge_expired_src_nodes(int waslocked
                                   &tree_src_tracking, cur);
                               locked = 1;
                       }
+                       pf_remove_src_node(cur);
               }
       }