- IPIPCTL_ALLOW - atomically accessed integer;
- IPIPCTL_STATS - per-CPU counters;

ok?

Index: sys/netinet/in_proto.c
===================================================================
RCS file: /cvs/src/sys/netinet/in_proto.c,v
diff -u -p -r1.110 in_proto.c
--- sys/netinet/in_proto.c	20 Aug 2024 07:47:25 -0000	1.110
+++ sys/netinet/in_proto.c	20 Aug 2024 14:51:59 -0000
@@ -230,7 +230,7 @@ const struct protosw inetsw[] = {
   .pr_type	= SOCK_RAW,
   .pr_domain	= &inetdomain,
   .pr_protocol	= IPPROTO_IPV4,
-  .pr_flags	= PR_ATOMIC|PR_ADDR|PR_MPSOCKET,
+  .pr_flags	= PR_ATOMIC|PR_ADDR|PR_MPSOCKET|PR_MPSYSCTL,
 #if NGIF > 0
   .pr_input	= in_gif_input,
 #else
Index: sys/netinet/ip_ipip.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_ipip.c,v
diff -u -p -r1.102 ip_ipip.c
--- sys/netinet/ip_ipip.c	17 May 2024 20:44:36 -0000	1.102
+++ sys/netinet/ip_ipip.c	20 Aug 2024 14:51:59 -0000
@@ -72,6 +72,11 @@
 #include <net/pfvar.h>
 #endif
 
+/*
+ * Locks used to protect data:
+ *	a	atomic
+ */
+
 #ifdef ENCDEBUG
 #define DPRINTF(fmt, args...)						\
 	do {								\
@@ -87,7 +92,7 @@
  * We can control the acceptance of IP4 packets by altering the sysctl
  * net.inet.ipip.allow value.  Zero means drop them, all else is acceptance.
  */
-int ipip_allow = 0;
+int ipip_allow = 0;	/* [a] */
 
 struct cpumem *ipipcounters;
 
@@ -106,7 +111,8 @@ ipip_input(struct mbuf **mp, int *offp, 
 	struct ifnet *ifp;
 
 	/* If we do not accept IP-in-IP explicitly, drop.  */
-	if (!ipip_allow && ((*mp)->m_flags & (M_AUTH|M_CONF)) == 0) {
+	if (atomic_load_int(&ipip_allow) == 0 &&
+	    ((*mp)->m_flags & (M_AUTH|M_CONF)) == 0) {
 		DPRINTF("dropped due to policy");
 		ipipstat_inc(ipips_pdrops);
 		m_freemp(mp);
@@ -271,7 +277,8 @@ ipip_input_if(struct mbuf **mp, int *off
 	}
 
 	/* Check for local address spoofing. */
-	if (!(ifp->if_flags & IFF_LOOPBACK) && ipip_allow != 2) {
+	if (!(ifp->if_flags & IFF_LOOPBACK) &&
+	    atomic_load_int(&ipip_allow) != 2) {
 		struct sockaddr_storage ss;
 		struct rtentry *rt;
 
@@ -584,19 +591,14 @@ int
 ipip_sysctl(int *name, u_int namelen, void *oldp, size_t *oldlenp, void *newp,
     size_t newlen)
 {
-	int error;
-
 	/* All sysctl names at this level are terminal. */
 	if (namelen != 1)
 		return (ENOTDIR);
 
 	switch (name[0]) {
 	case IPIPCTL_ALLOW:
-		NET_LOCK();
-		error = sysctl_int_bounded(oldp, oldlenp, newp, newlen,
-		    &ipip_allow, 0, 2);
-		NET_UNLOCK();
-		return (error);
+		return (sysctl_int_bounded(oldp, oldlenp, newp, newlen,
+		    &ipip_allow, 0, 2));
 	case IPIPCTL_STATS:
 		return (ipip_sysctl_ipipstat(oldp, oldlenp, newp));
 	default: