Use temporary local buffer for lockless IO within ip6_sysctl_soiikey().
Use existing `sysctl_lock' rwlock(9) to protect `ip6_soiikey'. Also,
replace temporary `ip_sysctl_lock' with `sysctl_lock' rwlock(9). It was
introduced as temporary to avoid recursive lock acquisition in
ip*_sysctl() until upcoming unlocked. The usage path are not the hot
paths, no reason to have dedicated locks for them.

I want to deny negative values for 'net.inet6.ip6.neighborgcthresh' and
'net.inet6.ip6.maxdynroutes' with the following diffs. Negative values
mean unlimited count of in-kernel objects and this is definitely bad
idea. During ip6_sysctl() unlocking I found some sysctl() variables
already modified, but these were forgotten.

const struct sysctl_bounded_args ipv6ctl_vars[] = {
    /* ... */
    { IPV6CTL_NEIGHBORGCTHRESH, &ip6_neighborgcthresh, -1, 5 * 2048},
    { IPV6CTL_MAXDYNROUTES, &ip6_maxdynroutes, -1, 5 * 4096},
};


Index: sys/netinet/ip_input.c
===================================================================
RCS file: /cvs/src/sys/netinet/ip_input.c,v
retrieving revision 1.424
diff -u -p -r1.424 ip_input.c
--- sys/netinet/ip_input.c	24 Jul 2025 22:31:19 -0000	1.424
+++ sys/netinet/ip_input.c	27 Jul 2025 18:58:30 -0000
@@ -1723,9 +1723,6 @@ ip_forward(struct mbuf *m, struct ifnet 
 
 #ifndef SMALL_KERNEL
 
-/* Temporary, to avoid sysctl_lock recursion. */
-struct rwlock ip_sysctl_lock = RWLOCK_INITIALIZER("ipslk");
-
 int
 ip_sysctl(int *name, u_int namelen, void *oldp, size_t *oldlenp, void *newp,
     size_t newlen)
@@ -1759,10 +1756,10 @@ ip_sysctl(int *name, u_int namelen, void
 		error = sysctl_int_bounded(oldp, oldlenp, newp, newlen,
 		    &newval, 0, INT_MAX);
 		if (error == 0 && oldval != newval) {
-			rw_enter_write(&ip_sysctl_lock);
+			rw_enter_write(&sysctl_lock);
 			atomic_store_int(&ip_mtudisc_timeout, newval);
 			rt_timer_queue_change(&ip_mtudisc_timeout_q, newval);
-			rw_exit_write(&ip_sysctl_lock);
+			rw_exit_write(&sysctl_lock);
 		}
 
 		return (error);
Index: sys/netinet/ip_var.h
===================================================================
RCS file: /cvs/src/sys/netinet/ip_var.h,v
retrieving revision 1.122
diff -u -p -r1.122 ip_var.h
--- sys/netinet/ip_var.h	21 Jun 2025 14:21:17 -0000	1.122
+++ sys/netinet/ip_var.h	27 Jul 2025 18:58:30 -0000
@@ -285,9 +285,5 @@ int	 rip_send(struct socket *, struct mb
 extern struct socket *ip_mrouter[];	/* multicast routing daemon */
 #endif
 
-#ifndef SMALL_KERNEL
-extern struct rwlock ip_sysctl_lock;
-#endif
-
 #endif /* _KERNEL */
 #endif /* _NETINET_IP_VAR_H_ */
Index: sys/netinet6/in6_proto.c
===================================================================
RCS file: /cvs/src/sys/netinet6/in6_proto.c,v
retrieving revision 1.148
diff -u -p -r1.148 in6_proto.c
--- sys/netinet6/in6_proto.c	27 Jul 2025 17:46:58 -0000	1.148
+++ sys/netinet6/in6_proto.c	27 Jul 2025 18:58:30 -0000
@@ -120,6 +120,7 @@ const struct protosw inet6sw[] = {
 {
   .pr_domain	= &inet6domain,
   .pr_protocol	= IPPROTO_IPV6,
+  .pr_flags	= PR_MPSYSCTL,
   .pr_init	= ip6_init,
   .pr_slowtimo	= frag6_slowtimo,
   .pr_sysctl	= ip6_sysctl
Index: sys/netinet6/ip6_input.c
===================================================================
RCS file: /cvs/src/sys/netinet6/ip6_input.c,v
retrieving revision 1.293
diff -u -p -r1.293 ip6_input.c
--- sys/netinet6/ip6_input.c	27 Jul 2025 17:46:58 -0000	1.293
+++ sys/netinet6/ip6_input.c	27 Jul 2025 18:58:30 -0000
@@ -1484,14 +1484,27 @@ ip6_sysctl_ip6stat(void *oldp, size_t *o
 int
 ip6_sysctl_soiikey(void *oldp, size_t *oldlenp, void *newp, size_t newlen)
 {
+	uint8_t soiikey[sizeof(ip6_soiikey)];
 	int error;
 
 	error = suser(curproc);
 	if (error != 0)
 		return (error);
 
-	return (sysctl_struct(oldp, oldlenp, newp, newlen, ip6_soiikey,
-	    sizeof(ip6_soiikey)));
+	rw_enter_read(&sysctl_lock);
+	memcpy(soiikey, ip6_soiikey, sizeof(ip6_soiikey));
+	rw_exit_read(&sysctl_lock);
+
+	error = sysctl_struct(oldp, oldlenp, newp, newlen, soiikey,
+	    sizeof(soiikey));
+
+	if (error == 0 && newp) {
+		rw_enter_write(&sysctl_lock);
+		memcpy(ip6_soiikey, soiikey, sizeof(soiikey));
+		rw_exit_write(&sysctl_lock);
+	}
+
+	return (error);
 }
 
 int
@@ -1533,10 +1546,10 @@ ip6_sysctl(int *name, u_int namelen, voi
 		error = sysctl_int_bounded(oldp, oldlenp, newp, newlen,
 		    &newval, 0, INT_MAX);
 		if (error == 0 && oldval != newval) {
-			rw_enter_write(&ip_sysctl_lock);
+			rw_enter_write(&sysctl_lock);
 			atomic_store_int(&ip6_mtudisc_timeout, newval);
 			rt_timer_queue_change(&icmp6_mtudisc_timeout_q, newval);
-			rw_exit_write(&ip_sysctl_lock);
+			rw_exit_write(&sysctl_lock);
 		}
 
 		return (error);
Index: sys/sys/sysctl.h
===================================================================
RCS file: /cvs/src/sys/sys/sysctl.h,v
retrieving revision 1.245
diff -u -p -r1.245 sysctl.h
--- sys/sys/sysctl.h	24 Jul 2025 19:42:41 -0000	1.245
+++ sys/sys/sysctl.h	27 Jul 2025 18:58:30 -0000
@@ -1030,6 +1030,8 @@ struct sysctl_bounded_args {
  */
 typedef int (sysctlfn)(int *, u_int, void *, size_t *, void *, size_t, struct proc *);
 
+extern struct rwlock sysctl_lock;
+
 int sysctl_vslock(void *, size_t);
 void sysctl_vsunlock(void *, size_t);