Lower the default limits on the IPv4 reassembly queue.

In particular, try to ensure that no bucket will have a reassembly
queue larger than approximately 100 items. This limits the cost to
find the correct reassembly queue when processing an incoming
fragment.

Due to the low limits on each bucket's length, increase the size of
the hash table from 64 to 1024.

Reviewed by:	jhb
Security:	FreeBSD-SA-18:10.ip
Security:	CVE-2018-6923

1 files changed, 21 insertions, 5 deletions

diff --git a/sys/netinet/ip_reass.c b/sys/netinet/ip_reass.c
index d47e13182606..c23ace87a0ee 100644
--- a/sys/netinet/ip_reass.c
+++ b/sys/netinet/ip_reass.c
@@ -64,7 +64,7 @@ SYSCTL_DECL(_net_inet_ip);
 /*
  * Reassembly headers are stored in hash buckets.
  */
-#define	IPREASS_NHASH_LOG2	6
+#define	IPREASS_NHASH_LOG2	10
 #define	IPREASS_NHASH		(1 << IPREASS_NHASH_LOG2)
 #define	IPREASS_HMASK		(IPREASS_NHASH - 1)
 
@@ -116,6 +116,22 @@ ipq_drop(struct ipqbucket *bucket, struct ipq *fp)
 	ipq_free(bucket, fp);
 }
 
+/*
+ * By default, limit the number of IP fragments across all reassembly
+ * queues to  1/32 of the total number of mbuf clusters.
+ *
+ * Limit the total number of reassembly queues per VNET to the
+ * IP fragment limit, but ensure the limit will not allow any bucket
+ * to grow above 100 items. (The bucket limit is
+ * IP_MAXFRAGPACKETS / (IPREASS_NHASH / 2), so the 50 is the correct
+ * multiplier to reach a 100-item limit.)
+ * The 100-item limit was chosen as brief testing seems to show that
+ * this produces "reasonable" performance on some subset of systems
+ * under DoS attack.
+ */
+#define	IP_MAXFRAGS		(nmbclusters / 32)
+#define	IP_MAXFRAGPACKETS	(imin(IP_MAXFRAGS, IPREASS_NHASH * 50))
+
 static int		maxfrags;
 static volatile u_int	nfrags;
 SYSCTL_INT(_net_inet_ip, OID_AUTO, maxfrags, CTLFLAG_RW,
@@ -513,12 +529,12 @@ ipreass_init(void)
 	V_maxfragsperpacket = 16;
 	V_ipq_zone = uma_zcreate("ipq", sizeof(struct ipq), NULL, NULL, NULL,
 	    NULL, UMA_ALIGN_PTR, 0);
-	max = nmbclusters / 32;
+	max = IP_MAXFRAGPACKETS;
 	max = uma_zone_set_max(V_ipq_zone, max);
 	V_ipreass_maxbucketsize = imax(max / (IPREASS_NHASH / 2), 1);
 
 	if (IS_DEFAULT_VNET(curvnet)) {
-		maxfrags = nmbclusters / 32;
+		maxfrags = IP_MAXFRAGS;
 		EVENTHANDLER_REGISTER(nmbclusters_change, ipreass_zone_change,
 		    NULL, EVENTHANDLER_PRI_ANY);
 	}
@@ -622,8 +638,8 @@ ipreass_zone_change(void *tag)
 	VNET_ITERATOR_DECL(vnet_iter);
 	int max;
 
-	maxfrags = nmbclusters / 32;
-	max = nmbclusters / 32;
+	maxfrags = IP_MAXFRAGS;
+	max = IP_MAXFRAGPACKETS;
 	VNET_LIST_RLOCK_NOSLEEP();
 	VNET_FOREACH(vnet_iter) {
 		CURVNET_SET(vnet_iter);