Lower the default limits on the IPv6 reassembly queue.

Currently, the limits are quite high. On machines with millions of
mbuf clusters, the reassembly queue limits can also run into
the millions. Lower these values.

Also, 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/netinet6/frag6.c b/sys/netinet6/frag6.c
index 674be961ba56..0f30801540a6 100644
--- a/sys/netinet6/frag6.c
+++ b/sys/netinet6/frag6.c
@@ -71,7 +71,7 @@ __FBSDID("$FreeBSD$");
 /*
  * Reassembly headers are stored in hash buckets.
  */
-#define	IP6REASS_NHASH_LOG2	6
+#define	IP6REASS_NHASH_LOG2	10
 #define	IP6REASS_NHASH		(1 << IP6REASS_NHASH_LOG2)
 #define	IP6REASS_HMASK		(IP6REASS_NHASH - 1)
 
@@ -107,6 +107,22 @@ VNET_DEFINE_STATIC(uint32_t, ip6q_hashseed);
 static MALLOC_DEFINE(M_FTABLE, "fragment", "fragment reassembly header");
 
 /*
+ * By default, limit the number of IP6 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
+ * IP6 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	IP6_MAXFRAGS		(nmbclusters / 32)
+#define	IP6_MAXFRAGPACKETS	(imin(IP6_MAXFRAGS, IP6REASS_NHASH * 50))
+
+/*
  * Initialise reassembly queue and fragment identifier.
  */
 void
@@ -123,11 +139,11 @@ frag6_change(void *tag)
 {
 	VNET_ITERATOR_DECL(vnet_iter);
 
-	ip6_maxfrags = nmbclusters / 4;
+	ip6_maxfrags = IP6_MAXFRAGS;
 	VNET_LIST_RLOCK_NOSLEEP();
 	VNET_FOREACH(vnet_iter) {
 		CURVNET_SET(vnet_iter);
-		V_ip6_maxfragpackets = nmbclusters / 4;
+		V_ip6_maxfragpackets = IP6_MAXFRAGPACKETS;
 		frag6_set_bucketsize();
 		CURVNET_RESTORE();
 	}
@@ -140,7 +156,7 @@ frag6_init(void)
 	struct ip6q *q6;
 	int i;
 
-	V_ip6_maxfragpackets = nmbclusters / 4;
+	V_ip6_maxfragpackets = IP6_MAXFRAGPACKETS;
 	frag6_set_bucketsize();
 	for (i = 0; i < IP6REASS_NHASH; i++) {
 		q6 = IP6Q_HEAD(i);
@@ -153,7 +169,7 @@ frag6_init(void)
 	if (!IS_DEFAULT_VNET(curvnet))
 		return;
 
-	ip6_maxfrags = nmbclusters / 4;
+	ip6_maxfrags = IP6_MAXFRAGS;
 	EVENTHANDLER_REGISTER(nmbclusters_change,
 	    frag6_change, NULL, EVENTHANDLER_PRI_ANY);
 }