Revert "net: increase fragment memory usage limits"

This reverts commit c2a93660. It
made denial of service attacks on the IP fragment handling easier to
carry out.

CVE-2018-5391
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

include/net/ipv6.h

@@ -327,8 +327,8 @@ static inline int ip6_frag_mem(struct net *net)
 }
 #endif
 
-#define IPV6_FRAG_HIGH_THRESH	(4 * 1024*1024)	/* 4194304 */
-#define IPV6_FRAG_LOW_THRESH	(3 * 1024*1024)	/* 3145728 */
+#define IPV6_FRAG_HIGH_THRESH	(256 * 1024)	/* 262144 */
+#define IPV6_FRAG_LOW_THRESH	(192 * 1024)	/* 196608 */
 #define IPV6_FRAG_TIMEOUT	(60 * HZ)	/* 60 seconds */
 
 int __ipv6_addr_type(const struct in6_addr *addr);

net/ipv4/ip_fragment.c

@@ -849,22 +849,14 @@ static void __init ip4_frags_ctl_register(void)
 
 static int __net_init ipv4_frags_init_net(struct net *net)
 {
-	/* Fragment cache limits.
-	 *
-	 * The fragment memory accounting code, (tries to) account for
-	 * the real memory usage, by measuring both the size of frag
-	 * queue struct (inet_frag_queue (ipv4:ipq/ipv6:frag_queue))
-	 * and the SKB's truesize.
-	 *
-	 * A 64K fragment consumes 129736 bytes (44*2944)+200
-	 * (1500 truesize == 2944, sizeof(struct ipq) == 200)
-	 *
-	 * We will commit 4MB at one time. Should we cross that limit
-	 * we will prune down to 3MB, making room for approx 8 big 64K
-	 * fragments 8x128k.
+	/*
+	 * Fragment cache limits. We will commit 256K at one time. Should we
+	 * cross that limit we will prune down to 192K. This should cope with
+	 * even the most extreme cases without allowing an attacker to
+	 * measurably harm machine performance.
 	 */
-	net->ipv4.frags.high_thresh = 4 * 1024 * 1024;
-	net->ipv4.frags.low_thresh  = 3 * 1024 * 1024;
+	net->ipv4.frags.high_thresh = 256 * 1024;
+	net->ipv4.frags.low_thresh = 192 * 1024;
 	/*
 	 * Important NOTE! Fragment queue must be destroyed before MSL expires.
 	 * RFC791 is wrong proposing to prolongate timer each fragment arrival