icmp: use per rate limit randomized jitter

Using the same random jitter for multiple rate limits allows an
attacker to use one rate limiter to figure out the current jitter
and then use this knowledge to de-randomize the other rate limiters.
This can be mitigated by using a separate randomized jitter for each
rate limiter.
This issue was reported as issue number 10 in Keyu Man et al.:
SCAD: Towards a Universal and Automated Network Side-Channel
Vulnerability Detection

Reviewed by:		rrs, Peter Lei, glebius
MFC after:		3 days
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D48804

sys/netinet/ip_icmp.c

@@ -88,7 +88,7 @@ SYSCTL_PROC(_net_inet_icmp, ICMPCTL_ICMPLIM, icmplim, CTLTYPE_UINT |
     &sysctl_icmplim_and_jitter, "IU",
     "Maximum number of ICMP responses per second");
 
-VNET_DEFINE_STATIC(int, icmplim_curr_jitter) = 0;
+VNET_DEFINE_STATIC(int, icmplim_curr_jitter[BANDLIM_MAX]) = {0};
 #define V_icmplim_curr_jitter		VNET(icmplim_curr_jitter)
 VNET_DEFINE_STATIC(u_int, icmplim_jitter) = 16;
 #define	V_icmplim_jitter		VNET(icmplim_jitter)
@@ -1108,14 +1108,16 @@ static const char *icmp_rate_descrs[BANDLIM_MAX] = {
 };
 
 static void
-icmplim_new_jitter(void)
+icmplim_new_jitter(int which)
 {
 	/*
 	 * Adjust limit +/- to jitter the measurement to deny a side-channel
 	 * port scan as in https://dl.acm.org/doi/10.1145/3372297.3417280
 	 */
+	KASSERT(which >= 0 && which < BANDLIM_MAX,
+	    ("%s: which %d", __func__, which));
 	if (V_icmplim_jitter > 0)
-		V_icmplim_curr_jitter =
+		V_icmplim_curr_jitter[which] =
 		    arc4random_uniform(V_icmplim_jitter * 2 + 1) -
 		    V_icmplim_jitter;
 }
@@ -1144,7 +1146,9 @@ sysctl_icmplim_and_jitter(SYSCTL_HANDLER_ARGS)
 				error = EINVAL;
 			else {
 				V_icmplim_jitter = new;
-				icmplim_new_jitter();
+				for (int i = 0; i < BANDLIM_MAX; i++) {
+					icmplim_new_jitter(i);
+				}
 			}
 		}
 	}
@@ -1160,8 +1164,8 @@ icmp_bandlimit_init(void)
 	for (int i = 0; i < BANDLIM_MAX; i++) {
 		V_icmp_rates[i].cr_rate = counter_u64_alloc(M_WAITOK);
 		V_icmp_rates[i].cr_ticks = ticks;
+		icmplim_new_jitter(i);
 	}
-	icmplim_new_jitter();
 }
 VNET_SYSINIT(icmp_bandlimit, SI_SUB_PROTO_DOMAIN, SI_ORDER_ANY,
     icmp_bandlimit_init, NULL);
@@ -1190,14 +1194,14 @@ badport_bandlim(int which)
 	    ("%s: which %d", __func__, which));
 
 	pps = counter_ratecheck(&V_icmp_rates[which], V_icmplim +
-	    V_icmplim_curr_jitter);
+	    V_icmplim_curr_jitter[which]);
 	if (pps > 0) {
 		if (V_icmplim_output)
 			log(LOG_NOTICE,
 			    "Limiting %s response from %jd to %d packets/sec\n",
 			    icmp_rate_descrs[which], (intmax_t )pps,
-			    V_icmplim + V_icmplim_curr_jitter);
+			    V_icmplim + V_icmplim_curr_jitter[which]);
-		icmplim_new_jitter();
+		icmplim_new_jitter(which);
 	}
 	if (pps == -1)
 		return (-1);

sys/netinet6/icmp6.c

@@ -2750,22 +2750,6 @@ SYSCTL_PROC(_net_inet6_icmp6, ICMPV6CTL_ERRPPSLIMIT, errppslimit,
     &sysctl_icmp6lim_and_jitter, "IU",
     "Maximum number of ICMPv6 error/reply messages per second");
 
-VNET_DEFINE_STATIC(int, icmp6lim_curr_jitter) = 0;
-#define	V_icmp6lim_curr_jitter	VNET(icmp6lim_curr_jitter)
-
-VNET_DEFINE_STATIC(u_int, icmp6lim_jitter) = 8;
-#define	V_icmp6lim_jitter	VNET(icmp6lim_jitter)
-SYSCTL_PROC(_net_inet6_icmp6, OID_AUTO, icmp6lim_jitter, CTLTYPE_UINT |
-    CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(icmp6lim_jitter), 0,
-    &sysctl_icmp6lim_and_jitter, "IU",
-    "Random errppslimit jitter adjustment limit");
-
-VNET_DEFINE_STATIC(int, icmp6lim_output) = 1;
-#define	V_icmp6lim_output	VNET(icmp6lim_output)
-SYSCTL_INT(_net_inet6_icmp6, OID_AUTO, icmp6lim_output,
-    CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(icmp6lim_output), 0,
-    "Enable logging of ICMPv6 response rate limiting");
-
 typedef enum {
 	RATELIM_PARAM_PROB = 0,
 	RATELIM_TOO_BIG,
@@ -2787,15 +2771,33 @@ static const char *icmp6_rate_descrs[RATELIM_MAX] = {
 	[RATELIM_OTHER] = "(other)",
 };
 
+VNET_DEFINE_STATIC(int, icmp6lim_curr_jitter[RATELIM_MAX]) = {0};
+#define	V_icmp6lim_curr_jitter	VNET(icmp6lim_curr_jitter)
+
+VNET_DEFINE_STATIC(u_int, icmp6lim_jitter) = 8;
+#define	V_icmp6lim_jitter	VNET(icmp6lim_jitter)
+SYSCTL_PROC(_net_inet6_icmp6, OID_AUTO, icmp6lim_jitter, CTLTYPE_UINT |
+    CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(icmp6lim_jitter), 0,
+    &sysctl_icmp6lim_and_jitter, "IU",
+    "Random errppslimit jitter adjustment limit");
+
+VNET_DEFINE_STATIC(int, icmp6lim_output) = 1;
+#define	V_icmp6lim_output	VNET(icmp6lim_output)
+SYSCTL_INT(_net_inet6_icmp6, OID_AUTO, icmp6lim_output,
+    CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(icmp6lim_output), 0,
+    "Enable logging of ICMPv6 response rate limiting");
+
 static void
-icmp6lim_new_jitter(void)
+icmp6lim_new_jitter(int which)
 {
 	/*
 	 * Adjust limit +/- to jitter the measurement to deny a side-channel
 	 * port scan as in https://dl.acm.org/doi/10.1145/3372297.3417280
 	 */
+	KASSERT(which >= 0 && which < RATELIM_MAX,
+	    ("%s: which %d", __func__, which));
 	if (V_icmp6lim_jitter > 0)
-		V_icmp6lim_curr_jitter =
+		V_icmp6lim_curr_jitter[which] =
 		    arc4random_uniform(V_icmp6lim_jitter * 2 + 1) -
 		    V_icmp6lim_jitter;
 }
@@ -2824,7 +2826,9 @@ sysctl_icmp6lim_and_jitter(SYSCTL_HANDLER_ARGS)
 				error = EINVAL;
 			else {
 				V_icmp6lim_jitter = new;
-				icmp6lim_new_jitter();
+				for (int i = 0; i < RATELIM_MAX; i++) {
+					icmp6lim_new_jitter(i);
+				}
 			}
 		}
 	}
@@ -2844,8 +2848,8 @@ icmp6_ratelimit_init(void)
 	for (int i = 0; i < RATELIM_MAX; i++) {
 		V_icmp6_rates[i].cr_rate = counter_u64_alloc(M_WAITOK);
 		V_icmp6_rates[i].cr_ticks = ticks;
+		icmp6lim_new_jitter(i);
 	}
-	icmp6lim_new_jitter();
 }
 VNET_SYSINIT(icmp6_ratelimit, SI_SUB_PROTO_DOMAIN, SI_ORDER_ANY,
     icmp6_ratelimit_init, NULL);
@@ -2907,14 +2911,14 @@ icmp6_ratelimit(const struct in6_addr *dst, const int type, const int code)
 	};
 
 	pps = counter_ratecheck(&V_icmp6_rates[which], V_icmp6errppslim +
-	    V_icmp6lim_curr_jitter);
+	    V_icmp6lim_curr_jitter[which]);
 	if (pps > 0) {
 		if (V_icmp6lim_output)
 			log(LOG_NOTICE, "Limiting ICMPv6 %s output from %jd "
 			    "to %d packets/sec\n", icmp6_rate_descrs[which],
 			    (intmax_t )pps, V_icmp6errppslim +
-			    V_icmp6lim_curr_jitter);
+			    V_icmp6lim_curr_jitter[which]);
-		icmp6lim_new_jitter();
+		icmp6lim_new_jitter(which);
 	}
 	if (pps == -1) {
 		ICMP6STAT_INC(icp6s_toofreq);