icmp: randomize the global rate limiter

[ Upstream commit b38e7819cae946e2edf869e604af1e65a5d241c5 ]

Keyu Man reported that the ICMP rate limiter could be used
by attackers to get useful signal. Details will be provided
in an upcoming academic publication.

Our solution is to add some noise, so that the attackers
no longer can get help from the predictable token bucket limiter.

Fixes: 4cdf507d5452 ("icmp: add a global rate limitation")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Keyu Man <kman001@ucr.edu>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 3bbe6fb2b086b3fbcaf28ea0220b36b9fee948b5..5f1e3dc567f1db2c27a8cc18b7b36044fe71c8a5 100644 (file)
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -905,12 +905,14 @@ icmp_ratelimit - INTEGER
 icmp_msgs_per_sec - INTEGER
        Limit maximal number of ICMP packets sent per second from this host.
        Only messages whose type matches icmp_ratemask (see below) are
-       controlled by this limit.
+       controlled by this limit. For security reasons, the precise count
+       of messages per second is randomized.
        Default: 1000
 
 icmp_msgs_burst - INTEGER
        icmp_msgs_per_sec controls number of ICMP packets sent per second,
        while icmp_msgs_burst controls the burst size of these packets.
+       For security reasons, the precise burst size is randomized.
        Default: 50
 
 icmp_ratemask - INTEGER

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 995ef3d23368987c0fe2e1964081552647f68a04..41c78a716b9aa6b895f6bb2944b109365f3f3b6a 100644 (file)
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -244,7 +244,7 @@ static struct {
 /**
  * icmp_global_allow - Are we allowed to send one more ICMP message ?
  *
- * Uses a token bucket to limit our ICMP messages to sysctl_icmp_msgs_per_sec.
+ * Uses a token bucket to limit our ICMP messages to ~sysctl_icmp_msgs_per_sec.
  * Returns false if we reached the limit and can not send another packet.
  * Note: called with BH disabled
  */
@@ -272,7 +272,10 @@ bool icmp_global_allow(void)
        }
        credit = min_t(u32, icmp_global.credit + incr, sysctl_icmp_msgs_burst);
        if (credit) {
-               credit--;
+               /* We want to use a credit of one in average, but need to randomize
+                * it for security reasons.
+                */
+               credit = max_t(int, credit - prandom_u32_max(3), 0);
                rc = true;
        }
        WRITE_ONCE(icmp_global.credit, credit);