From: Eric Dumazet <edumazet@google.com>

Date: Thu, 15 Oct 2020 11:42:00 -0700

Subject: icmp: randomize the global rate limiter

Patch-mainline: v5.10-rc1

Git-commit: b38e7819cae946e2edf869e604af1e65a5d241c5

References: CVE-2020-25705 bsc#1175721 git-fixes

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>

Acked-by: Michal Kubecek <mkubecek@suse.cz>

---

 Documentation/networking/ip-sysctl.txt | 4 +++-

 net/ipv4/icmp.c                        | 7 +++++--

 2 files changed, 8 insertions(+), 3 deletions(-)

--- a/Documentation/networking/ip-sysctl.txt

+++ b/Documentation/networking/ip-sysctl.txt

@@ -903,12 +903,14 @@ 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

--- 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

  */

@@ -271,7 +271,10 @@ 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;

 	}

 	icmp_global.credit = credit;