From d6c552505c0d1719dda42b4af2def0618bd7bf54 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Thu, 15 Oct 2020 11:42:00 -0700 Subject: [PATCH] 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 Reported-by: Keyu Man Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman --- Documentation/networking/ip-sysctl.txt | 4 +++- net/ipv4/icmp.c | 7 +++++-- 2 files changed, 8 insertions(+), 3 deletions(-) diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt index 7eb9366422f5..3c617d620b6f 100644 --- a/Documentation/networking/ip-sysctl.txt +++ b/Documentation/networking/ip-sysctl.txt @@ -934,12 +934,14 @@ 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 4efa5e33513e..59c0b1a86e51 100644 --- 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);