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icmp: randomize the global rate limiter

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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: 4cdf507d54 ("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>
This commit is contained in:
Eric Dumazet
2020-10-15 11:42:00 -07:00
committed by Jakub Kicinski
parent 137d23cea1
commit b38e7819ca
2 changed files with 8 additions and 3 deletions
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7
net/ipv4/icmp.c
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@@ -239,7 +239,7 @@ 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
*/
@@ -267,7 +267,10 @@ 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);