CVE-2024-47678: Observable Discrepancy

In the Linux kernel, the following vulnerability has been resolved:

icmp: change the order of rate limits

ICMP messages are ratelimited :

After the blamed commits, the two rate limiters are applied in this order:

1) host wide ratelimit (icmp_global_allow())

2) Per destination ratelimit (inetpeer based)

In order to avoid side-channels attacks, we need to apply the per destination check first.

This patch makes the following change :

1) icmp_global_allow() checks if the host wide limit is reached. But credits are not yet consumed. This is deferred to 3)

2) The per destination limit is checked/updated. This might add a new node in inetpeer tree.

3) icmp_global_consume() consumes tokens if prior operations succeeded.

This means that host wide ratelimit is still effective in keeping inetpeer tree small even under DDOS.

As a bonus, I removed icmp_global.lock as the fast path can use a lock-free operation.