CVE-2024-46828: sched: sch_cake: fix bulk flow accounting logic for host fairness

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

sched: schcake: fix bulk flow accounting logic for host fairness

In schcake, we keep track of the count of active bulk flows per host, when running in dst/src host fairness mode, which is used as the round-robin weight when iterating through flows. The count of active bulk flows is updated whenever a flow changes state.

This has a peculiar interaction with the hash collision handling: when a hash collision occurs (after the set-associative hashing), the state of the hash bucket is simply updated to match the new packet that collided, and if host fairness is enabled, that also means assigning new per-host state to the flow. For this reason, the bulk flow counters of the host(s) assigned to the flow are decremented, before new state is assigned (and the counters, which may not belong to the same host anymore, are incremented again).

Back when this code was introduced, the host fairness mode was always enabled, so the decrement was unconditional. When the configuration flags were introduced the increment was made conditional, but the decrement was not. Which of course can lead to a spurious decrement (and associated wrap-around to U16MAX).

AFAICT, when host fairness is disabled, the decrement and wrap-around happens as soon as a hash collision occurs (which is not that common in itself, due to the set-associative hashing). However, in most cases this is harmless, as the value is only used when host fairness mode is enabled. So in order to trigger an array overflow, schcake has to first be configured with host fairness disabled, and while running in this mode, a hash collision has to occur to cause the overflow. Then, the qdisc has to be reconfigured to enable host fairness, which leads to the array out-of-bounds because the wrapped-around value is retained and used as an array index. It seems that syzbot managed to trigger this, which is quite impressive in its own right.

This patch fixes the issue by introducing the same conditional check on decrement as is used on increment.

The original bug predates the upstreaming of cake, but the commit listed in the Fixes tag touched that code, meaning that this patch won't apply before that.