In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_route: free emptied bucket on filter move route4_change

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

net/sched: cls_route: free emptied bucket on filter move

route4_change can move an existing filter to a different top-level
bucket: route4_set_parms recomputes the handle from TCA_ROUTE4_TO/
FROM/IIF, and the handle-mismatch check is gated on the 'new' flag, so
for an existing filter the new handle may differ from the old one and
land in a different bucket. When this happens, the filter is unlinked
from the old bucket, but the bucket itself is never freed once it goes
empty. The stale empty bucket remains in head->table[], causing
route4_delete to report *last=false even after the last live filter is
gone. That pins the empty tcf_proto and causes a leak.

Fix this by refcounting the filters linked to a bucket and freeing the
bucket when the count drops to zero. The existing scan in route4_delete
goes away with it.

The count is updated at all sites that link or unlink a filter during add,
change and delete, and the bucket is dropped from head->table[] as soon as
it reaches zero.

Conditions to recreate the bug:
CONFIG_NET_CLS_ROUTE4=y, CONFIG_NET_SCH_INGRESS=y, CONFIG_NET_CLS_ACT=y.

tc qdisc replace dev lo clsact
tc filter add dev lo ingress protocol ip pref 100 route from 1 to 1
tc filter change dev lo ingress protocol ip pref 100 handle 0x10001
route from 1 to 2
tc filter del dev lo ingress protocol ip pref 100 handle 0x10002
route from 1 to 2
tc filter show dev lo ingress | grep -c 'pref 100 route chain 0 '