CVE-2024-41010 bpf: Fix too early release of tcx_entry

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

bpf: Fix too early release of tcx_entry

Pedro Pinto and later independently also Hyunwoo Kim and Wongi Lee reported
an issue that the tcx_entry can be released too early leading to a use
after free (UAF) when an active old-style ingress or clsact qdisc with a
shared tc block is later replaced by another ingress or clsact instance.

Essentially, the sequence to trigger the UAF (one example) can be as follows:

1. A network namespace is created

2. An ingress qdisc is created. This allocates a tcx_entry, and
   &tcx_entry->miniq is stored in the qdisc’s miniqp->p_miniq. At the
   same time, a tcf block with index 1 is created.

3. chain0 is attached to the tcf block. chain0 must be connected to
   the block linked to the ingress qdisc to later reach the function
   tcf_chain0_head_change_cb_del() which triggers the UAF.

4. Create and graft a clsact qdisc. This causes the ingress qdisc
   created in step 1 to be removed, thus freeing the previously linked
   tcx_entry:

   rtnetlink_rcv_msg()
   => tc_modify_qdisc()
   => qdisc_create()
   => clsact_init() [a]
   => qdisc_graft()
   => qdisc_destroy()
   => __qdisc_destroy()
   => ingress_destroy() [b]
   => tcx_entry_free()
   => kfree_rcu() // tcx_entry freed

5. Finally, the network namespace is closed. This registers the
   cleanup_net worker, and during the process of releasing the
   remaining clsact qdisc, it accesses the tcx_entry that was
   already freed in step 4, causing the UAF to occur:

   cleanup_net()
   => ops_exit_list()
   => default_device_exit_batch()
   => unregister_netdevice_many()
   => unregister_netdevice_many_notify()
   => dev_shutdown()
   => qdisc_put()
   => clsact_destroy() [c]
   => tcf_block_put_ext()
   => tcf_chain0_head_change_cb_del()
   => tcf_chain_head_change_item()
   => clsact_chain_head_change()
   => mini_qdisc_pair_swap() // UAF

There are also other variants, the gist is to add an ingress (or clsact)
qdisc with a specific shared block, then to replace that qdisc, waiting
for the tcx_entry kfree_rcu() to be executed and subsequently accessing
the current active qdisc’s miniq one way or another.

The correct fix is to turn the miniq_active boolean into a counter. What
can be observed, at step 2 above, the counter transitions from 0->1, at
step [a] from 1->2 (in order for the miniq object to remain active during
the replacement), then in [b] from 2->1 and finally [c] 1->0 with the
eventual release. The reference counter in general ranges from [0,2] and
it does not need to be atomic since all access to the counter is protected
by the rtnl mutex. With this in place, there is no longer a UAF happening
and the tcx_entry is freed at the correct time.