CVE: CVE-2024-41010
Published: 2024-07-17
Description: 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.
CVSS Score: 5.5 (MEDIUM)
CWE: CWE-416
References: https://git.kernel.org/stable/c/1cb6f0bae50441f4b4b32a28315853b279c7404e; https://git.kernel.org/stable/c/230bb13650b0f186f540500fd5f5f7096a822a2a; https://git.kernel.org/stable/c/f61ecf1bd5b562ebfd7d430ccb31619857e80857