CVE-2024-41010

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.