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 tcxentry

Pedro Pinto and later independently also Hyunwoo Kim and Wongi Lee reported an issue that the tcxentry 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 tcxentry, and &tcxentry->miniq is stored in the qdisc's miniqp->pminiq. 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 tcfchain0headchangecbdel() 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 tcxentry:

rtnetlinkrcvmsg() => tcmodifyqdisc() => qdisccreate() => clsactinit() [a] => qdiscgraft() => qdiscdestroy() => qdiscdestroy() => ingressdestroy() [b] => tcxentryfree() => kfreercu() // tcxentry freed

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

cleanupnet() => opsexitlist() => defaultdeviceexitbatch() => unregisternetdevicemany() => unregisternetdevicemanynotify() => devshutdown() => qdiscput() => clsactdestroy() [c] => tcfblockputext() => tcfchain0headchangecbdel() => tcfchainheadchangeitem() => clsactchainheadchange() => miniqdiscpairswap() // 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 tcxentry kfreercu() to be executed and subsequently accessing the current active qdisc's miniq one way or another.

The correct fix is to turn the miniqactive 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 tcxentry is freed at the correct time.