CVE-2025-38721

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

netfilter: ctnetlink: fix refcount leak on table dump

There is a reference count leak in ctnetlink_dump_table(): if (res < 0) { nf_conntrack_get(&ct->ct_general); // HERE cb->args[1] = (unsigned long)ct; …

While its very unlikely, its possible that ct == last. If this happens, then the refcount of ct was already incremented. This 2nd increment is never undone.

This prevents the conntrack object from being released, which in turn keeps prevents cnet->count from dropping back to 0.

This will then block the netns dismantle (or conntrack rmmod) as nf_conntrack_cleanup_net_list() will wait forever.

This can be reproduced by running conntrack_resize.sh selftest in a loop. It takes ~20 minutes for me on a preemptible kernel on average before I see a runaway kworker spinning in nf_conntrack_cleanup_net_list.

One fix would to change this to: if (res < 0) { if (ct != last) nf_conntrack_get(&ct->ct_general);

But this reference counting isnt needed in the first place. We can just store a cookie value instead.

A followup patch will do the same for ctnetlink_exp_dump_table, it looks to me as if this has the same problem and like ctnetlink_dump_table, we only need a skip hint, not the actual object so we can apply the same cookie strategy there as well.

Affected Software

References

• https://git.kernel.org/stable/c/19b909a4b1452fb97e477d2f08b97f8d04095619
• https://git.kernel.org/stable/c/30cf811058552b8cd0e98dff677ef3f89d6d34ce
• https://git.kernel.org/stable/c/41462f4cfc583513833f87f9ee55d12da651a7e3
• https://git.kernel.org/stable/c/586892e341fbf698e7cbaca293e1353957db725a
• https://git.kernel.org/stable/c/962518c6ca9f9a13df099cafa429f72f68ad61f0
• https://git.kernel.org/stable/c/a2cb4df7872de069f809de2f076ec8e54d649fe3
• https://git.kernel.org/stable/c/a62d6aa3f31f216b637a4c71b7a8bfc7c57f049b
• https://git.kernel.org/stable/c/de788b2e6227462b6dcd0e07474e72c089008f74
• https://git.kernel.org/stable/c/e14f72aa66c029db106921d621edcedef68e065b