On Wed, Dec 27, 2023 at 12:20 AM D. Wythe <alibuda@xxxxxxxxxxxxxxxxx> wrote:
>
>
> Hi Alexei,
>
>
> IMMO, nf_unregister_net_hook does not wait for the completion of the
> execution of the hook that is being removed,
> instead, it allocates a new array without the very hook to replace the
> old arrayvia rcu_assign_pointer() (in __nf_hook_entries_try_shrink),
> then it use call_rcu() to release the old one.
>
> You can find more details in commit
> 8c873e2199700c2de7dbd5eedb9d90d5f109462b.
>
> In other words, when nf_unregister_net_hook returns, there may still be
> contexts executing hooks on the
> old array, which means that the `link` may still be accessed after
> nf_unregister_net_hook returns.
>
> And that's the reason why we use kfree_rcu() to release the `link`.
> >> nf_hook_run_bpf
> >> const struct
> >> bpf_nf_link *nf_link = bpf_link;
> >>
> >> bpf_nf_link_release
> >> nf_unregister_net_hook(nf_link->net, &nf_link->hook_ops);
> >>
> >> bpf_nf_link_dealloc
> >> free(link)
> >> bpf_prog_run(link->prog);
Got it.
Sounds like it's an existing bug. If so it should be an independent
patch with Fixes tag.
Also please craft a test case to demonstrate UAF.
>
> I must admit that it is indeed feasible if we eliminate the mutex and
> use cmpxchg to swap the prog (we need to ensure that there is only one
> bpf_prog_put() on the old prog).
> However, when cmpxchg fails, it means that this context has not
> outcompeted the other one, and we have to return a failure. Maybe
> something like this:
>
> if (!cmpxchg(&link->prog, old_prog, new_prog)) {
> /* already replaced by another link_update */
> return -xxx;
> }
>
> As a comparison, The version with the mutex wouldn't encounter this
> error, every update would succeed. I think that it's too harsh for the
> user to receive a failure
> in that case since they haven't done anything wrong.
Disagree. The mutex doesn't prevent this issue.
There is always a race.
It happens when link_update.old_prog_fd and BPF_F_REPLACE
were specified.
One user space passes an FD of the old prog and
another user space doing the same. They both race and one of them
gets
if (old_prog && link->prog != old_prog) {
err = -EPERM;
it's no different with dropping the mutex and doing:
if (old_prog) {
if (!cmpxchg(&link->prog, old_prog, new_prog))
-EPERM
} else {
old_prog = xchg(&link->prog, new_prog);
}
