Re: [RFC nf-next v3 1/2] netfilter: bpf: support prog update

["D. Wythe" <alibuda@linux.alibaba.com>]

On 12/28/23 3:00 AM, Alexei Starovoitov wrote:
> On Wed, Dec 27, 2023 at 12:20 AM D. Wythe <alibuda@linux.alibaba.com> 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.
>

It is not an existing bug... Accessing the link within the hook was 
something I introduced here
to support updates😉, as previously there was no access to the link 
within the hook.
>> 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);
> }

Got it!  It's very helpful, Thanks very much! I will modify my patch 
accordingly.


Best wishes,
D. Wythe