Re: [PATCH bpf-next] bpf: Fix RCU usage in bpf_get_cgroup_classid_curr helper

[Daniel Borkmann <daniel@iogearbox.net>]

On 6/10/25 5:51 PM, Charalampos Mitrodimas wrote:
> Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
>> On Tue, Jun 10, 2025 at 8:23 AM Charalampos Mitrodimas
>> <charmitro@posteo.net> wrote:
>>> Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
>>>> On Tue, Jun 10, 2025 at 5:58 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>>>>> On 6/9/25 5:51 PM, Alexei Starovoitov wrote:
>>>>>> On Sun, Jun 8, 2025 at 8:35 AM Charalampos Mitrodimas
>>>>>> <charmitro@posteo.net> wrote:
>>>>>>>
>>>>>>> The commit ee971630f20f ("bpf: Allow some trace helpers for all prog
>>>>>>> types") made bpf_get_cgroup_classid_curr helper available to all BPF
>>>>>>> program types.  This helper used __task_get_classid() which calls
>>>>>>> task_cls_state() that requires rcu_read_lock_bh_held().
>>>>>>>
>>>>>>> This triggers an RCU warning when called from BPF syscall programs
>>>>>>> which run under rcu_read_lock_trace():
>>>>>>>
>>>>>>>     WARNING: suspicious RCU usage
>>>>>>>     6.15.0-rc4-syzkaller-g079e5c56a5c4 #0 Not tainted
>>>>>>>     -----------------------------
>>>>>>>     net/core/netclassid_cgroup.c:24 suspicious rcu_dereference_check() usage!
>>>>>>>
>>>>>>> Fix this by replacing __task_get_classid() with task_cls_classid()
>>>>>>> which handles RCU locking internally using regular rcu_read_lock() and
>>>>>>> is safe to call from any context.
>>>>>>>
>>>>>>> Reported-by: syzbot+b4169a1cfb945d2ed0ec@syzkaller.appspotmail.com
>>>>>>> Closes: https://syzkaller.appspot.com/bug?extid=b4169a1cfb945d2ed0ec
>>>>>>> Fixes: ee971630f20f ("bpf: Allow some trace helpers for all prog types")
>>>>>>> Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.net>
>>>>>>> ---
>>>>>>>    net/core/filter.c | 2 +-
>>>>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>>>>>> index 30e7d36790883b29174654315738e93237e21dd0..3b3f81cf674dde7d2bd83488450edad4e129bdac 100644
>>>>>>> --- a/net/core/filter.c
>>>>>>> +++ b/net/core/filter.c
>>>>>>> @@ -3083,7 +3083,7 @@ static const struct bpf_func_proto bpf_msg_pop_data_proto = {
>>>>>>>    #ifdef CONFIG_CGROUP_NET_CLASSID
>>>>>>>    BPF_CALL_0(bpf_get_cgroup_classid_curr)
>>>>>>>    {
>>>>>>> -       return __task_get_classid(current);
>>>>>>> +       return task_cls_classid(current);
>>>>>>>    }
>>>>>>
>>>>>> Daniel added this helper in
>>>>>> commit 5a52ae4e32a6 ("bpf: Allow to retrieve cgroup v1 classid from v2 hooks")
>>>>>> with intention to use it from networking hooks.
>>>>>>
>>>>>> But task_cls_classid() has
>>>>>>           if (in_interrupt())
>>>>>>                   return 0;
>>>>>>
>>>>>> which will trigger in softirq and tc hooks.
>>>>>> So this might break Daniel's use case.
>>>>>
>>>>> Yeap, we cannot break tc(x) BPF programs. It probably makes sense to have
>>>>> a new helper implementation for the more generic, non-networking case which
>>>>> then internally uses task_cls_classid().
>>>>
>>>> Instead of forking the helper I think we can :
>>>> rcu_read_lock_bh_held() || rcu_read_lock_held()
>>>> in task_cls_state().
>>>
>>> I tested your suggestion with,
>>>
>>>    rcu_read_lock_bh_held() || rcu_read_lock_held()
>>>
>>> but it still triggers the RCU warning because BPF syscall programs use
>>> rcu_read_lock_trace().
>>>
>>> Adding rcu_read_lock_trace_held() fixes it functionally but triggers a
>>> checkpatch warning:
>>>
>>>    WARNING: use of RCU tasks trace is incorrect outside BPF or core RCU code
>>
>> It's safe to ignore checkpatch in this case.
> 
> If that is the case I'll move forward with this. It was my initial fix
> for this[1] anyway, but checkpatch made me change it.

Agree that one is better!

> [1]: https://github.com/charmitro/linux/commit/e5c42d49bfb967c3c35f536971f397492d2f46bf

Thanks,
Daniel