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

[Daniel Borkmann <daniel@iogearbox.net>]

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().

Thanks,
Daniel