Re: [PATCH bpf] bpf/tracing: fix a deadlock in perf_event_detach_bpf_prog

[Yonghong Song <yhs@fb.com>]

On 4/9/18 9:47 AM, Alexei Starovoitov wrote:
> On 4/9/18 9:18 AM, Yonghong Song wrote:
>> syzbot reported a possible deadlock in perf_event_detach_bpf_prog.
> ...
>> @@ -985,16 +986,31 @@ int perf_event_query_prog_array(struct 
>> perf_event *event, void __user *info)
>>          return -EINVAL;
>>      if (copy_from_user(&query, uquery, sizeof(query)))
>>          return -EFAULT;
>> -    if (query.ids_len > BPF_TRACE_MAX_PROGS)
>> +
>> +    ids_len = query.ids_len;
>> +    if (ids_len > BPF_TRACE_MAX_PROGS)
>>          return -E2BIG;
>> +    ids = kcalloc(ids_len, sizeof(u32), GFP_USER | __GFP_NOWARN);
>> +    if (!ids)
>> +        return -ENOMEM;
>>
>>      mutex_lock(&bpf_event_mutex);
>>      ret = bpf_prog_array_copy_info(event->tp_event->prog_array,
>> -                       uquery->ids,
>> -                       query.ids_len,
>> -                       &uquery->prog_cnt);
>> +                       ids,
>> +                       ids_len,
>> +                       &prog_cnt);
>>      mutex_unlock(&bpf_event_mutex);
>>
>> +    if (!ret || ret == -ENOSPC) {
>> +        if (copy_to_user(&uquery->prog_cnt, &prog_cnt, 
>> sizeof(prog_cnt)) ||
>> +            copy_to_user(uquery->ids, ids, ids_len * sizeof(u32))) {
>> +            ret = -EFAULT;
>> +            goto out;
>> +        }
>> +    }
>> +
>> +out:
>> +    kfree(ids);
> 
> alloc/free just to avoid this locking dependency feels suboptimal.

We actually already did kcalloc/kfree in bpf_prog_array_copy_to_user.
In that function, we did not copy_to_user one id at a time.
We allocate a temporary array and store the result there
and at the end, we call one copy_to_user to copy to the user buffer.

The patch here just moved this allocation and associated copy_to_user
out of the function and bpf_event_mutex. It did not introduce new
allocations.

> 
> may be we can get rid of bpf_event_mutex in some cases?
> the perf event itself is locked via perf_event_ctx_lock() when we're
> calling perf_event_query_prog_array, perf_event_attach|detach_bpf_prog.
> I forgot what was the motivation for us to introduce it in the
> first place.

The original motivation for the lock to make sure bpf_prog_array
does not change during middle of attach/detach/query. it looks like
we have:
    . perf_event_attach|query under perf_event_ctx_lock
    . perf_event_detach not under perf_event_ctx_lock
Introducing perf_event_ctx_lock to perf_event_detach could still
have the deadlock.

>