Re: [RFC][BUG] io_uring: fix work corruption for poll_add

[Jens Axboe <axboe@kernel.dk>]

On 7/24/20 6:52 AM, Pavel Begunkov wrote:
> On 24/07/2020 15:46, Pavel Begunkov wrote:
>> On 24/07/2020 01:24, Jens Axboe wrote:
>>> On 7/23/20 4:16 PM, Jens Axboe wrote:
>>>> On 7/23/20 12:12 PM, Pavel Begunkov wrote:
>>>>> poll_add can have req->work initialised, which will be overwritten in
>>>>> __io_arm_poll_handler() because of the union. Luckily, hash_node is
>>>>> zeroed in the end, so the damage is limited to lost put for work.creds,
>>>>> and probably corrupted work.list.
>>>>>
>>>>> That's the easiest and really dirty fix, which rearranges members in the
>>>>> union, arm_poll*() modifies and zeroes only work.files and work.mm,
>>>>> which are never taken for poll add.
>>>>> note: io_kiocb is exactly 4 cachelines now.
>>>>
>>>> I don't think there's a way around moving task_work out, just like it
>>
>> +hash_node. I was thinking to do apoll alloc+memcpy as for rw, but this
>> one is ugly.
>>
>>>> was done on 5.9. The problem is that we could put the environment bits
>>>> before doing task_work_add(), but we might need them if the subsequent
>>>> queue ends up having to go async. So there's really no know when we can
>>>> put them, outside of when the request finishes. Hence, we are kind of
>>>> SOL here.
>>>
>>> Actually, if we do go async, then we can just grab the environment
>>> again. We're in the same task at that point. So maybe it'd be better to
>>> work on ensuring that the request is either in the valid work state, or
>>> empty work if using task_work.
>>>
>>> Only potential complication with that is doing io_req_work_drop_env()
>>> from the waitqueue handler, at least the ->needs_fs part won't like that
>>> too much.
>>
>> Considering that work->list is removed before executing io_wq_work, it
>> should work. And if done only for poll_add, which needs nothing and ends up
>> with creds, there shouldn't be any problems. I'll try this out
> 
> Except for custom ->creds assigned at the beginning with the personality
> feature. Does poll ever use it?

It's kind of annoying how we don't have a def->needs_creds, because lots
of things would never use it. For poll, it wouldn't be used at all,
which makes this issue doubly annoying.

-- 
Jens Axboe