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

[Pavel Begunkov <asml.silence@gmail.com>]

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?

-- 
Pavel Begunkov