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

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

On 24/07/2020 17:12, Jens Axboe wrote:
> 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.

Then we don't have to care which one it has, and the scheme should work
good enough for a quick fix.
I still don't like overwriting work.list until it leaves io-wq, but that's
to think about for 5.9

-- 
Pavel Begunkov