linux-kernel - Re: [PATCH 3/5] io_uring: let fast poll support multishot

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Message-ID: <e29d7079-ff52-db69-c215-7212682fd3fc@gmail.com>
Date:   Sat, 7 May 2022 10:26:07 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, Hao Xu <haoxu.linux@...il.com>,
        io-uring@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] io_uring: let fast poll support multishot

On 5/6/22 23:02, Jens Axboe wrote:
> On 5/6/22 11:19 AM, Pavel Begunkov wrote:
>> On 5/6/22 08:01, Hao Xu wrote:
>>> From: Hao Xu <howeyxu@...cent.com>
>>>
>>> For operations like accept, multishot is a useful feature, since we can
>>> reduce a number of accept sqe. Let's integrate it to fast poll, it may
>>> be good for other operations in the future.
>>>
>>> Signed-off-by: Hao Xu <howeyxu@...cent.com>
>>> ---
>>>    fs/io_uring.c | 41 ++++++++++++++++++++++++++---------------
>>>    1 file changed, 26 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index 8ebb1a794e36..d33777575faf 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -5952,7 +5952,7 @@ static void io_poll_remove_entries(struct io_kiocb *req)
>>>     * either spurious wakeup or multishot CQE is served. 0 when it's done with
>>>     * the request, then the mask is stored in req->cqe.res.
>>>     */
>>> -static int io_poll_check_events(struct io_kiocb *req, bool locked)
>>> +static int io_poll_check_events(struct io_kiocb *req, bool *locked)
>>>    {
>>>        struct io_ring_ctx *ctx = req->ctx;
>>>        int v;
>>> @@ -5981,17 +5981,26 @@ static int io_poll_check_events(struct io_kiocb *req, bool locked)
>>>              /* multishot, just fill an CQE and proceed */
>>>            if (req->cqe.res && !(req->apoll_events & EPOLLONESHOT)) {
>>> -            __poll_t mask = mangle_poll(req->cqe.res & req->apoll_events);
>>> -            bool filled;
>>> -
>>> -            spin_lock(&ctx->completion_lock);
>>> -            filled = io_fill_cqe_aux(ctx, req->cqe.user_data, mask,
>>> -                         IORING_CQE_F_MORE);
>>> -            io_commit_cqring(ctx);
>>> -            spin_unlock(&ctx->completion_lock);
>>> -            if (unlikely(!filled))
>>> -                return -ECANCELED;
>>> -            io_cqring_ev_posted(ctx);
>>> +            if (req->flags & REQ_F_APOLL_MULTISHOT) {
>>> +                io_tw_lock(req->ctx, locked);
>>> +                if (likely(!(req->task->flags & PF_EXITING)))
>>> +                    io_queue_sqe(req);
>>
>> That looks dangerous, io_queue_sqe() usually takes the request
>> ownership and doesn't expect that someone, i.e.
>> io_poll_check_events(), may still be actively using it.
> 
> I took a look at this, too. We do own the request at this point, but

Right, but we don't pass the ownership into io_queue_sqe(). IOW,
it can potentially free it / use tw / etc. inside and then we
return back to io_poll_check_events() with a broken req.

> it's still on the poll list. If io_accept() fails, then we do run the
> poll_clean.
> 
>> E.g. io_accept() fails on fd < 0, return an error, io_queue_sqe() ->
>> io_queue_async() -> io_req_complete_failed() kills it. Then
>> io_poll_check_events() and polling in general carry on using the freed
>> request => UAF. Didn't look at it too carefully, but there might other
>> similar cases.
> 
> But we better have done poll_clean() before returning the error. What am
> I missing here?

One scenario I'd be worry about is sth like:


io_apoll_task_func()                            |
-> io_poll_check_events()                       |
   // 1st iteration                              |
   -> io_queue_sqe()                             |
                                                 | poll cancel()
                                                 |   -> set IO_POLL_CANCEL_FLAG
     -> io_accept() fails                        |
       -> io_poll_clean()                        |
     -> io_req_complete_failed()                 |
   // 2nd iteration finds IO_POLL_CANCEL_FLAG    |
     return -ECANCELLED                          |
-> io_req_complete_failed(req, ret)             |


The problem in this example is double io_req_complete_failed()

-- 
Pavel Begunkov