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

[Jens Axboe]

On 5/6/22 11:19 AM, Pavel Begunkov wrote:
> On 5/6/22 08:01, Hao Xu wrote:
>> From: Hao Xu <howeyxu@xxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxx>
>> ---
>>   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
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?

-- 
Jens Axboe