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

[Pavel Begunkov]

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@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

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