linux-kernel - Re: [syzbot] KCSAN: data-race in __io_uring_cancel / io_uring_try_cancel

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Message-ID: <CANpmjNP1CKuoK82HCRYpDxDrvy4DgN9yVknfsxHSwfojx5Ttug@mail.gmail.com>
Date:   Wed, 26 May 2021 18:36:33 +0200
From:   Marco Elver <elver@...gle.com>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        syzbot <syzbot+73554e2258b7b8bf0bbf@...kaller.appspotmail.com>,
        io-uring@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [syzbot] KCSAN: data-race in __io_uring_cancel / io_uring_try_cancel_requests

On Wed, 26 May 2021 at 18:29, Pavel Begunkov <asml.silence@...il.com> wrote:
> On 5/26/21 4:52 PM, Marco Elver wrote:
> > Due to some moving around of code, the patch lost the actual fix (using
> > atomically read io_wq) -- so here it is again ... hopefully as intended.
> > :-)
>
> "fortify" damn it... It was synchronised with &ctx->uring_lock
> before, see io_uring_try_cancel_iowq() and io_uring_del_tctx_node(),
> so should not clear before *del_tctx_node()

Ah, so if I understand right, the property stated by the comment in
io_uring_try_cancel_iowq() was broken, and your patch below would fix
that, right?

> The fix should just move it after this sync point. Will you send
> it out as a patch?

Do you mean your move of write to io_wq goes on top of the patch I
proposed? (If so, please also leave your Signed-of-by so I can squash
it.)

So if I understand right, we do in fact have 2 problems:
1. the data race as I noted in my patch, and
2. the fact that io_wq does not live long enough.

Did I get it right?

Thanks,
-- Marco

> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 7db6aaf31080..b76ba26b4c6c 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -9075,11 +9075,12 @@ static void io_uring_clean_tctx(struct io_uring_task *tctx)
>         struct io_tctx_node *node;
>         unsigned long index;
>
> -       tctx->io_wq = NULL;
>         xa_for_each(&tctx->xa, index, node)
>                 io_uring_del_tctx_node(index);
> -       if (wq)
> +       if (wq) {
> +               tctx->io_wq = NULL;
>                 io_wq_put_and_exit(wq);
> +       }
>  }
>
>  static s64 tctx_inflight(struct io_uring_task *tctx, bool tracked)
>
>
> --
> Pavel Begunkov