Re: [PATCH] io_uring: don't issue reqs in iopoll mode when ctx is dying

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

On 06/02/2021 15:00, Xiaoguang Wang wrote:
> Abaci Robot reported following panic:
> ------------[ cut here ]------------
> refcount_t: underflow; use-after-free.
> WARNING: CPU: 1 PID: 195 at lib/refcount.c:28 refcount_warn_saturate+0x137/0x140
> Modules linked in:
> CPU: 1 PID: 195 Comm: kworker/u4:2 Not tainted 5.11.0-rc3+ #70
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.1-0-g0551a4be2c-prebuilt.qemu-project.or4
> Workqueue: events_unbound io_ring_exit_work
> RIP: 0010:refcount_warn_saturate+0x137/0x140
> Code: 05 ad 63 49 08 01 e8 45 0f 6f 00 0f 0b e9 16 ff ff ff e8 4c ba ae ff 48 c7 c7 28 2e 7c 82 c6 05 90 63 40
> RSP: 0018:ffffc900002e3cc8 EFLAGS: 00010282
> RAX: 0000000000000000 RBX: 0000000000000003 RCX: 0000000000000000
> RDX: ffff888102918000 RSI: ffffffff81150a34 RDI: ffff88813bd28570
> RBP: ffff8881075cd348 R08: 0000000000000001 R09: 0000000000000001
> R10: 0000000000000001 R11: 0000000000080000 R12: ffff8881075cd308
> R13: ffff8881075cd348 R14: ffff888122d33ab8 R15: ffff888104780300
> FS:  0000000000000000(0000) GS:ffff88813bd00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000108 CR3: 0000000107636005 CR4: 0000000000060ee0
> Call Trace:
>  io_dismantle_req+0x3f3/0x5b0
>  __io_free_req+0x2c/0x270
>  io_put_req+0x4c/0x70
>  io_wq_cancel_cb+0x171/0x470
>  ? io_match_task.part.0+0x80/0x80
>  __io_uring_cancel_task_requests+0xa0/0x190
>  io_ring_exit_work+0x32/0x3e0
>  process_one_work+0x2f3/0x720
>  worker_thread+0x5a/0x4b0
>  ? process_one_work+0x720/0x720
>  kthread+0x138/0x180
>  ? kthread_park+0xd0/0xd0
>  ret_from_fork+0x1f/0x30
> 
> Later system will panic for some memory corruption.
> 
> The io_identity's count is underflowed. It's because in io_put_identity,
> first argument tctx comes from req->task->io_uring, the second argument
> comes from the task context that calls io_req_init_async, so the compare
> in io_put_identity maybe meaningless. See below case:
>     task context A issue one polled req, then req->task = A.
>     task context B do iopoll, above req returns with EAGAIN error.
>     task context B re-issue req, call io_queue_async_work for req.
>     req->task->io_uring will set to task context B's identity, or cow new one.
> then for above case, in io_put_identity(), the compare is meaningless.
> 
> IIUC, req->task should indicates the initial task context that issues req,
> then if it gets EAGAIN error, we'll call io_prep_async_work() in req->task
> context, but iopoll reqs seems special, they maybe issued successfully and
> got re-issued in other task context because of EAGAIN error.

Looks as you say, but the patch doesn't solve the issue completely.
1. We must not do io_queue_async_work() under a different task context,
because of it potentially uses a different set of resources. So, I just
thought that it would be better to punt it to the right task context
via task_work. But...

2. ...iovec import from io_resubmit_prep() might happen after submit ends,
i.e. when iovec was freed in userspace. And that's not great at all.

> Currently for this panic, we can disable issuing reqs that are returned
> with EAGAIN error in iopoll mode when ctx is dying, but we may need to
> re-consider the io identity codes more.
> 
> Reported-by: Abaci Robot <abaci@linux.alibaba.com>
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
> ---
>  fs/io_uring.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 9db05171a774..e3b90426d72b 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2467,7 +2467,12 @@ static void io_iopoll_complete(struct io_ring_ctx *ctx, unsigned int *nr_events,
>  		int cflags = 0;
>  
>  		req = list_first_entry(done, struct io_kiocb, inflight_entry);
> -		if (READ_ONCE(req->result) == -EAGAIN) {
> +		/*
> +		 * If ctx is dying, don't need to issue reqs that are returned
> +		 * with EAGAIN error, since there maybe no users to reap them.
> +		 */
> +		if ((READ_ONCE(req->result) == -EAGAIN) &&
> +		    !percpu_ref_is_dying(&ctx->refs)) {
>  			req->result = 0;
>  			req->iopoll_completed = 0;
>  			list_move_tail(&req->inflight_entry, &again);
> 

-- 
Pavel Begunkov