Potiential nullptr derefence in io_do_iopoll

Potiential nullptr derefence in io_do_iopoll

[chase xd]

Dear maintainers,

I'm looking into io_uring internals and find there might be a bug in
io_do_iopoll, but I'm struggling to construct a POC due to my lack of
knowledge about the kernel internals. So maybe it's better to put it
here for discussion.

After [issuing an
SQE](https://elixir.bootlin.com/linux/v6.9/source/io_uring/io_uring.c#L1920)
in iopoll-enabled io_uring, if the return value is
`IOU_ISSUE_SKIP_COMPLETE` and the op supports `iopoll_queue`, the req
will be added to `ctx->iopoll_list` and later retrieved in
`io_do_iopoll`, where `iopoll` or `uring_cmd_iopoll` of the req file
op is [called](https://elixir.bootlin.com/linux/v6.9/source/io_uring/rw.c#L1167).

IMHO here we miss a check of whether `iopoll` or `uring_cmd_iopoll` is
implemented. A more understandable case for me is, the custom ioctl
function with [IORING_OP_URING_CMD](https://elixir.bootlin.com/linux/v6.11.7/source/io_uring/opdef.c#L416)
satisfies all the constraints and will go to this path if `uring_cmd`
returns `-EIOCBQUEUED`
[here](https://elixir.bootlin.com/linux/v6.9/source/io_uring/uring_cmd.c#L192).

So this requires that all the ops with `->uring_cmd` returning
`-EIOCBQUEUED` should support `->uring_cmd_iopoll` as well, which is
not the case for
[ublk_ch_fops](https://elixir.bootlin.com/linux/v6.9/source/drivers/block/ublk_drv.c#L1967)
resulting in a nullptr-deref in `io_do_poll`. I'm wondering if this is
legit.

Note that the related code changes a bit for [newer
kernel](https://elixir.bootlin.com/linux/v6.11/source/io_uring/uring_cmd.c#L261).

Best Regards

Re: Potiential nullptr derefence in io_do_iopoll

[chase xd]

Sorry, apparently I missed a check
[here](https://elixir.bootlin.com/linux/v6.9/source/io_uring/uring_cmd.c#L169).
this is not legit then.

On Sun, Nov 10, 2024 at 2:24 PM chase xd <sl1589472800@gmail.com> wrote:
>
> Dear maintainers,
>
> I'm looking into io_uring internals and find there might be a bug in
> io_do_iopoll, but I'm struggling to construct a POC due to my lack of
> knowledge about the kernel internals. So maybe it's better to put it
> here for discussion.
>
> After [issuing an
> SQE](https://elixir.bootlin.com/linux/v6.9/source/io_uring/io_uring.c#L1920)
> in iopoll-enabled io_uring, if the return value is
> `IOU_ISSUE_SKIP_COMPLETE` and the op supports `iopoll_queue`, the req
> will be added to `ctx->iopoll_list` and later retrieved in
> `io_do_iopoll`, where `iopoll` or `uring_cmd_iopoll` of the req file
> op is [called](https://elixir.bootlin.com/linux/v6.9/source/io_uring/rw.c#L1167).
>
> IMHO here we miss a check of whether `iopoll` or `uring_cmd_iopoll` is
> implemented. A more understandable case for me is, the custom ioctl
> function with [IORING_OP_URING_CMD](https://elixir.bootlin.com/linux/v6.11.7/source/io_uring/opdef.c#L416)
> satisfies all the constraints and will go to this path if `uring_cmd`
> returns `-EIOCBQUEUED`
> [here](https://elixir.bootlin.com/linux/v6.9/source/io_uring/uring_cmd.c#L192).
>
> So this requires that all the ops with `->uring_cmd` returning
> `-EIOCBQUEUED` should support `->uring_cmd_iopoll` as well, which is
> not the case for
> [ublk_ch_fops](https://elixir.bootlin.com/linux/v6.9/source/drivers/block/ublk_drv.c#L1967)
> resulting in a nullptr-deref in `io_do_poll`. I'm wondering if this is
> legit.
>
> Note that the related code changes a bit for [newer
> kernel](https://elixir.bootlin.com/linux/v6.11/source/io_uring/uring_cmd.c#L261).
>
> Best Regards