Re: [RFC PATCH] io_uring: add support for IORING_OP_IOCTL

[Pavel Begunkov]

On 14/12/2019 20:12, Jann Horn wrote:
> On Sat, Dec 14, 2019 at 4:30 PM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
>> This works almost like ioctl(2), except it doesn't support a bunch of
>> common opcodes, (e.g. FIOCLEX and FIBMAP, see ioctl.c), and goes
>> straight to a device specific implementation.
>>
>> The case in mind is dma-buf, drm and other ioctl-centric interfaces.
>>
>> Not-yet Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
>> ---
>>
>> It clearly needs some testing first, though works fine with dma-buf,
>> but I'd like to discuss whether the use cases are convincing enough,
>> and is it ok to desert some ioctl opcodes. For the last point it's
>> fairly easy to add, maybe except three requiring fd (e.g. FIOCLEX)
>>
>> P.S. Probably, it won't benefit enough to consider using io_uring
>> in drm/mesa, but anyway.
> [...]
>> +static int io_ioctl(struct io_kiocb *req,
>> +                   struct io_kiocb **nxt, bool force_nonblock)
>> +{
>> +       const struct io_uring_sqe *sqe = req->sqe;
>> +       unsigned int cmd = READ_ONCE(sqe->ioctl_cmd);
>> +       unsigned long arg = READ_ONCE(sqe->ioctl_arg);
>> +       int ret;
>> +
>> +       if (!req->file)
>> +               return -EBADF;
>> +       if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
>> +               return -EINVAL;
>> +       if (unlikely(sqe->ioprio || sqe->addr || sqe->buf_index
>> +               || sqe->rw_flags))
>> +               return -EINVAL;
>> +       if (force_nonblock)
>> +               return -EAGAIN;
>> +
>> +       ret = security_file_ioctl(req->file, cmd, arg);
>> +       if (!ret)
>> +               ret = (int)vfs_ioctl(req->file, cmd, arg);
> 
> This isn't going to work. For several of the syscalls that were added,
> special care had to be taken to avoid bugs - like for RECVMSG, for the
> upcoming OPEN/CLOSE stuff, and so on.
> 
> And in principle, ioctls handlers can do pretty much all of the things
> syscalls can do, and more. They can look at the caller's PID, they can
> open and close (well, technically that's slightly unsafe, but IIRC
> autofs does it anyway) things in the file descriptor table, they can
> give another process access to the calling process in some way, and so
> on. If you just allow calling arbitrary ioctls through io_uring, you
> will certainly get bugs, and probably security bugs, too.
> 
> Therefore, I would prefer to see this not happen at all; and if you do
> have a usecase where you think the complexity is worth it, then I
> think you'll have to add new infrastructure that allows each
> file_operations instance to opt in to having specific ioctls called
> via this mechanism, or something like that, and ensure that each of
> the exposed ioctls only performs operations that are safe from uring
> worker context.

Sounds like hell of a problem. Thanks for sorting this out!

> 
> Also, I'm not sure, but it might be a good idea to CC linux-api if you
> continue working on this.
> 

-- 
Pavel Begunkov

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