Re: [PATCH] io_uring: don't allow IORING_SETUP_NO_MMAP rings on highmem pages

[Jeff Moyer <jmoyer@redhat.com>]

Hi, Jens,

Jens Axboe <axboe@kernel.dk> writes:

> On at least arm32, but presumably any arch with highmem, if the
> application passes in memory that resides in highmem for the rings,
> then we should fail that ring creation. We fail it with -EINVAL, which
> is what kernels that don't support IORING_SETUP_NO_MMAP will do as well.
>
> Cc: stable@vger.kernel.org
> Fixes: 03d89a2de25b ("io_uring: support for user allocated memory for rings/sqes")
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>
> ---
>
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 783ed0fff71b..d839a80a6751 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -2686,7 +2686,7 @@ static void *__io_uaddr_map(struct page ***pages, unsigned short *npages,
>  {
>  	struct page **page_array;
>  	unsigned int nr_pages;
> -	int ret;
> +	int ret, i;
>  
>  	*npages = 0;
>  
> @@ -2716,6 +2716,20 @@ static void *__io_uaddr_map(struct page ***pages, unsigned short *npages,
>  	 */
>  	if (page_array[0] != page_array[ret - 1])
>  		goto err;
> +
> +	/*
> +	 * Can't support mapping user allocated ring memory on 32-bit archs
> +	 * where it could potentially reside in highmem. Just fail those with
> +	 * -EINVAL, just like we did on kernels that didn't support this
> +	 * feature.
> +	 */
> +	for (i = 0; i < nr_pages; i++) {
> +		if (PageHighMem(page_array[i])) {
> +			ret = -EINVAL;
> +			goto err;
> +		}
> +	}
> +

What do you think about throwing a printk_once in there that explains
the problem?  I'm worried that this will fail somewhat randomly, and it
may not be apparent to the user why.  We should also add documentation,
of course, and encourage developers to add fallbacks for this case.

-Jeff