Re: [PATCH] net: fm10k: check size from dma region

[Alexander Duyck <alexander.duyck@gmail.com>]

On Fri, Jul 3, 2020 at 11:21 AM Zekun Shen <bruceshenzk@gmail.com> wrote:
>
> Size is read from a dma region as input from device. Add sanity
> check of size before calling dma_sync_single_range_for_cpu
> with it.
>
> This would prevent DMA-API warning: device driver tries to sync DMA
> memory it has not allocated.
>
> Signed-off-by: Zekun Shen <bruceshenzk@gmail.com>
> ---
>  drivers/net/ethernet/intel/fm10k/fm10k_main.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/net/ethernet/intel/fm10k/fm10k_main.c b/drivers/net/ethernet/intel/fm10k/fm10k_main.c
> index 17738b0a9..e020b346b 100644
> --- a/drivers/net/ethernet/intel/fm10k/fm10k_main.c
> +++ b/drivers/net/ethernet/intel/fm10k/fm10k_main.c
> @@ -304,6 +304,11 @@ static struct sk_buff *fm10k_fetch_rx_buffer(struct fm10k_ring *rx_ring,
>         struct fm10k_rx_buffer *rx_buffer;
>         struct page *page;
>
> +       if (unlikely(size > PAGE_SIZE)) {
> +               dev_err(rx_ring->dev, "size %d exceeds PAGE_SIZE\n", size);
> +               return NULL;
> +       }
> +
>         rx_buffer = &rx_ring->rx_buffer[rx_ring->next_to_clean];
>         page = rx_buffer->page;
>         prefetchw(page);

The upper limitation for the size should be 2K or FM10K_RX_BUFSZ, not
PAGE_SIZE. Otherwise you are still capable of going out of bounds
because the offset is used within the page to push the start of the
region up by 2K.

If this is actually fixing the warning it makes me wonder if the code
performing the check is broken itself since we would still be
accessing outside of the accessible DMA range.