linux-kernel - Re: [PATCH] virtio_ring: aovid reading flag from the descriptor ring

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Date:   Wed, 23 Feb 2022 11:19:03 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     mst <mst@...hat.com>, jasowang <jasowang@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] virtio_ring: aovid reading flag from the descriptor ring

On Mon, Nov 8, 2021 at 4:13 PM Jason Wang <jasowang@...hat.com> wrote:
>
> Commit 72b5e8958738 ("virtio-ring: store DMA metadata in desc_extra
> for split virtqueue") tries to make it possible for the driver to not
> read from the descriptor ring to prevent the device from corrupting
> the descriptor ring. But it still read the descriptor flag from the
> descriptor ring during buffer detach.
>
> This patch fixes by always store the descriptor flag no matter whether
> DMA API is used and then we can avoid reading descriptor flag from the
> descriptor ring. This eliminates the possibly of unexpected next
> descriptor caused by the wrong flag (e.g the next flag).
>
> Signed-off-by: Jason Wang <jasowang@...hat.com>

Michael, any comment for this?

Thanks

> ---
>  drivers/virtio/virtio_ring.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 00f64f2f8b72..28734f4e57d3 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -583,7 +583,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>         }
>         /* Last one doesn't continue. */
>         desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> -       if (!indirect && vq->use_dma_api)
> +       if (!indirect)
>                 vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
>                         ~VRING_DESC_F_NEXT;
>
> @@ -713,7 +713,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
>         /* Put back on free list: unmap first-level descriptors and find end */
>         i = head;
>
> -       while (vq->split.vring.desc[i].flags & nextflag) {
> +       while (vq->split.desc_extra[i].flags & nextflag) {
>                 vring_unmap_one_split(vq, i);
>                 i = vq->split.desc_extra[i].next;
>                 vq->vq.num_free++;
> --
> 2.25.1
>