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

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Message-Id: <20211108081324.14204-1-jasowang@redhat.com>
Date:   Mon,  8 Nov 2021 16:13:24 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     mst@...hat.com, jasowang@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] virtio_ring: aovid reading flag from the descriptor ring

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>
---
 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