[PATCH] virtio: virtio_ring: prevent potential null dereference on per-desc state

When accessing the per-descriptor state structure, we could overflow
desc_state[] because id can be accessed by the host and the guest
at the same time, as described by the packed implementation.

'commit 32fe1de5c124 ("virtio_ring: Add READ_ONCE annotations for device-writable fields")'

The commit mentions these are legitimate races, so I assume id might
get corrupted? "id" is actually just ""last_used"" in some contexts,
and at the path where last_used gets manipulated, KCSAN reports these races.

I'm not sure if this is the right approach though, maybe we should handle it
differently other than just return?

Signed-off-by: Roi L <[email protected]>
---
 drivers/virtio/virtio_ring.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index b438dc2ce1b8..1d99b75f3bd7 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -2025,6 +2025,8 @@ static void detach_buf_packed_in_order(struct vring_virtqueue *vq,
 	unsigned int i, curr;
 
 	state = &vq->packed.desc_state[id];
+	if (unlikely(!state))
+		return;
 
 	/* Clear data ptr. */
 	state->data = NULL;
-- 
2.55.0