kmsan: unpoison virtio input buffers when adding them to to virtqueue

It's hard (and too late already) to figure out the memory range to
be unpoisoned when we receive something via virtio.
Instead, we unpoison the input buffer before putting it into the
virtqueue.

drivers/net/virtio_net.c

@@ -1197,17 +1197,13 @@ static int virtnet_receive(struct receive_queue *rq, int budget, bool *xdp_xmit)
 
 		while (received < budget &&
 		       (buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx))) {
-			r_bytes = receive_buf(vi, rq, buf, len, ctx, xdp_xmit);
-			kmsan_unpoison_shadow(buf, r_bytes);
-			bytes += r_bytes;
+			bytes += receive_buf(vi, rq, buf, len, ctx, xdp_xmit);
 			received++;
 		}
 	} else {
 		while (received < budget &&
 		       (buf = virtqueue_get_buf(rq->vq, &len)) != NULL) {
-			r_bytes = receive_buf(vi, rq, buf, len, NULL, xdp_xmit);
-			kmsan_unpoison_shadow(buf, r_bytes);
-			bytes += r_bytes;
+			bytes += receive_buf(vi, rq, buf, len, NULL, xdp_xmit);
 			received++;
 		}
 	}

drivers/virtio/virtio_ring.c

@@ -363,6 +363,10 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 			desc[i].flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_NEXT | VRING_DESC_F_WRITE);
 			desc[i].addr = cpu_to_virtio64(_vq->vdev, addr);
 			desc[i].len = cpu_to_virtio32(_vq->vdev, sg->length);
+			// It's hard to figure out the buffer's address upon receive.
+			// Instead we unpoison it once, when exposing it to the device, and hope nobody
+			// else will write to it.
+			kmsan_unpoison_shadow(sg_virt(sg), sg->length);
 			prev = i;
 			i = virtio16_to_cpu(_vq->vdev, desc[i].next);
 		}