[PATCH net-next] virtio-net: fix possible unsigned integer overflow

[Heng Qi <hengqi@linux.alibaba.com>]

When the single-buffer xdp is loaded and after xdp_linearize_page()
is called, *num_buf becomes 0 and (*num_buf - 1) may overflow into
a large integer in virtnet_build_xdp_buff_mrg(), resulting in
unexpected packet dropping.

Fixes: ef75cb51f139 ("virtio-net: build xdp_buff with multi buffers")
Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 drivers/net/virtio_net.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index aaa6fe9b214a..a8e9462903fa 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1007,6 +1007,9 @@ static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
 	xdp_prepare_buff(xdp, buf - VIRTIO_XDP_HEADROOM,
 			 VIRTIO_XDP_HEADROOM + vi->hdr_len, len - vi->hdr_len, true);
 
+	if (!*num_buf)
+		return 0;
+
 	if (*num_buf > 1) {
 		/* If we want to build multi-buffer xdp, we need
 		 * to specify that the flags of xdp_buff have the
@@ -1020,10 +1023,10 @@ static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
 		shinfo->xdp_frags_size = 0;
 	}
 
-	if ((*num_buf - 1) > MAX_SKB_FRAGS)
+	if (*num_buf > MAX_SKB_FRAGS + 1)
 		return -EINVAL;
 
-	while ((--*num_buf) >= 1) {
+	while (--*num_buf) {
 		buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
 		if (unlikely(!buf)) {
 			pr_debug("%s: rx error: %d buffers out of %d missing\n",
-- 
2.19.1.6.gb485710b