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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 31 Jan 2023 02:50:49 -0500

On Tue, Jan 31, 2023 at 03:40:32PM +0800, Heng Qi wrote:
> On Tue, Jan 31, 2023 at 02:20:49AM -0500, Michael S. Tsirkin wrote:
> > On Tue, Jan 31, 2023 at 11:43:37AM +0800, Heng Qi wrote:
> > > 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 <[email-masked]>
> > > Reviewed-by: Xuan Zhuo <[email-masked]>
> > 
> > Given the confusion, just make num_buf an int?
> 
> In the structure virtio_net_hdr_mrg_rxbuf, \field{num_buffers} is unsigned int,
> which matches each other.

Because hardware buffers are all unsigned. Does not mean we need
to do all math unsigned now.

> And num_buf is used in many different places, it seems
> to be a lot of work to modify it to int.

Are you sure?

Signed-off-by: Michael S. Tsirkin <[email-masked]>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 18b3de854aeb..97f2e9bfc6f9 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -722,7 +722,7 @@ static unsigned int virtnet_get_headroom(struct virtnet_info *vi)
  * have enough headroom.
  */
 static struct page *xdp_linearize_page(struct receive_queue *rq,
-				       u16 *num_buf,
+				       int *num_buf,
 				       struct page *p,
 				       int offset,
 				       int page_off,
@@ -822,7 +822,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
 		if (unlikely(xdp_headroom < virtnet_get_headroom(vi))) {
 			int offset = buf - page_address(page) + header_offset;
 			unsigned int tlen = len + vi->hdr_len;
-			u16 num_buf = 1;
+			int num_buf = 1;
 
 			xdp_headroom = virtnet_get_headroom(vi);
 			header_offset = VIRTNET_RX_PAD + xdp_headroom;
@@ -948,7 +948,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 					 struct virtnet_rq_stats *stats)
 {
 	struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
-	u16 num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
+	int num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
 	struct page *page = virt_to_head_page(buf);
 	int offset = buf - page_address(page);
 	struct sk_buff *head_skb, *curr_skb;

Feel free to reuse.

> > 
> > > ---
> > >  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
> > 
> > 
> > This means truesize won't be set.
> 
> Do you mean xdp_frags_truesize please? If yes, the answer is yes, this fix
> is only for single-buffer xdp, which doesn't need xdp_frags_truesize, and
> already set it to 0 in its wrapper receive_mergeable().

It seems cleaner to always set the value not rely on caller to do so.

> > 
> > > @@ -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;
> > 
> > Admittedly this is cleaner.
> > 
> > >  
> > > -	while ((--*num_buf) >= 1) {
> > > +	while (--*num_buf) {
> > 
> > A bit more fragile, > 0 would be better.
> 
> Sure.
> 
> Thanks.
> 
> > 
> > >  		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