Subject: Re: [PATCH net-next v2] virtio-net: fix possible unsigned integer overflow

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 2 Feb 2023 05:31:24 -0500

On Thu, Feb 02, 2023 at 05:07:04PM +0800, Heng Qi wrote:
> 
> 
> 在 2023/2/2 下午4:16, Michael S. Tsirkin 写道:
> > On Thu, Feb 02, 2023 at 04:14:51PM +0800, Heng Qi wrote:
> > > 
> > > 在 2023/2/2 下午4:07, Michael S. Tsirkin 写道:
> > > > On Tue, Jan 31, 2023 at 04:50:04PM +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]>
> > > > > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > > > > ---
> > > > > v1->v2:
> > > > > - Change the type of num_buf from unsigned int to int. @Michael S . Tsirkin
> > > > > - Some cleaner codes. @Michael S . Tsirkin
> > > > > 
> > > > >    drivers/net/virtio_net.c | 15 +++++++++------
> > > > >    1 file changed, 9 insertions(+), 6 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > > index aaa6fe9b214a..8102861785a2 100644
> > > > > --- a/drivers/net/virtio_net.c
> > > > > +++ b/drivers/net/virtio_net.c
> > > > > @@ -716,7 +716,7 @@ 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,
> > > > > @@ -816,7 +816,7 @@ 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;
> > > > > @@ -989,7 +989,7 @@ static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
> > > > >    				      void *buf,
> > > > >    				      unsigned int len,
> > > > >    				      unsigned int frame_sz,
> > > > > -				      u16 *num_buf,
> > > > > +				      int *num_buf,
> > > > >    				      unsigned int *xdp_frags_truesize,
> > > > >    				      struct virtnet_rq_stats *stats)
> > > > >    {
> > > > > @@ -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
> > > > Ouch. Why is this here? Merged so pls remove by a follow up patch, the
> > > > rest of the code handles 0 fine. I'm not sure this introduces a bug by
> > > > we don't want spaghetti code.
> > > Yes it would work without this, but I was keeping this because I wanted it
> > > to handle 0 early and exit early.
> > > 
> > > Do you want to remove this?
> > > 
> > > Thanks.
> > why do you want to exit early?
> 
> If num_buf is 0, we don't need to judge the subsequent process, because the
> latter process
> is used to build multi-buffer xdp, but this fix solves the possible problems
> of single-buffer xdp.
> 
> Thanks.

An optimization then? As any optimization I'd like to see some numbers.

Should have been documented as such not as part of a bugfix.

> > 
> > > > > @@ -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 > 0) {
> > > > >    		buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
> > > > >    		if (unlikely(!buf)) {
> > > > >    			pr_debug("%s: rx error: %d buffers out of %d missing\n",
> > > > > @@ -1076,7 +1079,7 @@ 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;
> > > > > -- 
> > > > > 2.19.1.6.gb485710b