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

Heng Qi
2023-01-31T07:40:43Z

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 <hengqi@linux.alibaba.com>
> > Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
>
> 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. And num_buf is used in many different places, it seems
to be a lot of work to modify it to int.

>
> > ---
> > 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().

>
> > @@ -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