Re: [PATCH net v2] virtio-net: fix received length check in big packets

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 10, 2025 at 06:44:03PM +0800, Jason Wang wrote:
> On Thu, Jul 10, 2025 at 5:57 PM Paolo Abeni <pabeni@redhat.com> wrote:
> >
> > On 7/8/25 4:42 PM, Bui Quang Minh wrote:
> > > Since commit 4959aebba8c0 ("virtio-net: use mtu size as buffer length
> > > for big packets"), the allocated size for big packets is not
> > > MAX_SKB_FRAGS * PAGE_SIZE anymore but depends on negotiated MTU. The
> > > number of allocated frags for big packets is stored in
> > > vi->big_packets_num_skbfrags. This commit fixes the received length
> > > check corresponding to that change. The current incorrect check can lead
> > > to NULL page pointer dereference in the below while loop when erroneous
> > > length is received.
> > >
> > > Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big packets")
> > > Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
> > > ---
> > > Changes in v2:
> > > - Remove incorrect give_pages call
> > > ---
> > >  drivers/net/virtio_net.c | 9 ++++++---
> > >  1 file changed, 6 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 5d674eb9a0f2..3a7f435c95ae 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -823,7 +823,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
> > >  {
> > >       struct sk_buff *skb;
> > >       struct virtio_net_common_hdr *hdr;
> > > -     unsigned int copy, hdr_len, hdr_padded_len;
> > > +     unsigned int copy, hdr_len, hdr_padded_len, max_remaining_len;
> > >       struct page *page_to_free = NULL;
> > >       int tailroom, shinfo_size;
> > >       char *p, *hdr_p, *buf;
> > > @@ -887,12 +887,15 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
> > >        * tries to receive more than is possible. This is usually
> > >        * the case of a broken device.
> > >        */
> > > -     if (unlikely(len > MAX_SKB_FRAGS * PAGE_SIZE)) {
> > > +     BUG_ON(offset >= PAGE_SIZE);
> >
> > Minor nit (not intended to block this patch): since you are touching
> > this, you could consider replacing the BUG_ON() with a:
> >
> >  if (WARN_ON_ONCE()) <goto error path>.
> 
> I'm not sure I get this, but using BUG_ON() can help to prevent bugs
> from being explored.
> 
> Thanks


You mean exploited.

Paolo what's your thought here? Why do you want to work around this
one, specifically? I don't see how we can get offset >= PAGE_SIZE.


> >
> > /P
> >