Re: [PATCH net] vhost/net: complete zerocopy ubufs only once

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

On Thu, Jun 04, 2026 at 12:55:49PM +0200, Paolo Abeni wrote:
> On 6/1/26 12:43 PM, Qing Ming wrote:
> > vhost-net initializes one ubuf_info per outstanding zerocopy TX
> > descriptor and hands it to the backend socket.  The networking stack may
> > then clone a zerocopy skb before all skb references are released.  For
> > example, batman-adv fragmentation reaches skb_split(), which calls
> > skb_zerocopy_clone() and increments the same ubuf_info refcount.
> > 
> > vhost_zerocopy_complete() currently treats every ubuf callback as a
> > completed vhost descriptor.  It dereferences ubuf->ctx, writes the
> > descriptor completion state, and drops the vhost_net_ubuf_ref even when
> > the callback only releases a cloned skb reference.  A backend reset can
> > therefore wait for and free the vhost_net_ubuf_ref while another cloned
> > skb still carries the same ubuf_info.  A later completion then
> > dereferences the freed ubufs pointer.
> > 
> > KASAN reports the stale completion as:
> > 
> >   BUG: KASAN: slab-use-after-free in vhost_zerocopy_complete+0x1d7/0x1f0
> >   BUG: KASAN: slab-use-after-free in vhost_zerocopy_complete+0x101/0x1f0
> >   vhost_zerocopy_complete
> >   skb_copy_ubufs
> >   __dev_forward_skb2
> >   veth_xmit
> > 
> > The freed object was allocated from vhost_net_ioctl() while setting the
> > backend and freed through kfree_rcu()/kvfree_rcu_bulk after backend
> > removal, while delayed skb completion still reached
> > vhost_zerocopy_complete().
> > 
> > Honor the generic ubuf_info refcount before touching vhost state, and run
> > the vhost descriptor completion only for the final ubuf reference.  This
> > matches the msg_zerocopy_complete() ownership rule for cloned zerocopy
> > skbs.
> > 
> > Fixes: bab632d69ee4 ("vhost: vhost TX zero-copy support")
> > Signed-off-by: Qing Ming <a0yami@mailbox.org>
> 
> The patch LGTM.
> 
> @Michael: to you want to take it via your tree?
> 
> /P


I wan't copied) Alright then.