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

[Paolo Abeni <pabeni@redhat.com>]

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