Re: [PATCH net] net: skbuff: fix missing zerocopy reference in pskb_carve helpers

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Willem de Bruijn wrote:
> lazyming wrote:
> > pskb_carve_inside_header() and pskb_carve_inside_nonlinear() both copy
> > the old skb_shared_info header into a new buffer via memcpy(), which
> > includes the destructor_arg pointer (uarg) for MSG_ZEROCOPY skbs.
> 
> These functions are not supposed to maintain zerocopy frags.
> 
> Both call skb_orphan_frags.
> 
> I think what may need to happen is to invert the order of that call
> and the memcpy. Current code:
> 
>         memcpy((struct skb_shared_info *)(data + size),
>                skb_shinfo(skb), offsetof(struct skb_shared_info, frags[0]));
>         if (skb_orphan_frags(skb, gfp_mask)) {
>                 skb_kfree_head(data);
>                 return -ENOMEM;
>         }

Never mind. This actually corresponds to the first Sashiko report you
mentioned: if zerocopy skbs are converted, then the memcpy prior to
that call will have stale state.

For skbs where skb_orphan_frags does not do a deep copy, we do need to
take this extra reference.

Reviewed-by: Willem de Bruijn <willemb@google.com>