netdev - Re: [PATCH net] skbuff: in skb_copy_ubufs unclone before releasing zerocopy

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Date:   Thu, 28 Dec 2017 14:30:03 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     willemdebruijn.kernel@...il.com
Cc:     netdev@...r.kernel.org, willemb@...gle.com
Subject: Re: [PATCH net] skbuff: in skb_copy_ubufs unclone before releasing
 zerocopy

From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Date: Thu, 28 Dec 2017 12:38:13 -0500

> From: Willem de Bruijn <willemb@...gle.com>
> 
> skb_copy_ubufs must unclone before it is safe to modify its
> skb_shared_info with skb_zcopy_clear.
> 
> Commit b90ddd568792 ("skbuff: skb_copy_ubufs must release uarg even
> without user frags") ensures that all skbs release their zerocopy
> state, even those without frags.
> 
> But I forgot an edge case where such an skb arrives that is cloned.
> 
> The stack does not build such packets. Vhost/tun skbs have their
> frags orphaned before cloning. TCP skbs only attach zerocopy state
> when a frag is added.
> 
> But if TCP packets can be trimmed or linearized, this might occur.
> Tracing the code I found no instance so far (e.g., skb_linearize
> ends up calling skb_zcopy_clear if !skb->data_len).
> 
> Still, it is non-obvious that no path exists. And it is fragile to
> rely on this.
> 
> Fixes: b90ddd568792 ("skbuff: skb_copy_ubufs must release uarg even without user frags")
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>

Applied and queued up for -stable.