[PATCH net v4 2/3] net: skbuff: don't skb_tx_error() the source skb in skb_zerocopy()

[Norbert Szetei <norbert@doyensec.com>]

skb_zerocopy() copies frags from @from into @to. On an
skb_orphan_frags() failure it calls skb_tx_error(@from), a destructive
operation on the source skb the copy helper does not own. That completes
@from's zerocopy uarg and clears SKBFL_ALL_ZEROCOPY, including the
SKBFL_SHARED_FRAG page-ownership marker.

Both callers already report the failure on their own drop path.
nfnetlink_queue does it at nla_put_failure, and Open vSwitch does it in
the flow-miss drop arm of ovs_dp_process_packet(), so nothing is lost by
dropping it here.

On Open vSwitch's OVS_ACTION_ATTR_USERSPACE path the skb is not freed on
this error: do_execute_actions() ignores output_userspace()'s return
value and, unless the upcall was the last action, keeps forwarding the
same skb through the flow's remaining actions. The uarg is completed
while that skb is still in flight, telling the producer its buffers are
free, and SKBFL_SHARED_FRAG is cleared on an skb the rest of the stack
still handles. That flag is what makes esp_input() call skb_cow_data()
instead of decrypting in place, so a later local ESP delivery can
decrypt over frags the skb does not own privately.

Leave error reporting to the callers.

Fixes: 36d5fe6a0007 ("core, nfqueue, openvswitch: Orphan frags in skb_zerocopy and handle errors")
Cc: stable@vger.kernel.org
Suggested-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Norbert Szetei <norbert@doyensec.com>
Reviewed-by: Ilya Maximets <i.maximets@ovn.org>
---
 net/core/skbuff.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index d4382b68d56e..ab3d161247b9 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3914,7 +3914,6 @@ skb_zerocopy(struct sk_buff *to, struct sk_buff *from, int len, int hlen)
 	skb_len_add(to, len + plen);
 
 	if (unlikely(skb_orphan_frags(from, GFP_ATOMIC))) {
-		skb_tx_error(from);
 		if (j > 0)
 			put_page(virt_to_head_page(from->head));
 		return -ENOMEM;
-- 
2.55.0