netdev - [PATCH net 1/2] skbuff: orphan frags before zerocopy clone

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Date:   Wed, 20 Dec 2017 17:37:49 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH net 1/2] skbuff: orphan frags before zerocopy clone

From: Willem de Bruijn <willemb@...gle.com>

Call skb_zerocopy_clone after skb_orphan_frags, to avoid duplicate
calls to skb_uarg(skb)->callback for the same data.

skb_zerocopy_clone associates skb_shinfo(skb)->uarg from frag_skb
with each segment. This is only safe for uargs that do refcounting,
which is those that pass skb_orphan_frags without dropping their
shared frags. For others, skb_orphan_frags drops the user frags and
sets the uarg to NULL, after which sock_zerocopy_clone has no effect.

Qemu hangs were reported due to duplicate vhost_net_zerocopy_callback
calls for the same data causing the vhost_net_ubuf_ref_>refcount to
drop below zero.

Link: http://lkml.kernel.org/r/<CAF=yD-LWyCD4Y0aJ9O0e_CHLR+3JOeKicRRTEVCPxgw4XOcqGQ@...l.gmail.com>
Fixes: 1f8b977ab32d ("sock: enable MSG_ZEROCOPY")
Reported-by: Andreas Hartmann <andihartmann@...19freenet.de>
Reported-by: David Hill <dhill@...hat.com>
Signed-off-by: Willem de Bruijn <willemb@...gle.com>

---

This fix causes skb_zerocopy_clone to be called for each frag in the
array. I will follow-up with a patch to net-next that will call both
skb_orphan_frags and skb_zerocopy_clone once per skb only.
---
 net/core/skbuff.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a592ca025fc4..edf40ac0cd07 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3654,8 +3654,6 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 
 		skb_shinfo(nskb)->tx_flags |= skb_shinfo(head_skb)->tx_flags &
 					      SKBTX_SHARED_FRAG;
-		if (skb_zerocopy_clone(nskb, head_skb, GFP_ATOMIC))
-			goto err;
 
 		while (pos < offset + len) {
 			if (i >= nfrags) {
@@ -3681,6 +3679,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 
 			if (unlikely(skb_orphan_frags(frag_skb, GFP_ATOMIC)))
 				goto err;
+			if (skb_zerocopy_clone(nskb, frag_skb, GFP_ATOMIC))
+				goto err;
 
 			*nskb_frag = *frag;
 			__skb_frag_ref(nskb_frag);
-- 
2.15.1.620.gb9897f4670-goog