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Date:	Tue, 3 Mar 2015 16:26:11 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	<netdev@...r.kernel.org>
CC:	David Vrabel <david.vrabel@...rix.com>,
	<xen-devel@...ts.xenproject.org>,
	Ian Campbell <ian.campbell@...rix.com>,
	Wei Liu <wei.liu2@...rix.com>
Subject: [PATCHv1 2/2] xen-netback: unref frags when handling a from-guest skb with a frag list

Every time a VIF is destroyed up-to 256 pages may be leaked if packets
with more than MAX_SKB_FRAGS frags where transmitted from the guest.
Even worse, if another user of ballooned pages allocated one of these
ballooned pages it would not handle the the unexpectedly non-zero page
count (e.g., gntdev would deadlock when unmapping a grant because the
page count would never reach 1).

When handling a from-guest skb with a frag list, unref the frags
before releasing them so they are freed correctly when the VIF is
destroyed.

Also swap over to the new (local) frags /after/ calling the skb
destructor.  This isn't strictly necessary but it's less confusing.

Signed-off-by: David Vrabel <david.vrabel@...rix.com>
---
 drivers/net/xen-netback/netback.c |   43 +++++++++++++++++++------------------
 1 file changed, 22 insertions(+), 21 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index f7a31d2..3d06eeb 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -1343,7 +1343,7 @@ static int xenvif_handle_frag_list(struct xenvif_queue *queue, struct sk_buff *s
 {
 	unsigned int offset = skb_headlen(skb);
 	skb_frag_t frags[MAX_SKB_FRAGS];
-	int i;
+	int i, nr_frags;
 	struct ubuf_info *uarg;
 	struct sk_buff *nskb = skb_shinfo(skb)->frag_list;
 
@@ -1357,17 +1357,16 @@ static int xenvif_handle_frag_list(struct xenvif_queue *queue, struct sk_buff *s
 	skb->data_len += nskb->len;
 
 	/* create a brand new frags array and coalesce there */
-	for (i = 0; offset < skb->len; i++) {
+	for (nr_frags = 0; offset < skb->len; nr_frags++) {
 		struct page *page;
 		unsigned int len;
 
-		BUG_ON(i >= MAX_SKB_FRAGS);
+		BUG_ON(nr_frags >= MAX_SKB_FRAGS);
 		page = alloc_page(GFP_ATOMIC);
 		if (!page) {
-			int j;
 			skb->truesize += skb->data_len;
-			for (j = 0; j < i; j++)
-				put_page(frags[j].page.p);
+			for (i = 0; i < nr_frags; i++)
+				put_page(frags[i].page.p);
 			return -ENOMEM;
 		}
 
@@ -1379,27 +1378,29 @@ static int xenvif_handle_frag_list(struct xenvif_queue *queue, struct sk_buff *s
 			BUG();
 
 		offset += len;
-		frags[i].page.p = page;
-		frags[i].page_offset = 0;
-		skb_frag_size_set(&frags[i], len);
+		frags[nr_frags].page.p = page;
+		frags[nr_frags].page_offset = 0;
+		skb_frag_size_set(&frags[nr_frags], len);
 	}
-	/* swap out with old one */
-	memcpy(skb_shinfo(skb)->frags,
-	       frags,
-	       i * sizeof(skb_frag_t));
-	skb_shinfo(skb)->nr_frags = i;
-	skb->truesize += i * PAGE_SIZE;
-
-	/* remove traces of mapped pages and frag_list */
+
+	/* Copied all the bits from the frag list -- free it. */
 	skb_frag_list_init(skb);
-	uarg = skb_shinfo(skb)->destructor_arg;
-	/* increase inflight counter to offset decrement in callback */
+	xenvif_skb_zerocopy_prepare(queue, nskb);
+	kfree_skb(nskb);
+
+	/* Release all the original (foreign) frags. */
+	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
+		skb_frag_unref(skb, i);
 	atomic_inc(&queue->inflight_packets);
+	uarg = skb_shinfo(skb)->destructor_arg;
 	uarg->callback(uarg, true);
 	skb_shinfo(skb)->destructor_arg = NULL;
 
-	xenvif_skb_zerocopy_prepare(queue, nskb);
-	kfree_skb(nskb);
+	/* Fill the skb with the new (local) frags. */
+	memcpy(skb_shinfo(skb)->frags, frags,
+	       nr_frags * sizeof(skb_frag_t));
+	skb_shinfo(skb)->nr_frags = nr_frags;
+	skb->truesize += nr_frags * PAGE_SIZE;
 
 	return 0;
 }
-- 
1.7.10.4

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