[Xen-devel] [PATCHv1 2/2] xen-netback: unref frags when handling a from-guest skb with a frag list

[David Vrabel <david.vrabel@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
 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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