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[Xen-devel] [PATCH net-next] xen-netfront: try linearizing SKB if it occupies too many slots

Some workload, such as Redis can generate SKBs which make use of
compound pages. Netfront doesn't quite like that because it doesn't want
to send packet that occupies exessive slots to the backend as backend
might deem it malicious. On the flip side these packets are actually
legit, the size check at the beginning of xennet_start_xmit ensures that
packet size is below 64K.

So we linearize SKB if it occupies too many slots. If the linearization
fails then the SKB is dropped.

Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
Cc: Konrad Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
Cc: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
 drivers/net/xen-netfront.c |   17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 895355d..b378dcd 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -573,9 +573,20 @@ static int xennet_start_xmit(struct sk_buff *skb, struct 
net_device *dev)
        slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
        if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
-               net_alert_ratelimited(
-                       "xennet: skb rides the rocket: %d slots\n", slots);
-               goto drop;
+               if (skb_linearize(skb)) {
+                       net_alert_ratelimited(
+                               "xennet: failed to linearize skb, skb 
+                       goto drop;
+               }
+               data = skb->data;
+               offset = offset_in_page(data);
+               len = skb_headlen(skb);
+               slots = DIV_ROUND_UP(offset + len, PAGE_SIZE);
+               if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
+                       net_alert_ratelimited(
+                               "xennet: still too many slots after 
linerization: %d", slots);
+                       goto drop;
+               }
        spin_lock_irqsave(&np->tx_lock, flags);

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