[Xen-devel] [PATCH net] xen-netback: make sure that hashes are not send to unaware frontends

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

In the case when a frontend only negotiates a single queue with xen-
netback it is possible for a skbuff with a s/w hash to result in a
hash extra_info segment being sent to the frontend even when no hash
algorithm has been configured. (The ndo_select_queue() entry point makes
sure the hash is not set if no algorithm is configured, but this entry
point is not called when there is only a single queue). This can result
in a frontend that isunable to handle extra_info segments being given
such a segment, causing it to crash.

This patch fixes the problem by gating whether the extra_info is sent
not only on the presence of a s/w hash, but also on whether the hash
algorithm has been configured.

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
---
 drivers/net/xen-netback/interface.c | 13 ++-----------
 drivers/net/xen-netback/netback.c   | 23 ++++++++++++++---------
 2 files changed, 16 insertions(+), 20 deletions(-)

diff --git a/drivers/net/xen-netback/interface.c 
b/drivers/net/xen-netback/interface.c
index fb50c6d..1034139 100644
--- a/drivers/net/xen-netback/interface.c
+++ b/drivers/net/xen-netback/interface.c
@@ -149,17 +149,8 @@ static u16 xenvif_select_queue(struct net_device *dev, 
struct sk_buff *skb,
        struct xenvif *vif = netdev_priv(dev);
        unsigned int size = vif->hash.size;
 
-       if (vif->hash.alg == XEN_NETIF_CTRL_HASH_ALGORITHM_NONE) {
-               u16 index = fallback(dev, skb) % dev->real_num_tx_queues;
-
-               /* Make sure there is no hash information in the socket
-                * buffer otherwise it would be incorrectly forwarded
-                * to the frontend.
-                */
-               skb_clear_hash(skb);
-
-               return index;
-       }
+       if (vif->hash.alg == XEN_NETIF_CTRL_HASH_ALGORITHM_NONE)
+               return fallback(dev, skb) % dev->real_num_tx_queues;
 
        xenvif_set_skb_hash(vif, skb);
 
diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index 3d0c989..2cd4a8e 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -168,6 +168,10 @@ static bool xenvif_rx_ring_slots_available(struct 
xenvif_queue *queue)
        needed = DIV_ROUND_UP(skb->len, XEN_PAGE_SIZE);
        if (skb_is_gso(skb))
                needed++;
+       /* Assume the frontend is capable of handling the hash
+        * extra_info at this point. This will only ever lead to an
+        * accurate value or over-estimation.
+        */
        if (skb->sw_hash)
                needed++;
 
@@ -378,9 +382,8 @@ static void xenvif_gop_frag_copy(struct xenvif_queue 
*queue, struct sk_buff *skb
                .npo = npo,
                .head = *head,
                .gso_type = XEN_NETIF_GSO_TYPE_NONE,
-               /* xenvif_set_skb_hash() will have either set a s/w
-                * hash or cleared the hash depending on
-                * whether the the frontend wants a hash for this skb.
+               /* xenvif_rx_action() will have cleared any hash if
+                * the frontend is not capable of handling it.
                 */
                .hash_present = skb->sw_hash,
        };
@@ -593,6 +596,14 @@ static void xenvif_rx_action(struct xenvif_queue *queue)
               && (skb = xenvif_rx_dequeue(queue)) != NULL) {
                queue->last_rx_time = jiffies;
 
+               /* If there is no hash algorithm configured make sure
+                * there is no hash information in the socket buffer
+                * otherwise it would be incorrectly forwarded to the
+                * frontend.
+                */
+               if (vif->hash.alg == XEN_NETIF_CTRL_HASH_ALGORITHM_NONE)
+                       skb_clear_hash(skb);
+
                XENVIF_RX_CB(skb)->meta_slots_used = xenvif_gop_skb(skb, &npo, 
queue);
 
                __skb_queue_tail(&rxq, skb);
@@ -667,12 +678,6 @@ static void xenvif_rx_action(struct xenvif_queue *queue)
                }
 
                if (skb->sw_hash) {
-                       /* Since the skb got here via xenvif_select_queue()
-                        * we know that the hash has been re-calculated
-                        * according to a configuration set by the frontend
-                        * and therefore we know that it is legitimate to
-                        * pass it to the frontend.
-                        */
                        if (resp->flags & XEN_NETRXF_extra_info)
                                extra->flags |= XEN_NETIF_EXTRA_FLAG_MORE;
                        else
-- 
2.1.4


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