[Xen-devel] [PATCH net v2] xen-netback: fix fragment detection in checksum setup

[Paul Durrant <paul.durrant@xxxxxxxxxx>]

The code to detect fragments in checksum_setup() was missing for IPv4 and
too eager for IPv6. (It transpires that Windows seems to send IPv6 packets
with a fragment header even if they are not a fragment - i.e. offset is zero,
and M bit is not set).

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
---
v2

- Added comments noting what fragment/offset masks mean
  
 drivers/net/xen-netback/netback.c |   33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index 919b650..c7464d8 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -1165,15 +1165,28 @@ static int checksum_setup_ip(struct xenvif *vif, struct 
sk_buff *skb,
        struct iphdr *iph = (void *)skb->data;
        unsigned int header_size;
        unsigned int off;
+       bool fragment;
        int err = -EPROTO;
 
+       fragment = false;
+
        off = sizeof(struct iphdr);
 
        header_size = skb->network_header + off + MAX_IPOPTLEN;
        maybe_pull_tail(skb, header_size);
 
+       /* 3fff -> fragment offset != 0 OR more fragments */
+       if (ntohs(iph->frag_off) & 0x3fff)
+               fragment = true;
+
        off = iph->ihl * 4;
 
+       if (fragment) {
+               if (net_ratelimit())
+                       netdev_err(vif->dev, "Packet is a fragment!\n");
+               goto out;
+       }
+
        switch (iph->protocol) {
        case IPPROTO_TCP:
                if (!skb_partial_csum_set(skb, off,
@@ -1237,6 +1250,7 @@ static int checksum_setup_ipv6(struct xenvif *vif, struct 
sk_buff *skb,
        bool fragment;
        bool done;
 
+       fragment = false;
        done = false;
 
        off = sizeof(struct ipv6hdr);
@@ -1275,9 +1289,22 @@ static int checksum_setup_ipv6(struct xenvif *vif, 
struct sk_buff *skb,
                        off += (hp->hdrlen+2)<<2;
                        break;
                }
-               case IPPROTO_FRAGMENT:
-                       fragment = true;
-                       /* fall through */
+               case IPPROTO_FRAGMENT: {
+                       struct frag_hdr *hp = (void *)(skb->data + off);
+
+                       header_size = skb->network_header +
+                               off +
+                               sizeof(struct frag_hdr);
+                       maybe_pull_tail(skb, header_size);
+
+                       /* fff9 -> fragment offset != 0 OR more fragments */
+                       if (ntohs(hp->frag_off) & 0xfff9)
+                               fragment = true;
+
+                       nexthdr = hp->nexthdr;
+                       off += sizeof(struct frag_hdr);
+                       break;
+               }
                default:
                        done = true;
                        break;
-- 
1.7.10.4


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