mpls: In mpls_egress verify the packet length.

Reobert Shearman noticed that mpls_egress is failing to verify that
the bytes to be examined are in fact present in the packet before
mpls_egress reads those bytes.

As suggested by David Miller reduce this to a single pskb_may_pull
call so that we don't do unnecessary work in the fast path.

Reported-by: Robert Shearman <rshearma@brocade.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
index 0ad8f71..db8a2ea 100644 (file)
--- a/net/mpls/af_mpls.c
+++ b/net/mpls/af_mpls.c
@@ -92,9 +92,24 @@ static bool mpls_egress(struct mpls_route *rt, struct sk_buff *skb,
         * The strange cases if we choose to support them will require
         * manual configuration.
         */
-       struct iphdr *hdr4 = ip_hdr(skb);
+       struct iphdr *hdr4;
        bool success = true;
 
+       /* The IPv4 code below accesses through the IPv4 header
+        * checksum, which is 12 bytes into the packet.
+        * The IPv6 code below accesses through the IPv6 hop limit
+        * which is 8 bytes into the packet.
+        *
+        * For all supported cases there should always be at least 12
+        * bytes of packet data present.  The IPv4 header is 20 bytes
+        * without options and the IPv6 header is always 40 bytes
+        * long.
+        */
+       if (!pskb_may_pull(skb, 12))
+               return false;
+
+       /* Use ip_hdr to find the ip protocol version */
+       hdr4 = ip_hdr(skb);
        if (hdr4->version == 4) {
                skb->protocol = htons(ETH_P_IP);
                csum_replace2(&hdr4->check,