udp: In udp_flow_src_port use random hash value if skb_get_hash fails

In the unlikely event that skb_get_hash is unable to deduce a hash
in udp_flow_src_port we use a consistent random value instead.
This is specified in GRE/UDP draft section 3.2.1:
https://tools.ietf.org/html/draft-ietf-tsvwg-gre-in-udp-encap-04

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 25 insertions, 6 deletions

diff --git a/include/net/udp.h b/include/net/udp.h
index 07f9b70962f6..32d8d9f07f76 100644
--- a/include/net/udp.h
+++ b/include/net/udp.h
@@ -194,6 +194,8 @@ int udp_lib_get_port(struct sock *sk, unsigned short snum,
                      int (*)(const struct sock *, const struct sock *),
                      unsigned int hash2_nulladdr);
 
+u32 udp_flow_hashrnd(void);
+
 static inline __be16 udp_flow_src_port(struct net *net, struct sk_buff *skb,
                                        int min, int max, bool use_eth)
 {
@@ -205,12 +207,19 @@ static inline __be16 udp_flow_src_port(struct net *net, struct sk_buff *skb,
         }
 
         hash = skb_get_hash(skb);
-        if (unlikely(!hash) && use_eth) {
-                /* Can't find a normal hash, caller has indicated an Ethernet
-                 * packet so use that to compute a hash.
-                 */
-                hash = jhash(skb->data, 2 * ETH_ALEN,
-                             (__force u32) skb->protocol);
+        if (unlikely(!hash)) {
+                if (use_eth) {
+                        /* Can't find a normal hash, caller has indicated an
+                         * Ethernet packet so use that to compute a hash.
+                         */
+                        hash = jhash(skb->data, 2 * ETH_ALEN,
+                                     (__force u32) skb->protocol);
+                } else {
+                        /* Can't derive any sort of hash for the packet, set
+                         * to some consistent random value.
+                         */
+                        hash = udp_flow_hashrnd();
+                }
         }
 
         /* Since this is being sent on the wire obfuscate hash a bit
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 97ef1f8b7be8..0224f930c613 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2525,6 +2525,16 @@ void __init udp_table_init(struct udp_table *table, const char *name)
         }
 }
 
+u32 udp_flow_hashrnd(void)
+{
+        static u32 hashrnd __read_mostly;
+
+        net_get_random_once(&hashrnd, sizeof(hashrnd));
+
+        return hashrnd;
+}
+EXPORT_SYMBOL(udp_flow_hashrnd);
+
 void __init udp_init(void)
 {
         unsigned long limit;