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>

1 files changed, 10 insertions, 0 deletions

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;