ipv4: Switch inet_addr_hash() to less predictable hash.

Recently, commit 4a0ec2a ("ipv6: switch inet6_addr_hash()
to less predictable hash") and commit 4daf4dc ("ipv6: switch
inet6_acaddr_hash() to less predictable hash") hardened IPv6
address hash functions.

inet_addr_hash() is also highly predictable, and a malicious use
could abuse a specific bucket.

Let's follow the change on IPv4 by using jhash_1word().

Signed-off-by: Kuniyuki Iwashima <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: q2ven

include/net/ip.h

@@ -690,6 +690,11 @@ static inline unsigned int ipv4_addr_hash(__be32 ip)
 	return (__force unsigned int) ip;
 }
 
+static inline u32 __ipv4_addr_hash(const __be32 ip, const u32 initval)
+{
+	return jhash_1word((__force u32)ip, initval);
+}
+
 static inline u32 ipv4_portaddr_hash(const struct net *net,
 				     __be32 saddr,
 				     unsigned int port)

net/ipv4/devinet.c

@@ -121,7 +121,7 @@ struct inet_fill_args {
 
 static u32 inet_addr_hash(const struct net *net, __be32 addr)
 {
-	u32 val = (__force u32) addr ^ net_hash_mix(net);
+	u32 val = __ipv4_addr_hash(addr, net_hash_mix(net));
 
 	return hash_32(val, IN4_ADDR_HSIZE_SHIFT);
 }