ipv4: Fix data-races around sysctl_fib_multipath_hash_fields.

While reading sysctl_fib_multipath_hash_fields, it can be changed
concurrently.  Thus, we need to add READ_ONCE() to its readers.

Fixes: ce5c9c20 ("ipv4: Add a sysctl to control multipath hash fields")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c

@@ -10342,7 +10342,7 @@ static void mlxsw_sp_mp4_hash_init(struct mlxsw_sp *mlxsw_sp,
 		mlxsw_sp_mp_hash_inner_l3(config);
 		break;
 	case 3:
-		hash_fields = net->ipv4.sysctl_fib_multipath_hash_fields;
+		hash_fields = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_fields);
 		/* Outer */
 		MLXSW_SP_MP_HASH_HEADER_SET(headers, IPV4_EN_NOT_TCP_NOT_UDP);
 		MLXSW_SP_MP_HASH_HEADER_SET(headers, IPV4_EN_TCP_UDP);

net/ipv4/route.c

@@ -1929,7 +1929,7 @@ static u32 fib_multipath_custom_hash_outer(const struct net *net,
 					   const struct sk_buff *skb,
 					   bool *p_has_inner)
 {
-	u32 hash_fields = net->ipv4.sysctl_fib_multipath_hash_fields;
+	u32 hash_fields = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_fields);
 	struct flow_keys keys, hash_keys;
 
 	if (!(hash_fields & FIB_MULTIPATH_HASH_FIELD_OUTER_MASK))
@@ -1958,7 +1958,7 @@ static u32 fib_multipath_custom_hash_inner(const struct net *net,
 					   const struct sk_buff *skb,
 					   bool has_inner)
 {
-	u32 hash_fields = net->ipv4.sysctl_fib_multipath_hash_fields;
+	u32 hash_fields = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_fields);
 	struct flow_keys keys, hash_keys;
 
 	/* We assume the packet carries an encapsulation, but if none was
@@ -2018,7 +2018,7 @@ static u32 fib_multipath_custom_hash_skb(const struct net *net,
 static u32 fib_multipath_custom_hash_fl4(const struct net *net,
 					 const struct flowi4 *fl4)
 {
-	u32 hash_fields = net->ipv4.sysctl_fib_multipath_hash_fields;
+	u32 hash_fields = READ_ONCE(net->ipv4.sysctl_fib_multipath_hash_fields);
 	struct flow_keys hash_keys;
 
 	if (!(hash_fields & FIB_MULTIPATH_HASH_FIELD_OUTER_MASK))