net: Add default_mtu() methods to blackhole dst_ops

When an IPSEC SA is still being set up, __xfrm_lookup() will return
-EREMOTE and so ip_route_output_flow() will return a blackhole route.
This can happen in a sndmsg call, and after d33e455337ea ("net: Abstract
default MTU metric calculation behind an accessor.") this leads to a
crash in ip_append_data() because the blackhole dst_ops have no
default_mtu() method and so dst_mtu() calls a NULL pointer.

Fix this by adding default_mtu() methods (that simply return 0, matching
the old behavior) to the blackhole dst_ops.

The IPv4 part of this patch fixes a crash that I saw when using an IPSEC
VPN; the IPv6 part is untested because I don't have an IPv6 VPN, but it
looks to be needed as well.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -2707,6 +2707,11 @@ static struct dst_entry *ipv4_blackhole_dst_check(struct dst_entry *dst, u32 coo
 	return NULL;
 }
 
+static unsigned int ipv4_blackhole_default_mtu(const struct dst_entry *dst)
+{
+	return 0;
+}
+
 static void ipv4_rt_blackhole_update_pmtu(struct dst_entry *dst, u32 mtu)
 {
 }
@@ -2716,6 +2721,7 @@ static struct dst_ops ipv4_dst_blackhole_ops = {
 	.protocol		=	cpu_to_be16(ETH_P_IP),
 	.destroy		=	ipv4_dst_destroy,
 	.check			=	ipv4_blackhole_dst_check,
+	.default_mtu		=	ipv4_blackhole_default_mtu,
 	.update_pmtu		=	ipv4_rt_blackhole_update_pmtu,
 };
 

net/ipv6/route.c

@@ -113,6 +113,11 @@ static struct dst_ops ip6_dst_ops_template = {
 	.local_out		=	__ip6_local_out,
 };
 
+static unsigned int ip6_blackhole_default_mtu(const struct dst_entry *dst)
+{
+	return 0;
+}
+
 static void ip6_rt_blackhole_update_pmtu(struct dst_entry *dst, u32 mtu)
 {
 }
@@ -122,6 +127,7 @@ static struct dst_ops ip6_dst_blackhole_ops = {
 	.protocol		=	cpu_to_be16(ETH_P_IPV6),
 	.destroy		=	ip6_dst_destroy,
 	.check			=	ip6_dst_check,
+	.default_mtu		=	ip6_blackhole_default_mtu,
 	.update_pmtu		=	ip6_rt_blackhole_update_pmtu,
 };