Subject: [PATCH bpf-next 3/3] bpf: Add mtu checking to FIB forwarding helper

From: David Ahern <[email-masked]>
Date: Thu, 17 May 2018 09:09:30 -0700

Add check that egress MTU can handle packet to be forwarded. If
the MTU is less than the packet lenght, return 0 meaning the
packet is expected to continue up the stack for help - eg.,
fragmenting the packet or sending an ICMP.

Signed-off-by: David Ahern <[email-masked]>
---
 net/core/filter.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 6d0d1560bd70..c47c47a75d4b 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4098,6 +4098,7 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	struct fib_nh *nh;
 	struct flowi4 fl4;
 	int err;
+	u32 mtu;
 
 	dev = dev_get_by_index_rcu(net, params->ifindex);
 	if (unlikely(!dev))
@@ -4149,6 +4150,10 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	if (res.fi->fib_nhs > 1)
 		fib_select_path(net, &res, &fl4, NULL);
 
+	mtu = ip_mtu_from_fib_result(&res, params->ipv4_dst);
+	if (params->tot_len > mtu)
+		return 0;
+
 	nh = &res.fi->fib_nh[res.nh_sel];
 
 	/* do not handle lwt encaps right now */
@@ -4188,6 +4193,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	struct flowi6 fl6;
 	int strict = 0;
 	int oif;
+	u32 mtu;
 
 	/* link local addresses are never forwarded */
 	if (rt6_need_strict(dst) || rt6_need_strict(src))
@@ -4250,6 +4256,10 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 						       fl6.flowi6_oif, NULL,
 						       strict);
 
+	mtu = ip6_mtu_from_fib6(f6i, dst, src);
+	if (params->tot_len > mtu)
+		return 0;
+
 	if (f6i->fib6_nh.nh_lwtstate)
 		return 0;
 
-- 
2.11.0