[PATCH 1/2] ipv4/icmp: fix rt dst dev null pointer dereference

[Hangbin Liu <liuhangbin@gmail.com>]

In __icmp_send() there is a possibility that the rt->dst.dev is NULL,
e,g, with tunnel collect_md mode, which will cause kernel crash.
Here is what the code path looks like, for GRE:

- ip6gre_tunnel_xmit
  - ip6gre_xmit_ipv4
    - __gre6_xmit
      - ip6_tnl_xmit
        - if skb->len - t->tun_hlen - eth_hlen > mtu; return -EMSGSIZE
    - icmp_send
      - net = dev_net(rt->dst.dev); <-- here

The reason is __metadata_dst_init() init dst->dev to NULL by default.
We could not fix it in __metadata_dst_init() as there is no dev supplied.
On the other hand, the reason we need rt->dst.dev is to get the net.
So we can just get it from skb->dev, just like commit 8d9336704521
("ipv6: make icmp6_send() robust against null skb->dev") did.

Fixes: c8b34e680a09 ("ip_tunnel: Add tnl_update_pmtu in ip_md_tunnel_xmit")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
---
 net/ipv4/icmp.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 1510e951f451..5f00c9d18b02 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -582,7 +582,10 @@ void __icmp_send(struct sk_buff *skb_in, int type, int code, __be32 info,
 
 	if (!rt)
 		goto out;
-	net = dev_net(rt->dst.dev);
+
+	if (!skb_in->dev)
+		goto out;
+	net = dev_net(skb_in->dev);
 
 	/*
 	 *	Find the original header. It is expected to be valid, of course.
-- 
2.19.2