ipv4: fix null-deref in ipv4_link_failure

Currently, we assume the skb is associated with a device before calling
__ip_options_compile, which is not always the case if it is re-routed by
ipvs.
When skb->dev is NULL, dev_net(skb->dev) will become null-dereference.
This patch adds a check for the edge case and switch to use the net_device
from the rtable when skb->dev is NULL.

Fixes: ed0de45a ("ipv4: recompile ip options in ipv4_link_failure")
Suggested-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Kyle Zeng <zengyhkyle@gmail.com>
Cc: Stephen Suryaputra <ssuryaextr@gmail.com>
Cc: Vadim Fedorenko <vfedorenko@novek.ru>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1213,6 +1213,7 @@ EXPORT_INDIRECT_CALLABLE(ipv4_dst_check);
 
 static void ipv4_send_dest_unreach(struct sk_buff *skb)
 {
+	struct net_device *dev;
 	struct ip_options opt;
 	int res;
 
@@ -1230,7 +1231,8 @@ static void ipv4_send_dest_unreach(struct sk_buff *skb)
 		opt.optlen = ip_hdr(skb)->ihl * 4 - sizeof(struct iphdr);
 
 		rcu_read_lock();
-		res = __ip_options_compile(dev_net(skb->dev), &opt, skb, NULL);
+		dev = skb->dev ? skb->dev : skb_rtable(skb)->dst.dev;
+		res = __ip_options_compile(dev_net(dev), &opt, skb, NULL);
 		rcu_read_unlock();
 
 		if (res)