ip_gre: set dev->hard_header_len and dev->needed_headroom properly

GRE tunnel has its own header_ops, ipgre_header_ops, and sets it
conditionally. When it is set, it assumes the outer IP header is
already created before ipgre_xmit().

This is not true when we send packets through a raw packet socket,
where L2 headers are supposed to be constructed by user. Packet
socket calls dev_validate_header() to validate the header. But
GRE tunnel does not set dev->hard_header_len, so that check can
be simply bypassed, therefore uninit memory could be passed down
to ipgre_xmit(). Similar for dev->needed_headroom.

dev->hard_header_len is supposed to be the length of the header
created by dev->header_ops->create(), so it should be used whenever
header_ops is set, and dev->needed_headroom should be used when it
is not set.

Reported-and-tested-by: syzbot+4a2c52677a8a1aa283cb@syzkaller.appspotmail.com
Cc: William Tu <u9012063@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Xie He <xie.he.0141@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/ip_gre.c

@@ -625,9 +625,7 @@ static netdev_tx_t ipgre_xmit(struct sk_buff *skb,
 	}
 
 	if (dev->header_ops) {
-		/* Need space for new headers */
-		if (skb_cow_head(skb, dev->needed_headroom -
-				      (tunnel->hlen + sizeof(struct iphdr))))
+		if (skb_cow_head(skb, 0))
 			goto free_skb;
 
 		tnl_params = (const struct iphdr *)skb->data;
@@ -748,7 +746,11 @@ static void ipgre_link_update(struct net_device *dev, bool set_mtu)
 	len = tunnel->tun_hlen - len;
 	tunnel->hlen = tunnel->hlen + len;
 
-	dev->needed_headroom = dev->needed_headroom + len;
+	if (dev->header_ops)
+		dev->hard_header_len += len;
+	else
+		dev->needed_headroom += len;
+
 	if (set_mtu)
 		dev->mtu = max_t(int, dev->mtu - len, 68);
 
@@ -944,6 +946,7 @@ static void __gre_tunnel_init(struct net_device *dev)
 	tunnel->parms.iph.protocol = IPPROTO_GRE;
 
 	tunnel->hlen = tunnel->tun_hlen + tunnel->encap_hlen;
+	dev->needed_headroom = tunnel->hlen + sizeof(tunnel->parms.iph);
 
 	dev->features		|= GRE_FEATURES;
 	dev->hw_features	|= GRE_FEATURES;
@@ -987,10 +990,14 @@ static int ipgre_tunnel_init(struct net_device *dev)
 				return -EINVAL;
 			dev->flags = IFF_BROADCAST;
 			dev->header_ops = &ipgre_header_ops;
+			dev->hard_header_len = tunnel->hlen + sizeof(*iph);
+			dev->needed_headroom = 0;
 		}
 #endif
 	} else if (!tunnel->collect_md) {
 		dev->header_ops = &ipgre_header_ops;
+		dev->hard_header_len = tunnel->hlen + sizeof(*iph);
+		dev->needed_headroom = 0;
 	}
 
 	return ip_tunnel_init(dev);