ip6_gre: Fix skb_under_panic in __gre6_xmit()

Feng reported an skb_under_panic BUG triggered by running
test_ip6gretap() in tools/testing/selftests/bpf/test_tunnel.sh:

[   82.492551] skbuff: skb_under_panic: text:ffffffffb268bb8e len:403 put:12 head:ffff9997c5480000 data:ffff9997c547fff8 tail:0x18b end:0x2c0 dev:ip6gretap11
<...>
[   82.607380] Call Trace:
[   82.609389]  <TASK>
[   82.611136]  skb_push.cold.109+0x10/0x10
[   82.614289]  __gre6_xmit+0x41e/0x590
[   82.617169]  ip6gre_tunnel_xmit+0x344/0x3f0
[   82.620526]  dev_hard_start_xmit+0xf1/0x330
[   82.623882]  sch_direct_xmit+0xe4/0x250
[   82.626961]  __dev_queue_xmit+0x720/0xfe0
<...>
[   82.633431]  packet_sendmsg+0x96a/0x1cb0
[   82.636568]  sock_sendmsg+0x30/0x40
<...>

The following sequence of events caused the BUG:

1. During ip6gretap device initialization, tunnel->tun_hlen (e.g. 4) is
   calculated based on old flags (see ip6gre_calc_hlen());
2. packet_snd() reserves header room for skb A, assuming
   tunnel->tun_hlen is 4;
3. Later (in clsact Qdisc), the eBPF program sets a new tunnel key for
   skb A using bpf_skb_set_tunnel_key() (see _ip6gretap_set_tunnel());
4. __gre6_xmit() detects the new tunnel key, and recalculates
   "tun_hlen" (e.g. 12) based on new flags (e.g. TUNNEL_KEY and
   TUNNEL_SEQ);
5. gre_build_header() calls skb_push() with insufficient reserved header
   room, triggering the BUG.

As sugguested by Cong, fix it by moving the call to skb_cow_head() after
the recalculation of tun_hlen.

Reproducer:

  OBJ=$LINUX/tools/testing/selftests/bpf/test_tunnel_kern.o

  ip netns add at_ns0
  ip link add veth0 type veth peer name veth1
  ip link set veth0 netns at_ns0
  ip netns exec at_ns0 ip addr add 172.16.1.100/24 dev veth0
  ip netns exec at_ns0 ip link set dev veth0 up
  ip link set dev veth1 up mtu 1500
  ip addr add dev veth1 172.16.1.200/24

  ip netns exec at_ns0 ip addr add ::11/96 dev veth0
  ip netns exec at_ns0 ip link set dev veth0 up
  ip addr add dev veth1 ::22/96
  ip link set dev veth1 up

  ip netns exec at_ns0 \
  	ip link add dev ip6gretap00 type ip6gretap seq flowlabel 0xbcdef key 2 \
  	local ::11 remote ::22

  ip netns exec at_ns0 ip addr add dev ip6gretap00 10.1.1.100/24
  ip netns exec at_ns0 ip addr add dev ip6gretap00 fc80::100/96
  ip netns exec at_ns0 ip link set dev ip6gretap00 up

  ip link add dev ip6gretap11 type ip6gretap external
  ip addr add dev ip6gretap11 10.1.1.200/24
  ip addr add dev ip6gretap11 fc80::200/24
  ip link set dev ip6gretap11 up

  tc qdisc add dev ip6gretap11 clsact
  tc filter add dev ip6gretap11 egress bpf da obj $OBJ sec ip6gretap_set_tunnel
  tc filter add dev ip6gretap11 ingress bpf da obj $OBJ sec ip6gretap_get_tunnel

  ping6 -c 3 -w 10 -q ::11

Fixes: 6712abc168 ("ip6_gre: add ip6 gre and gretap collect_md mode")
Reported-by: Feng Zhou <zhoufeng.zf@bytedance.com>
Co-developed-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -733,9 +733,6 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
 	else
 		fl6->daddr = tunnel->parms.raddr;
 
-	if (skb_cow_head(skb, dev->needed_headroom ?: tunnel->hlen))
-		return -ENOMEM;
-
 	/* Push GRE header. */
 	protocol = (dev->type == ARPHRD_ETHER) ? htons(ETH_P_TEB) : proto;
 
@@ -763,6 +760,9 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
 			(TUNNEL_CSUM | TUNNEL_KEY | TUNNEL_SEQ);
 		tun_hlen = gre_calc_hlen(flags);
 
+		if (skb_cow_head(skb, dev->needed_headroom ?: tun_hlen + tunnel->encap_hlen))
+			return -ENOMEM;
+
 		gre_build_header(skb, tun_hlen,
 				 flags, protocol,
 				 tunnel_id_to_key32(tun_info->key.tun_id),
@@ -773,6 +773,9 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
 		if (tunnel->parms.o_flags & TUNNEL_SEQ)
 			tunnel->o_seqno++;
 
+		if (skb_cow_head(skb, dev->needed_headroom ?: tunnel->hlen))
+			return -ENOMEM;
+
 		gre_build_header(skb, tunnel->tun_hlen, tunnel->parms.o_flags,
 				 protocol, tunnel->parms.o_key,
 				 htonl(tunnel->o_seqno));