ip6_gre: fix use-after-free in ip6gre_tunnel_lookup()

In the datapath, the ip6gre_tunnel_lookup() is used and it internally uses
fallback tunnel device pointer, which is fb_tunnel_dev.
This pointer variable should be set to NULL when a fb interface is deleted.
But there is no routine to set fb_tunnel_dev pointer to NULL.
So, this pointer will be still used after interface is deleted and
it eventually results in the use-after-free problem.

Test commands:
    ip netns add A
    ip netns add B
    ip link add eth0 type veth peer name eth1
    ip link set eth0 netns A
    ip link set eth1 netns B

    ip netns exec A ip link set lo up
    ip netns exec A ip link set eth0 up
    ip netns exec A ip link add ip6gre1 type ip6gre local fc:0::1 \
	    remote fc:0::2
    ip netns exec A ip -6 a a fc:100::1/64 dev ip6gre1
    ip netns exec A ip link set ip6gre1 up
    ip netns exec A ip -6 a a fc:0::1/64 dev eth0
    ip netns exec A ip link set ip6gre0 up

    ip netns exec B ip link set lo up
    ip netns exec B ip link set eth1 up
    ip netns exec B ip link add ip6gre1 type ip6gre local fc:0::2 \
	    remote fc:0::1
    ip netns exec B ip -6 a a fc:100::2/64 dev ip6gre1
    ip netns exec B ip link set ip6gre1 up
    ip netns exec B ip -6 a a fc:0::2/64 dev eth1
    ip netns exec B ip link set ip6gre0 up
    ip netns exec A ping fc:100::2 -s 60000 &
    ip netns del B

Splat looks like:
[   73.087285][    C1] BUG: KASAN: use-after-free in ip6gre_tunnel_lookup+0x1064/0x13f0 [ip6_gre]
[   73.088361][    C1] Read of size 4 at addr ffff888040559218 by task ping/1429
[   73.089317][    C1]
[   73.089638][    C1] CPU: 1 PID: 1429 Comm: ping Not tainted 5.7.0+ torvalds#602
[   73.090531][    C1] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[   73.091725][    C1] Call Trace:
[   73.092160][    C1]  <IRQ>
[   73.092556][    C1]  dump_stack+0x96/0xdb
[   73.093122][    C1]  print_address_description.constprop.6+0x2cc/0x450
[   73.094016][    C1]  ? ip6gre_tunnel_lookup+0x1064/0x13f0 [ip6_gre]
[   73.094894][    C1]  ? ip6gre_tunnel_lookup+0x1064/0x13f0 [ip6_gre]
[   73.095767][    C1]  ? ip6gre_tunnel_lookup+0x1064/0x13f0 [ip6_gre]
[   73.096619][    C1]  kasan_report+0x154/0x190
[   73.097209][    C1]  ? ip6gre_tunnel_lookup+0x1064/0x13f0 [ip6_gre]
[   73.097989][    C1]  ip6gre_tunnel_lookup+0x1064/0x13f0 [ip6_gre]
[   73.098750][    C1]  ? gre_del_protocol+0x60/0x60 [gre]
[   73.099500][    C1]  gre_rcv+0x1c5/0x1450 [ip6_gre]
[   73.100199][    C1]  ? ip6gre_header+0xf00/0xf00 [ip6_gre]
[   73.100985][    C1]  ? rcu_read_lock_sched_held+0xc0/0xc0
[   73.101830][    C1]  ? ip6_input_finish+0x5/0xf0
[   73.102483][    C1]  ip6_protocol_deliver_rcu+0xcbb/0x1510
[   73.103296][    C1]  ip6_input_finish+0x5b/0xf0
[   73.103920][    C1]  ip6_input+0xcd/0x2c0
[   73.104473][    C1]  ? ip6_input_finish+0xf0/0xf0
[   73.105115][    C1]  ? rcu_read_lock_held+0x90/0xa0
[   73.105783][    C1]  ? rcu_read_lock_sched_held+0xc0/0xc0
[   73.106548][    C1]  ipv6_rcv+0x1f1/0x300
[ ... ]

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Fixes: c12b395 ("gre: Support GRE over IPv6")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -127,6 +127,7 @@ static struct ip6_tnl *ip6gre_tunnel_lookup(struct net_device *dev,
 			gre_proto == htons(ETH_P_ERSPAN2)) ?
 		       ARPHRD_ETHER : ARPHRD_IP6GRE;
 	int score, cand_score = 4;
+	struct net_device *ndev;
 
 	for_each_ip_tunnel_rcu(t, ign->tunnels_r_l[h0 ^ h1]) {
 		if (!ipv6_addr_equal(local, &t->parms.laddr) ||
@@ -238,9 +239,9 @@ static struct ip6_tnl *ip6gre_tunnel_lookup(struct net_device *dev,
 	if (t && t->dev->flags & IFF_UP)
 		return t;
 
-	dev = ign->fb_tunnel_dev;
-	if (dev && dev->flags & IFF_UP)
-		return netdev_priv(dev);
+	ndev = READ_ONCE(ign->fb_tunnel_dev);
+	if (ndev && ndev->flags & IFF_UP)
+		return netdev_priv(ndev);
 
 	return NULL;
 }
@@ -413,6 +414,8 @@ static void ip6gre_tunnel_uninit(struct net_device *dev)
 
 	ip6gre_tunnel_unlink_md(ign, t);
 	ip6gre_tunnel_unlink(ign, t);
+	if (ign->fb_tunnel_dev == dev)
+		WRITE_ONCE(ign->fb_tunnel_dev, NULL);
 	dst_cache_reset(&t->dst_cache);
 	dev_put(dev);
 }