ip_tunnel: fix use-after-free in ip_tunnel_lookup()

In the datapath, the ip_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 gre1 type gre local 10.0.0.1 \
	    remote 10.0.0.2
    ip netns exec A ip link set gre1 up
    ip netns exec A ip a a 10.0.100.1/24 dev gre1
    ip netns exec A ip a a 10.0.0.1/24 dev eth0

    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 gre1 type gre local 10.0.0.2 \
	    remote 10.0.0.1
    ip netns exec B ip link set gre1 up
    ip netns exec B ip a a 10.0.100.2/24 dev gre1
    ip netns exec B ip a a 10.0.0.2/24 dev eth1
    ip netns exec A hping3 10.0.100.2 -2 --flood -d 60000 &
    ip netns del B

Splat looks like:
[  133.319668][    C3] BUG: KASAN: use-after-free in ip_tunnel_lookup+0x9d6/0xde0
[  133.343852][    C3] Read of size 4 at addr ffff8880b1701c84 by task hping3/1222
[  133.344724][    C3]
[  133.345002][    C3] CPU: 3 PID: 1222 Comm: hping3 Not tainted 5.7.0+ torvalds#591
[  133.345814][    C3] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[  133.373336][    C3] Call Trace:
[  133.374792][    C3]  <IRQ>
[  133.375205][    C3]  dump_stack+0x96/0xdb
[  133.375789][    C3]  print_address_description.constprop.6+0x2cc/0x450
[  133.376720][    C3]  ? ip_tunnel_lookup+0x9d6/0xde0
[  133.377431][    C3]  ? ip_tunnel_lookup+0x9d6/0xde0
[  133.378130][    C3]  ? ip_tunnel_lookup+0x9d6/0xde0
[  133.378851][    C3]  kasan_report+0x154/0x190
[  133.379494][    C3]  ? ip_tunnel_lookup+0x9d6/0xde0
[  133.380200][    C3]  ip_tunnel_lookup+0x9d6/0xde0
[  133.380894][    C3]  __ipgre_rcv+0x1ab/0xaa0 [ip_gre]
[  133.381630][    C3]  ? rcu_read_lock_sched_held+0xc0/0xc0
[  133.382429][    C3]  gre_rcv+0x304/0x1910 [ip_gre]
[ ... ]

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Fixes: c544193 ("GRE: Refactor GRE tunneling code.")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>

Author: TaeheeYoo

net/ipv4/ip_tunnel.c

@@ -85,9 +85,10 @@ struct ip_tunnel *ip_tunnel_lookup(struct ip_tunnel_net *itn,
 				   __be32 remote, __be32 local,
 				   __be32 key)
 {
-	unsigned int hash;
 	struct ip_tunnel *t, *cand = NULL;
 	struct hlist_head *head;
+	struct net_device *ndev;
+	unsigned int hash;
 
 	hash = ip_tunnel_hash(key, remote);
 	head = &itn->tunnels[hash];
@@ -162,8 +163,9 @@ struct ip_tunnel *ip_tunnel_lookup(struct ip_tunnel_net *itn,
 	if (t && t->dev->flags & IFF_UP)
 		return t;
 
-	if (itn->fb_tunnel_dev && itn->fb_tunnel_dev->flags & IFF_UP)
-		return netdev_priv(itn->fb_tunnel_dev);
+	ndev = READ_ONCE(itn->fb_tunnel_dev);
+	if (ndev && ndev->flags & IFF_UP)
+		return netdev_priv(ndev);
 
 	return NULL;
 }
@@ -1260,8 +1262,9 @@ void ip_tunnel_uninit(struct net_device *dev)
 
 	itn = net_generic(net, tunnel->ip_tnl_net_id);
 	/* fb_tunnel_dev will be unregisted in net-exit call. */
-	if (itn->fb_tunnel_dev != dev)
-		ip_tunnel_del(itn, netdev_priv(dev));
+	ip_tunnel_del(itn, netdev_priv(dev));
+	if (itn->fb_tunnel_dev == dev)
+		WRITE_ONCE(itn->fb_tunnel_dev, NULL);
 
 	dst_cache_reset(&tunnel->dst_cache);
 }