udp: fix jump label misuse

The commit 60fb9567 ("udp: implement complete book-keeping for
encap_needed") introduced a severe misuse of jump label APIs, which
syzbot, as reported by Eric, was able to exploit.

When multiple sockets/process can concurrently request (and than
disable) the udp encap, we need to track the activation counter with
*_inc()/*_dec() jump label variants, or we can experience bad things
at disable time.

Fixes: 60fb9567 ("udp: implement complete book-keeping for encap_needed")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -587,7 +587,7 @@ static inline bool __udp_is_mcast_sock(struct net *net, struct sock *sk,
 DEFINE_STATIC_KEY_FALSE(udp_encap_needed_key);
 void udp_encap_enable(void)
 {
-	static_branch_enable(&udp_encap_needed_key);
+	static_branch_inc(&udp_encap_needed_key);
 }
 EXPORT_SYMBOL(udp_encap_enable);
 
@@ -2524,7 +2524,7 @@ void udp_destroy_sock(struct sock *sk)
 				encap_destroy(sk);
 		}
 		if (up->encap_enabled)
-			static_branch_disable(&udp_encap_needed_key);
+			static_branch_dec(&udp_encap_needed_key);
 	}
 }
 

net/ipv6/udp.c

@@ -448,7 +448,7 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 DEFINE_STATIC_KEY_FALSE(udpv6_encap_needed_key);
 void udpv6_encap_enable(void)
 {
-	static_branch_enable(&udpv6_encap_needed_key);
+	static_branch_inc(&udpv6_encap_needed_key);
 }
 EXPORT_SYMBOL(udpv6_encap_enable);
 
@@ -1579,7 +1579,7 @@ void udpv6_destroy_sock(struct sock *sk)
 				encap_destroy(sk);
 		}
 		if (up->encap_enabled)
-			static_branch_disable(&udpv6_encap_needed_key);
+			static_branch_dec(&udpv6_encap_needed_key);
 	}
 
 	inet6_destroy_sock(sk);