[PATCH net-next] udp: fix jump label misuse

[Paolo Abeni]

The commit 60fb9567bf30 ("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: 60fb9567bf30 ("udp: implement complete book-keeping for encap_needed")
Reported-by: Eric Dumazet <eric.duma...@gmail.com>
Signed-off-by: Paolo Abeni <pab...@redhat.com>
---
 net/ipv4/udp.c | 4 ++--
 net/ipv6/udp.c | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 6f8890c5bc7e..aff2a8e99e01 100644
--- a/net/ipv4/udp.c
+++ b/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);
        }
 }
 
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index dde51fc7ac16..09cba4cfe31f 100644
--- a/net/ipv6/udp.c
+++ b/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);
-- 
2.17.2