net/tcp: Disable TCP-MD5 static key on tcp_md5sig_info destruction

To do that, separate two scenarios:
- where it's the first MD5 key on the system, which means that enabling
  of the static key may need to sleep;
- copying of an existing key from a listening socket to the request
  socket upon receiving a signed TCP segment, where static key was
  already enabled (when the key was added to the listening socket).

Now the life-time of the static branch for TCP-MD5 is until:
- last tcp_md5sig_info is destroyed
- last socket in time-wait state with MD5 key is closed.

Which means that after all sockets with TCP-MD5 keys are gone, the
system gets back the performance of disabled md5-key static branch.

While at here, provide static_key_fast_inc() helper that does ref
counter increment in atomic fashion (without grabbing cpus_read_lock()
on CONFIG_JUMP_LABEL=y). This is needed to add a new user for
a static_key when the caller controls the lifetime of another user.

Signed-off-by: Dmitry Safonov <dima@arista.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

4 files changed, 73 insertions, 23 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 24602a5184b0..001947136b0a 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -4464,11 +4464,8 @@ bool tcp_alloc_md5sig_pool(void)
 	if (unlikely(!READ_ONCE(tcp_md5sig_pool_populated))) {
 		mutex_lock(&tcp_md5sig_mutex);
 
-		if (!tcp_md5sig_pool_populated) {
+		if (!tcp_md5sig_pool_populated)
 			__tcp_alloc_md5sig_pool();
-			if (tcp_md5sig_pool_populated)
-				static_branch_inc(&tcp_md5_needed);
-		}
 
 		mutex_unlock(&tcp_md5sig_mutex);
 	}
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index c72e53835397..5d83a332f1dd 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1053,7 +1053,7 @@ static void tcp_v4_reqsk_destructor(struct request_sock *req)
  * We need to maintain these in the sk structure.
  */
 
-DEFINE_STATIC_KEY_FALSE(tcp_md5_needed);
+DEFINE_STATIC_KEY_DEFERRED_FALSE(tcp_md5_needed, HZ);
 EXPORT_SYMBOL(tcp_md5_needed);
 
 static bool better_md5_match(struct tcp_md5sig_key *old, struct tcp_md5sig_key *new)
@@ -1166,9 +1166,6 @@ static int tcp_md5sig_info_add(struct sock *sk, gfp_t gfp)
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct tcp_md5sig_info *md5sig;
 
-	if (rcu_dereference_protected(tp->md5sig_info, lockdep_sock_is_held(sk)))
-		return 0;
-
 	md5sig = kmalloc(sizeof(*md5sig), gfp);
 	if (!md5sig)
 		return -ENOMEM;
@@ -1180,9 +1177,9 @@ static int tcp_md5sig_info_add(struct sock *sk, gfp_t gfp)
 }
 
 /* This can be called on a newly created socket, from other files */
-int tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr,
-		   int family, u8 prefixlen, int l3index, u8 flags,
-		   const u8 *newkey, u8 newkeylen, gfp_t gfp)
+static int __tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr,
+			    int family, u8 prefixlen, int l3index, u8 flags,
+			    const u8 *newkey, u8 newkeylen, gfp_t gfp)
 {
 	/* Add Key to the list */
 	struct tcp_md5sig_key *key;
@@ -1209,9 +1206,6 @@ int tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr,
 		return 0;
 	}
 
-	if (tcp_md5sig_info_add(sk, gfp))
-		return -ENOMEM;
-
 	md5sig = rcu_dereference_protected(tp->md5sig_info,
 					   lockdep_sock_is_held(sk));
 
@@ -1235,8 +1229,59 @@ int tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr,
 	hlist_add_head_rcu(&key->node, &md5sig->head);
 	return 0;
 }
+
+int tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr,
+		   int family, u8 prefixlen, int l3index, u8 flags,
+		   const u8 *newkey, u8 newkeylen)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (!rcu_dereference_protected(tp->md5sig_info, lockdep_sock_is_held(sk))) {
+		if (tcp_md5sig_info_add(sk, GFP_KERNEL))
+			return -ENOMEM;
+
+		if (!static_branch_inc(&tcp_md5_needed.key)) {
+			struct tcp_md5sig_info *md5sig;
+
+			md5sig = rcu_dereference_protected(tp->md5sig_info, lockdep_sock_is_held(sk));
+			rcu_assign_pointer(tp->md5sig_info, NULL);
+			kfree_rcu(md5sig);
+			return -EUSERS;
+		}
+	}
+
+	return __tcp_md5_do_add(sk, addr, family, prefixlen, l3index, flags,
+				newkey, newkeylen, GFP_KERNEL);
+}
 EXPORT_SYMBOL(tcp_md5_do_add);
 
+int tcp_md5_key_copy(struct sock *sk, const union tcp_md5_addr *addr,
+		     int family, u8 prefixlen, int l3index,
+		     struct tcp_md5sig_key *key)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (!rcu_dereference_protected(tp->md5sig_info, lockdep_sock_is_held(sk))) {
+		if (tcp_md5sig_info_add(sk, sk_gfp_mask(sk, GFP_ATOMIC)))
+			return -ENOMEM;
+
+		if (!static_key_fast_inc_not_disabled(&tcp_md5_needed.key.key)) {
+			struct tcp_md5sig_info *md5sig;
+
+			md5sig = rcu_dereference_protected(tp->md5sig_info, lockdep_sock_is_held(sk));
+			net_warn_ratelimited("Too many TCP-MD5 keys in the system\n");
+			rcu_assign_pointer(tp->md5sig_info, NULL);
+			kfree_rcu(md5sig);
+			return -EUSERS;
+		}
+	}
+
+	return __tcp_md5_do_add(sk, addr, family, prefixlen, l3index,
+				key->flags, key->key, key->keylen,
+				sk_gfp_mask(sk, GFP_ATOMIC));
+}
+EXPORT_SYMBOL(tcp_md5_key_copy);
+
 int tcp_md5_do_del(struct sock *sk, const union tcp_md5_addr *addr, int family,
 		   u8 prefixlen, int l3index, u8 flags)
 {
@@ -1323,7 +1368,7 @@ static int tcp_v4_parse_md5_keys(struct sock *sk, int optname,
 		return -EINVAL;
 
 	return tcp_md5_do_add(sk, addr, AF_INET, prefixlen, l3index, flags,
-			      cmd.tcpm_key, cmd.tcpm_keylen, GFP_KERNEL);
+			      cmd.tcpm_key, cmd.tcpm_keylen);
 }
 
 static int tcp_v4_md5_hash_headers(struct tcp_md5sig_pool *hp,
@@ -1580,8 +1625,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 		 * memory, then we end up not copying the key
 		 * across. Shucks.
 		 */
-		tcp_md5_do_add(newsk, addr, AF_INET, 32, l3index, key->flags,
-			       key->key, key->keylen, GFP_ATOMIC);
+		tcp_md5_key_copy(newsk, addr, AF_INET, 32, l3index, key);
 		sk_gso_disable(newsk);
 	}
 #endif
@@ -2273,6 +2317,7 @@ void tcp_v4_destroy_sock(struct sock *sk)
 		tcp_clear_md5_list(sk);
 		kfree_rcu(rcu_dereference_protected(tp->md5sig_info, 1), rcu);
 		tp->md5sig_info = NULL;
+		static_branch_slow_dec_deferred(&tcp_md5_needed);
 	}
 #endif
 
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index c375f603a16c..6908812d50d3 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -291,13 +291,19 @@ void tcp_time_wait(struct sock *sk, int state, int timeo)
 		 */
 		do {
 			tcptw->tw_md5_key = NULL;
-			if (static_branch_unlikely(&tcp_md5_needed)) {
+			if (static_branch_unlikely(&tcp_md5_needed.key)) {
 				struct tcp_md5sig_key *key;
 
 				key = tp->af_specific->md5_lookup(sk, sk);
 				if (key) {
 					tcptw->tw_md5_key = kmemdup(key, sizeof(*key), GFP_ATOMIC);
-					BUG_ON(tcptw->tw_md5_key && !tcp_alloc_md5sig_pool());
+					if (!tcptw->tw_md5_key)
+						break;
+					BUG_ON(!tcp_alloc_md5sig_pool());
+					if (!static_key_fast_inc_not_disabled(&tcp_md5_needed.key.key)) {
+						kfree(tcptw->tw_md5_key);
+						tcptw->tw_md5_key = NULL;
+					}
 				}
 			}
 		} while (0);
@@ -337,11 +343,13 @@ EXPORT_SYMBOL(tcp_time_wait);
 void tcp_twsk_destructor(struct sock *sk)
 {
 #ifdef CONFIG_TCP_MD5SIG
-	if (static_branch_unlikely(&tcp_md5_needed)) {
+	if (static_branch_unlikely(&tcp_md5_needed.key)) {
 		struct tcp_timewait_sock *twsk = tcp_twsk(sk);
 
-		if (twsk->tw_md5_key)
+		if (twsk->tw_md5_key) {
 			kfree_rcu(twsk->tw_md5_key, rcu);
+			static_branch_slow_dec_deferred(&tcp_md5_needed);
+		}
 	}
 #endif
 }
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 894410dc9293..71d01cf3c13e 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -766,7 +766,7 @@ static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
 
 	*md5 = NULL;
 #ifdef CONFIG_TCP_MD5SIG
-	if (static_branch_unlikely(&tcp_md5_needed) &&
+	if (static_branch_unlikely(&tcp_md5_needed.key) &&
 	    rcu_access_pointer(tp->md5sig_info)) {
 		*md5 = tp->af_specific->md5_lookup(sk, sk);
 		if (*md5) {
@@ -922,7 +922,7 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
 
 	*md5 = NULL;
 #ifdef CONFIG_TCP_MD5SIG
-	if (static_branch_unlikely(&tcp_md5_needed) &&
+	if (static_branch_unlikely(&tcp_md5_needed.key) &&
 	    rcu_access_pointer(tp->md5sig_info)) {
 		*md5 = tp->af_specific->md5_lookup(sk, sk);
 		if (*md5) {