[PATCH] tls: fix missing memory barrier in tls_init

["Dae R. Jeong" <threeearcat@gmail.com>]

In tls_init(), a write memory barrier is missing, and store-store
reordering may cause NULL dereference in tls_{setsockopt,getsockopt}.

CPU0                               CPU1
-----                              -----
// In tls_init()
// In tls_ctx_create()
ctx = kzalloc()
ctx->sk_proto = READ_ONCE(sk->sk_prot) -(1)

// In update_sk_prot()
WRITE_ONCE(sk->sk_prot, tls_prots)     -(2)

                                   // In sock_common_setsockopt()
                                   READ_ONCE(sk->sk_prot)->setsockopt()

                                   // In tls_{setsockopt,getsockopt}()
                                   ctx->sk_proto->setsockopt()    -(3)

In the above scenario, when (1) and (2) are reordered, (3) can observe
the NULL value of ctx->sk_proto, causing NULL dereference.

To fix it, we rely on rcu_assign_pointer() which implies the release
barrier semantic. By moving rcu_assign_pointer() after ctx is fully
initialized, we can ensure that all fields of ctx are visible when
changing sk->sk_prot.

Also, as Sabrina suggested, this patch gets rid of tls_ctx_create(),
and move all that into tls_init().

Signed-off-by: Dae R. Jeong <threeearcat@gmail.com>
---
 net/tls/tls_main.c | 32 +++++++++++++++-----------------
 1 file changed, 15 insertions(+), 17 deletions(-)

diff --git a/net/tls/tls_main.c b/net/tls/tls_main.c
index 1c2c6800949d..235fa93dc7ef 100644
--- a/net/tls/tls_main.c
+++ b/net/tls/tls_main.c
@@ -806,22 +806,6 @@ static int tls_setsockopt(struct sock *sk, int level, int optname,
 	return do_tls_setsockopt(sk, optname, optval, optlen);
 }
 
-struct tls_context *tls_ctx_create(struct sock *sk)
-{
-	struct inet_connection_sock *icsk = inet_csk(sk);
-	struct tls_context *ctx;
-
-	ctx = kzalloc(sizeof(*ctx), GFP_ATOMIC);
-	if (!ctx)
-		return NULL;
-
-	mutex_init(&ctx->tx_lock);
-	rcu_assign_pointer(icsk->icsk_ulp_data, ctx);
-	ctx->sk_proto = READ_ONCE(sk->sk_prot);
-	ctx->sk = sk;
-	return ctx;
-}
-
 static void build_proto_ops(struct proto_ops ops[TLS_NUM_CONFIG][TLS_NUM_CONFIG],
 			    const struct proto_ops *base)
 {
@@ -933,6 +917,7 @@ static void build_protos(struct proto prot[TLS_NUM_CONFIG][TLS_NUM_CONFIG],
 
 static int tls_init(struct sock *sk)
 {
+	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tls_context *ctx;
 	int rc = 0;
 
@@ -954,14 +939,27 @@ static int tls_init(struct sock *sk)
 
 	/* allocate tls context */
 	write_lock_bh(&sk->sk_callback_lock);
-	ctx = tls_ctx_create(sk);
+	ctx = kzalloc(sizeof(*ctx), GFP_ATOMIC);
 	if (!ctx) {
 		rc = -ENOMEM;
 		goto out;
 	}
 
+	mutex_init(&ctx->tx_lock);
+	ctx->sk_proto = READ_ONCE(sk->sk_prot);
+	ctx->sk = sk;
 	ctx->tx_conf = TLS_BASE;
 	ctx->rx_conf = TLS_BASE;
+	/* rcu_assign_pointer() should be called after initialization of
+	 * all fields of ctx. It ensures that all fields of ctx are
+	 * visible before changing sk->sk_prot, and prevents reading of
+	 * uninitialized fields in tls_{getsockopt,setsockopt}. Note that
+	 * we do not need a read barrier in tls_{getsockopt,setsockopt} as
+	 * there is an address dependency between
+	 * sk->sk_proto->{getsockopt,setsockopt} and ctx->sk_proto.
+	 */
+	rcu_assign_pointer(icsk->icsk_ulp_data, ctx);
+
 	update_sk_prot(sk, ctx);
 out:
 	write_unlock_bh(&sk->sk_callback_lock);
-- 
2.42.1