net: tls, fix sk_write_space NULL write when tx disabled

[ Upstream commit d85f01775850a35eae47a0090839baf510c1ef12 ]

The ctx->sk_write_space pointer is only set when TLS tx mode is enabled.
When running without TX mode its a null pointer but we still set the
sk sk_write_space pointer on close().

Fix the close path to only overwrite sk->sk_write_space when the current
pointer is to the tls_write_space function indicating the tls module should
clean it up properly as well.

Reported-by: Hillf Danton <hdanton@sina.com>
Cc: Ying Xue <ying.xue@windriver.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Fixes: 57c722e932cfb ("net/tls: swap sk_write_space on close")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/tls/tls_main.c

@@ -301,7 +301,8 @@ static void tls_sk_proto_close(struct sock *sk, long timeout)
 #else
 	{
 #endif
-		sk->sk_write_space = ctx->sk_write_space;
+		if (sk->sk_write_space == tls_write_space)
+			sk->sk_write_space = ctx->sk_write_space;
 		tls_ctx_free(ctx);
 		ctx = NULL;
 	}