Revert "Reduce stack usage in tls13_hkdf_expand"

This reverts commit ec0c5f56.

SSL_export_keying_material() may use longer label lengths.

Fixes #7712
Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7755)

ssl/tls13_enc.c

@@ -13,14 +13,7 @@
 #include <openssl/evp.h>
 #include <openssl/kdf.h>
 
-/*
- * RFC 8446, 7.1 Key Schedule, says:
- * Note: With common hash functions, any label longer than 12 characters
- * requires an additional iteration of the hash function to compute.
- * The labels in this specification have all been chosen to fit within
- * this limit.
- */
-#define TLS13_MAX_LABEL_LEN     12
+#define TLS13_MAX_LABEL_LEN     246
 
 /* Always filled with zeros */
 static const unsigned char default_zeros[EVP_MAX_MD_SIZE];
@@ -36,15 +29,14 @@ int tls13_hkdf_expand(SSL *s, const EVP_MD *md, const unsigned char *secret,
                              const unsigned char *data, size_t datalen,
                              unsigned char *out, size_t outlen)
 {
-    static const unsigned char label_prefix[] = "tls13 ";
+    const unsigned char label_prefix[] = "tls13 ";
     EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new_id(EVP_PKEY_HKDF, NULL);
     int ret;
     size_t hkdflabellen;
     size_t hashlen;
     /*
-     * 2 bytes for length of derived secret + 1 byte for length of combined
-     * prefix and label + bytes for the label itself + 1 byte length of hash
-     * + bytes for the hash itself
+     * 2 bytes for length of whole HkdfLabel + 1 byte for length of combined
+     * prefix and label + bytes for the label itself + bytes for the hash
      */
     unsigned char hkdflabel[sizeof(uint16_t) + sizeof(uint8_t) +
                             + sizeof(label_prefix) + TLS13_MAX_LABEL_LEN