Reset the HKDF state between operations

Fixes #7497
Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7501)

(cherry picked from commit ca55d70be031746daddd8bd0611db54ed81f1737)

crypto/kdf/hkdf.c

@@ -175,6 +175,18 @@ static int pkey_hkdf_ctrl_str(EVP_PKEY_CTX *ctx, const char *type,
     return -2;
 }
 
+static int pkey_hkdf_derive_init(EVP_PKEY_CTX *ctx)
+{
+    HKDF_PKEY_CTX *kctx = ctx->data;
+
+    OPENSSL_clear_free(kctx->key, kctx->key_len);
+    OPENSSL_clear_free(kctx->salt, kctx->salt_len);
+    OPENSSL_cleanse(kctx->info, kctx->info_len);
+    memset(kctx, 0, sizeof(*kctx));
+
+    return 1;
+}
+
 static int pkey_hkdf_derive(EVP_PKEY_CTX *ctx, unsigned char *key,
                             size_t *keylen)
 {
@@ -236,7 +248,7 @@ const EVP_PKEY_METHOD hkdf_pkey_meth = {
 
     0, 0,
 
-    0,
+    pkey_hkdf_derive_init,
     pkey_hkdf_derive,
     pkey_hkdf_ctrl,
     pkey_hkdf_ctrl_str