Update HMAC to fail when null value is passed to out parameter (#1662)

### Issues:
CryptoAlg-2522

### Description of changes: 

Currently, `NULL` passed as the value of `out` can result in a
segmentation fault. This change adds checks to the HMAC one-shot API and
HMAC_final function to handle scenarios where `NULL` is passed as a
value to the `out` parameter to return from the functions and prevent
further computation.

### Testing:

Added additional test case in `crypto/hmac_extra/hmac_test.cc` to verify
the behavior when `NULL` is passed as a value to `out` in both
functions.

### Call-outs:

- OpenSSL supports this differently by allowing the computation to occur
but allocating a throw-away array [OpenSSL
implementation](https://github.com/openssl/openssl/blob/1977c00f00ad0546421a5ec0b40c1326aee4cddb/crypto/hmac/hmac.c#L233)

- We should also evaluate if this change should be back-ported to the
FIPS branches

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

Co-authored-by: Sean McGrail <[email protected]>

crypto/fipsmodule/hmac/hmac.c

@@ -185,6 +185,11 @@ OPENSSL_STATIC_ASSERT(HMAC_STATE_UNINITIALIZED == 0, HMAC_STATE_UNINITIALIZED_is
 uint8_t *HMAC(const EVP_MD *evp_md, const void *key, size_t key_len,
               const uint8_t *data, size_t data_len, uint8_t *out,
               unsigned int *out_len) {
+
+  if (out == NULL) {
+    // Prevent further work from being done
+    return NULL;
+  }
 
   HMAC_CTX ctx;
   OPENSSL_memset(&ctx, 0, sizeof(HMAC_CTX));
@@ -341,6 +346,10 @@ int HMAC_Update(HMAC_CTX *ctx, const uint8_t *data, size_t data_len) {
 }
 
 int HMAC_Final(HMAC_CTX *ctx, uint8_t *out, unsigned int *out_len) {
+  if (out == NULL) {
+    return 0;
+  }
+
   const HmacMethods *methods = ctx->methods;
   if (!hmac_ctx_is_initialized(ctx)) {
     return 0;

crypto/hmac_extra/hmac_test.cc

@@ -351,3 +351,28 @@ TEST(HMACTest, EVP_DigestVerify) {
   EXPECT_EQ(EVP_R_OPERATION_NOT_SUPPORTED_FOR_THIS_KEYTYPE,
             ERR_GET_REASON(ERR_get_error()));
 }
+
+TEST(HMACTest, HandlesNullOutputParameters) {
+  bssl::ScopedHMAC_CTX ctx;
+  const EVP_MD *digest = EVP_sha256();
+  // make key and input valid
+  const uint8_t key[32] = {0};
+  const uint8_t input[16] = {0};
+
+  // Test one-shot API with out and out_len as NULL
+  ASSERT_FALSE(HMAC(digest, &key[0], sizeof(key), &input[0], sizeof(input),
+                  nullptr,nullptr));
+  unsigned mac_len;
+  // Test one-shot API with only out as NULL
+  ASSERT_FALSE(HMAC(digest, &key[0], sizeof(key), &input[0], sizeof(input),
+                  nullptr, &mac_len));
+
+  // Test HMAC_ctx
+  ASSERT_TRUE(HMAC_Init_ex(ctx.get(), &key[0], sizeof(key), digest, nullptr));
+  ASSERT_TRUE(HMAC_Update(ctx.get(), &input[0], sizeof(input)));
+  ASSERT_FALSE(HMAC_Final(ctx.get(), nullptr, nullptr));
+
+  ASSERT_TRUE(HMAC_Init_ex(ctx.get(), &key[0], sizeof(key), digest, nullptr));
+  ASSERT_TRUE(HMAC_Update(ctx.get(), &input[0], sizeof(input)));
+  ASSERT_FALSE(HMAC_Final(ctx.get(), nullptr, &mac_len));
+}