Premaster secret handling fixes

From BoringSSL
- Send an alert when the client key exchange isn't correctly formatted.
- Reject overly short RSA ciphertexts to avoid a (benign) out-of-bounds memory access.
Reviewed-by: NKurt Roeckx <kurt@openssl.org>

ssl/s3_srvr.c

@@ -2276,6 +2276,7 @@ int ssl3_get_client_key_exchange(SSL *s)
 		unsigned char rand_premaster_secret[SSL_MAX_MASTER_KEY_LENGTH];
 		int decrypt_len;
 		unsigned char decrypt_good, version_good;
+		size_t j;
 
 		/* FIX THIS UP EAY EAY EAY EAY */
 		if (s->s3->tmp.use_rsa_tmp)
@@ -2314,8 +2315,9 @@ int ssl3_get_client_key_exchange(SSL *s)
 				{
 				if (!(s->options & SSL_OP_TLS_D5_BUG))
 					{
+					al = SSL_AD_DECODE_ERROR;
 					SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE,SSL_R_TLS_RSA_ENCRYPTED_VALUE_LENGTH_IS_WRONG);
-					goto err;
+					goto f_err;
 					}
 				else
 					p-=2;
@@ -2324,6 +2326,20 @@ int ssl3_get_client_key_exchange(SSL *s)
 				n=i;
 			}
 
+		/*
+		 * Reject overly short RSA ciphertext because we want to be sure
+		 * that the buffer size makes it safe to iterate over the entire
+		 * size of a premaster secret (SSL_MAX_MASTER_KEY_LENGTH). The
+		 * actual expected size is larger due to RSA padding, but the
+		 * bound is sufficient to be safe.
+		 */
+		if (n < SSL_MAX_MASTER_KEY_LENGTH)
+			{
+			al = SSL_AD_DECRYPT_ERROR;
+			SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE, SSL_R_TLS_RSA_ENCRYPTED_VALUE_LENGTH_IS_WRONG);
+			goto f_err;
+			}
+
 		/* We must not leak whether a decryption failure occurs because
 		 * of Bleichenbacher's attack on PKCS #1 v1.5 RSA padding (see
 		 * RFC 2246, section 7.4.7.1). The code follows that advice of
@@ -2371,19 +2387,23 @@ int ssl3_get_client_key_exchange(SSL *s)
 		 * to remain non-zero (0xff). */
 		decrypt_good &= version_good;
 
-		/* Now copy rand_premaster_secret over p using
-		 * decrypt_good_mask. */
-		for (i = 0; i < (int) sizeof(rand_premaster_secret); i++)
+		/*
+		 * Now copy rand_premaster_secret over from p using
+		 * decrypt_good_mask. If decryption failed, then p does not
+		 * contain valid plaintext, however, a check above guarantees
+		 * it is still sufficiently large to read from.
+		 */
+		for (j = 0; j < sizeof(rand_premaster_secret); j++)
 			{
-			p[i] = constant_time_select_8(decrypt_good, p[i],
-						      rand_premaster_secret[i]);
+			p[j] = constant_time_select_8(decrypt_good, p[j],
+						      rand_premaster_secret[j]);
 			}
 
 		s->session->master_key_length=
 			s->method->ssl3_enc->generate_master_secret(s,
 				s->session->master_key,
-				p,i);
-		OPENSSL_cleanse(p,i);
+				p,sizeof(rand_premaster_secret));
+		OPENSSL_cleanse(p,sizeof(rand_premaster_secret));
 		}
 	else
 #endif