evm: evm_verify_hmac must not return INTEGRITY_UNKNOWN

If EVM is not supported or enabled, evm_verify_hmac() returns
INTEGRITY_UNKNOWN, which ima_appraise_measurement() ignores and sets
the appraisal status based solely on the security.ima verification.

evm_verify_hmac() also returns INTEGRITY_UNKNOWN for other failures, such
as temporary failures like -ENOMEM, resulting in possible attack vectors.
This patch changes the default return code for temporary/unexpected
failures, like -ENOMEM, from INTEGRITY_UNKNOWN to INTEGRITY_FAIL, making
evm_verify_hmac() fail safe.

As a result, failures need to be re-evaluated in order to catch both
temporary errors, such as the -ENOMEM, as well as errors that have been
resolved in fix mode.

Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@nokia.com>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>

security/integrity/evm/evm_main.c

@@ -56,13 +56,15 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
 	struct evm_ima_xattr_data xattr_data;
 	int rc;
 
-	if (iint->hmac_status != INTEGRITY_UNKNOWN)
+	if (iint->hmac_status == INTEGRITY_PASS)
 		return iint->hmac_status;
 
+	/* if status is not PASS, try to check again - against -ENOMEM */
+
 	rc = evm_calc_hmac(dentry, xattr_name, xattr_value,
 			   xattr_value_len, xattr_data.digest);
 	if (rc < 0)
-		return INTEGRITY_UNKNOWN;
+		goto err_out;
 
 	xattr_data.type = EVM_XATTR_HMAC;
 	rc = vfs_xattr_cmp(dentry, XATTR_NAME_EVM, (u8 *)&xattr_data,
@@ -77,11 +79,8 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
 	case -ENODATA:		/* file not labelled */
 		iint->hmac_status = INTEGRITY_NOLABEL;
 		break;
-	case -EINVAL:
-		iint->hmac_status = INTEGRITY_FAIL;
-		break;
 	default:
-		iint->hmac_status = INTEGRITY_UNKNOWN;
+		iint->hmac_status = INTEGRITY_FAIL;
 	}
 	return iint->hmac_status;
 }