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>

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 0fa8261..bfe44df 100644 (file)
--- a/security/integrity/evm/evm_main.c
+++ b/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 @@ err_out:
        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;
 }