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>

1 files changed, 5 insertions, 6 deletions

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 0fa8261c3655..bfe44dff61bb 100644
--- 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;
 }