[PATCH 1/7] ima: Avoid measurement and audit if access to the file will be denied

[Roberto Sassu]

Currently, IMA adds a measurement entry and an audit log even if access
to the file will be denied when appraisal is in enforce mode. This gives
the false indication to a verifier analyzing the data that file access
occurred. It also has the undesirable effect of unnecessarily revoking
access to TPM-protected objects sealed to PCRs.

Given that a potentially corrupted file will not be accessed by any user
space process, it is safe for the purpose of integrity evaluation to avoid
to record a measurement or audit log in this case.

Signed-off-by: Roberto Sassu <roberto.sa...@huawei.com>
---
 security/integrity/ima/ima_main.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/security/integrity/ima/ima_main.c 
b/security/integrity/ima/ima_main.c
index 9ef748ea829f..0faddcb8c71a 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -328,16 +328,14 @@ static int process_measurement(struct file *file, const 
struct cred *cred,
        hash_algo = ima_get_hash_algo(xattr_value, xattr_len);
 
        rc = ima_collect_measurement(iint, file, buf, size, hash_algo, modsig);
-       if (rc != 0 && rc != -EBADF && rc != -EINVAL)
+       if (rc != 0 && rc != -EBADF && rc != -EINVAL) {
+               action &= ~(IMA_MEASURE | IMA_AUDIT);
                goto out_locked;
+       }
 
        if (!pathbuf)   /* ima_rdwr_violation possibly pre-fetched */
                pathname = ima_d_path(&file->f_path, &pathbuf, filename);
 
-       if (action & IMA_MEASURE)
-               ima_store_measurement(iint, file, pathname,
-                                     xattr_value, xattr_len, modsig, pcr,
-                                     template_desc);
        if (rc == 0 && (action & IMA_APPRAISE_SUBMASK)) {
                rc = ima_check_blacklist(iint, modsig, pcr);
                if (rc != -EPERM) {
@@ -351,15 +349,21 @@ static int process_measurement(struct file *file, const 
struct cred *cred,
                        rc = mmap_violation_check(func, file, &pathbuf,
                                                  &pathname, filename);
        }
-       if (action & IMA_AUDIT)
-               ima_audit_measurement(iint, pathname);
-
        if ((file->f_flags & O_DIRECT) && (iint->flags & IMA_PERMIT_DIRECTIO))
                rc = 0;
 out_locked:
        if ((mask & MAY_WRITE) && test_bit(IMA_DIGSIG, &iint->atomic_flags) &&
             !(iint->flags & IMA_NEW_FILE))
                rc = -EACCES;
+       if (must_appraise)
+               if (rc && (ima_appraise & IMA_APPRAISE_ENFORCE))
+                       action &= ~(IMA_MEASURE | IMA_AUDIT);
+       if (action & IMA_MEASURE)
+               ima_store_measurement(iint, file, pathname,
+                                     xattr_value, xattr_len, modsig, pcr,
+                                     template_desc);
+       if (action & IMA_AUDIT)
+               ima_audit_measurement(iint, pathname);
        mutex_unlock(&iint->mutex);
        kfree(xattr_value);
        ima_free_modsig(modsig);
-- 
2.26.2