ima: process_measurement() needlessly takes inode_lock() on MAY_READ

On IMA policy update, if a measure rule exists in the policy,
IMA_MEASURE is set for ima_policy_flags which makes the violation_check
variable always true. Coupled with a no-action on MAY_READ for a
FILE_CHECK call, we're always taking the inode_lock().

This becomes a performance problem for extremely heavy read-only workloads.
Therefore, prevent this only in the case there's no action to be taken.

Signed-off-by: Frederick Lawler <fred@cloudflare.com>
Acked-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

security/integrity/ima/ima_main.c

@@ -245,7 +245,9 @@ static int process_measurement(struct file *file, const struct cred *cred,
 				&allowed_algos);
 	violation_check = ((func == FILE_CHECK || func == MMAP_CHECK ||
 			    func == MMAP_CHECK_REQPROT) &&
-			   (ima_policy_flag & IMA_MEASURE));
+			   (ima_policy_flag & IMA_MEASURE) &&
+			   ((action & IMA_MEASURE) ||
+			    (file->f_mode & FMODE_WRITE)));
 	if (!action && !violation_check)
 		return 0;