On 12/4/18 11:03 AM, BMK wrote:
Hello, I am currently struggling with a strange SELinux problem, for which I am not able to find an answer by reading the documentation and researching online. The problem is, that some AVC denial log entries seem to get lost in permissive mode, in other words, they are not logged... I've already deactivated all dont audit rules and I know for sure that the denial actually occurs, because I can trace it via strace... Although I can't see a corresponding entry in the audit.log. By the way, in enforcing mode I can see suddenly the missing denial entry... If the permissive mode lacks/drops some denials which we can only see in enforcing mode, then this would be truly terrible for the policy writers... Otherwise I am out of ideas, what other things could cause the loss of SELinux denials... I hope you can point me to right direction with this matter and I thank you in advance for your help.
Permissive mode only logs the first instance of a denial by design to avoid flooding the logs with repeated instances of the same denial. So if you triggered the denial a while ago and repeat the operation, you might not see the denial again. To be precise, in permissive mode, upon the first denial of a permission, the permission is audited and then added to the AVC entry so that subsequent denials using that cache entry won't keep producing a denial. You can flush the cache to force denials to re-appear by reloading policy (load_policy) or by switching back and forth between permissive and enforcing mode (setenforce 1 && setenforce 0).
If that doesn't explain the behavior you are seeing, then we can't really help without more information about the problem, e.g. the denial message you say is visible in enforcing mode but not permissive mode, your kernel version, possibly the strace output, a reproducer if you have one, what distro / policy you are using, etc.
There are cases where the audit system could drop records due to OOM conditions, its ratelimit, or its backlog limit. In those cases, you should have a audit: message logged indicating that messages were lost. Check your dmesg or journalctl logs for such messages from the audit system. Those are audit system issues rather than SELinux. You can configure the limits via auditctl and/or the audit configuration. But generally those only apply when the audit system is under heavy load from many denials (or many other audit messages) and you should see at least some of them.
