Re: [PATCH v3 4/4] selinux: Create new booleans and class dirs out of tree

Stephen Smalley
2020-08-20T15:23:56Z

On 8/19/20 3:59 PM, Daniel Burgener wrote:

> In order to avoid concurrency issues around selinuxfs resource availability
> during policy load, we first create new directories out of tree for
> reloaded resources, then swap them in, and finally delete the old versions.
>
> This fix focuses on concurrency in each of the two subtrees swapped, and
> not concurrency between the trees. This means that it is still possible
> that subsequent reads to eg the booleans directory and the class directory
> during a policy load could see the old state for one and the new for the other.
> The problem of ensuring that policy loads are fully atomic from the perspective
> of userspace is larger than what is dealt with here. This commit focuses on
> ensuring that the directories contents always match either the new or the old
> policy state from the perspective of userspace.
>
> In the previous implementation, on policy load /sys/fs/selinux is updated
> by deleting the previous contents of
> /sys/fs/selinux/{class,booleans} and then recreating them. This means
> that there is a period of time when the contents of these directories do not
> exist which can cause race conditions as userspace relies on them for
> information about the policy. In addition, it means that error recovery in
> the event of failure is challenging.
>
> In order to demonstrate the race condition that this series fixes, you
> can use the following commands:
>
> while true; do cat /sys/fs/selinux/class/service/perms/status
>> /dev/null; done &
> while true; do load_policy; done;
>
> In the existing code, this will display errors fairly often as the class
> lookup fails. (In normal operation from systemd, this would result in a
> permission check which would be allowed or denied based on policy settings
> around unknown object classes.) After applying this patch series you
> should expect to no longer see such error messages.
>
> Signed-off-by: Daniel Burgener <dburgener@linux.microsoft.com>

Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>