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

[Paul Moore <paul@paul-moore.com>]

On Wed, Aug 19, 2020 at 3:59 PM Daniel Burgener
<dburgener@linux.microsoft.com> 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>
> ---
>  security/selinux/selinuxfs.c | 113 ++++++++++++++++++++++++++++-------
>  1 file changed, 90 insertions(+), 23 deletions(-)

Merged into selinux/next, thanks!

-- 
paul moore
www.paul-moore.com