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selinux: Re: access decision API

Re: access decision API

From: michel m <michel.mcgregor_at_nospam>
Date: Wed Jan 27 2010 - 14:40:29 GMT
To: Stephen Smalley <sds@tycho.nsa.gov>


thanks for guidance, but here I am with a question. what should be used as object class in avc_has_perm(3) when using it for inter-object. is there any sample for inter-object access decision? can it be null?

on the other hand, access decision taken by avc_has_perm(), does it include MLS too?

regards.

On Wed, Jan 27, 2010 at 1:26 AM, Stephen Smalley <sds@tycho.nsa.gov> wrote:

> On Tue, 2010-01-26 at 16:34 -0500, Stephen Smalley wrote:
> > On Wed, 2010-01-27 at 00:50 +0330, michel m wrote:
> > > Hi,
> > > I have some data in userspace that I am keeping its security
> > > context.but as I want to write this data on some file in OS, I need to
> > > consult with security server if such data is allowed to be written on
> > > the file.
> > > I would like to know if there is any API that let me do such a
> > > scenario, that is input source and destination security context (both
> > > of them security context,not a domain context) and returns access
> > > decision. I am familiar with avc_has_perm(3), but seems to be not
> > > correct solution because it gets domain context as the first argument.
> >
> > avc_has_perm() can be used with any pair of security contexts. Typical
> > usage is to pass the security context of a subject/process as the first
> > argument, but not always (e.g. there are some permission checks that
> > control inter-object relationships), and that is not a requirement.
>
> Example usage of the userspace AVC can be found in the XSELinux
> extension, see:
> http://cgit.freedesktop.org/xorg/xserver/tree/Xext/xselinux_hooks.c
> http://cgit.freedesktop.org/xorg/xserver/tree/Xext/xselinuxint.h
>
> The modern usage is to call:
> 1) selinux_set_callback() to set up the callback functions for
> libselinux to use for logging/auditing or to notify the application of
> changes to enforcing mode or policy.
> 2) selinux_set_mapping() to set up a dynamic mapping for the object
> classes and permissions so that your application does not depend on
> fixed values for them in the policy, and
> 3) avc_open() to initialize the AVC.
>
> Then you can proceed to call avc_context_to_sid() and avc_has_perm() as
> appropriate.
>
> If you don't need caching, you can directly call security_compute_av();
> however this pushes responsibility for checking the particular
> permission bit, checking for and handling permissive mode/domains, and
> generating an avc audit message to your application. There has been a
> prior request to introduce a simpler interface for programs that do not
> require caching, see:
> https://bugzilla.redhat.com/show_bug.cgi?id=518268
>
> --
> Stephen Smalley
> National Security Agency
>
>
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