On Mon, 2007-12-10 at 21:08 +0000, David Howells wrote:
> Stephen Smalley <sds@tycho.nsa.gov> wrote:
>
> > Otherwise, only other issue I have with this interface is it won't
> > generalize to dealing with nfsd, where we want to set the acting context
> > to a context we obtain from or determine based upon the client.
>
> Are you speaking of security_kernel_act_as() and security_create_files_as()
> specifically? Or the task_struct::act_as override pointer in general?

security_kernel_act_as()

> I don't really know how nfsd wants to obtain and set its LSM context, so it's
> a bit difficult for me to make something that works for nfsd as well as
> cachefiles.

It would get a context from the client or from a local configuration that would map security-unaware clients to a default context, and then want to assume that context for the particular operation. No transition involved.

> > Why can't cachefilesd just push a context into the kernel and pass that
> > into the hook as the acting context,
>
> How does cachefilesd come up with such a context? Grab it from
> /etc/cachefilesd.conf?

>From a config file whose pathname would be provided by libselinux (ala
the way in which dbusd imports contexts), or directly as a context returned by a libselinux function. Has to be done that way so that it can be set differently for different policy types (strict, targeted, mls).

Naturally, cachefiles (the kernel module) would invoke a security hook to check whether the daemon is allowed to set the specified context.

> I use to do that, but someone objected... Possibly Karl MacMillan.

Yes, but I think I disagreed then too.

> > and then nfsd can do likewise using the context provided by the client or
> > obtained locally from exports for ordinary clients? Avoids the transition
> > SID computation altogether within the kernel and makes this more generic.
>
> I seem to remember that I was told that it should be done this way, possibly
> by Karl MacMillan, but I don't remember exactly.
>
> Now it's configured by cachefilesd.te:
>
> type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t;

It doesn't fit with how other users of security_kernel_act_as() will likely want to work (they will want to just set the context to a specified value, whether one obtained from the client or from some local source), nor with how type transitions normally work (exec, with the program type as the second type field). I think it will just cause confusion and subtle breakage.

-- 
Stephen Smalley
National Security Agency


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