Re: implement-file-posix-capabilities.patch

[Casey Schaufler <casey@schaufler-ca.com>]

--- "Serge E. Hallyn" <serue@us.ibm.com> wrote:

> Quoting Casey Schaufler (casey@schaufler-ca.com):
> > 
> > --- Andrew Morgan <morgan@kernel.org> wrote:
> > 
> > > -----BEGIN PGP SIGNED MESSAGE-----
> > > Hash: SHA1
> > > 
> > > Serge E. Hallyn wrote:
> > > >> Does that explain it?
> > > > 
> > > > Yes, thanks, but then it still could come in handy to have fE be a full
> > > > bitset, so the application gets some eff caps automatically, while
> > > > others it has to manually set...
> > > 
> > > [We touched on this a number of emails back.]
> > > 
> > > If an application is capability aware, it can manipulate its own
> > > capabilities and should have fE=0.
> > > 
> > > If an application is not capability aware, it needs to have *all* of its
> > > capabilities enabled at exec() time. Otherwise, it won't work.
> > 
> > The intent of the fE vector in the POSIX draft is that those capabilities
> > are set on exec (lower vectors permitting). There are cases where it
> > does make sense to raise just some (e.g. ping). 
> > 
> > > The only reason for having an fE bitmap is to allow a capability-aware
> > > program (you really trust to do its privileged operations carefully) to
> > > be lazy and get some of its capabilities raised for free. Perhaps you
> > > can clarify why this is a desirable thing? :-)
> > 
> > No, it's to allow you to grant a subset of the available capabilities
> > to a program that is not aware of capabilities. You can give "date"
> > the capability to reset the clock without giving it the capability
> > to remove other people's files without changing the code or running
> > it setuid.
> 
> Would there be a difference between that and setting either fI or fP
> (depending on your intent) to those caps, and setting fE=1 in Andrew's
> scheme?

Arg, you're making me think. The POSIX group went through this,
let me see if I can reconstruct the logic.

The main issue is one if there being a possible case where you
have a capability ignorant program that you want to exec with
a different fP and fE. On first glance it seems that since the
program is capability ignorant it can't matter. But what if your
capability ignorant program exec's a capability aware program
to perform a helper function? You may well want the first program
to have a capability that it does not use in fP (but not fE)
to pass along to the helper program. True, you could probably
come up with a way to set the capabilities on the helper program
to account for this use, but there may be design and security
constraints that make doing so complicated. 



Casey Schaufler
casey@schaufler-ca.com