Re: [RFC] Capabilities still can't be inherited by normal programs

From: Serge Hallyn
Date: Wed Dec 05 2012 - 17:20:38 EST

Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx):
> On Wed, Dec 5, 2012 at 1:05 PM, Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx> wrote:
> > Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx):
> >> On Tue, Dec 4, 2012 at 5:54 AM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> >> > Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx):
> >> >> >> d) If I really wanted, I could emulate execve without actually doing
> >> >> >> execve, and capabilities would be inherited.
> >> >> >
> >> >> > If you could modify the executable properties of the binary that has
> >> >> > the privilege to wield a privilege then you are either exploiting an
> >> >> > app bug, or doing something the privileged binary has been trusted to
> >> >> > do.
> >> >>
> >> >> That's not what I mean. I would:
> >> >>
> >> >> fork()
> >> >> munmap everything
> >> >> mmap
> >> >> set up a fake initial stack and the right fd or mapping or whatever
> >> >> just to
> >> >>
> >> >> That's almost execve, and privilege inheritance works.
> >> >
> >> > But of course that is why you only want to fill fI on programs you trust
> >> > not to do that. What you are arguing is that you want to give fI on
> >> > programs you don't trust anyway, and so heck why not just give it on
> >> > everything.
> >> >
> >>
> >> Huh? I'd set fP on a program I expect to do *exactly* that (or use
> >> actual in-kernel capability inheritance, which I would find vastly
> >> more pleasant). If I give a program a capability (via fP or fI & pI),
> >> then I had better trust it not to abuse that capability. Having it
> >> pass that capability on to a child helper process would be just fine
> >> with me *because it already has that capability*.
> >>
> >> The problem with the current inheritance mechanism is that it's very
> >> difficult to understand what it means for an fI bit or a pI bit to be
> >> set. Saying "set a pI bit using pam if you want to grant permission
> >> to that user to run a particular program with fI set" is crap -- it
> >> only works if there is exactly one binary on the system with that bit
> >> set. In any case, a different administrator or package might use it
> >> for something different.
> >>
> >> Suppose I use the (apparently) current suggested approach: I install a
> >> fI=cap_net_raw copy of tcpdump somewhere. Then I write a helper that
> >> has fP=cap_new_raw and invokes that copy of tcpdump after appropriate
> >> validation of parameters. All is well.
> >
> > Since you're writing a special helper, you can surely have it validate
> > the userid and make it so the calling user doesn't have to have
> > cap_net_raw in pI?
> I can and did.

Oh, oops, I mis-understood what you meant was the problem.

Yup, that is a real limitation.

Yes, with the posix file caps you will be disappointed unless you see
pI=X as "this user may run any program which is Inh-trusted with X" and
fI=X as "this program may be run with X by any user Inh-trusted with X".

It almost makes me want to say that there should be an execve-analogue
to prctl(PR_SET_KEEPCAPS), which says caps will remain unchanged for one
execve. Or perhaps an intermediate securebits state between
!SECBIT_NOROOT and SECBIT_NOROOT, which automatically transitions after
the first execve to SECBIT_NOROOT.

> The mere presence of a cap_net_raw+i tcpdump binary is more or less
> equivalent to saying that users with cap_net_raw in pI can capture
> packets. I've just prevented pI=cap_net_raw from meaning anything
> less than "can capture packets". So I think we should bite the bullet
> and just let programs opt in (via some appropriately careful
> mechanism) to real capability inheritance.

By real you mean more precise. I think it'd be very interesting to get
together with Markku and learn more from the N9 experiment!

Markku, are there any post-mortem analysis papers we can read for
starters? Andy would not be trying to restrict root in general, so
the ramification you cited may not necessarily be relevant.

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