Re: [PATCH v2 1/2] capabilities: Ambient capabilities

[Andy Lutomirski]

On May 15, 2015 11:31 PM, "Christoph Lameter" <cl@xxxxxxxxx> wrote:
>
> It would be best to start a complete new thread about this. You
> replied to earlier posts about ambient capabilities and
> people may not see it as a new release.
>
> > pA obeys the invariant that no bit can ever be set in pA if it is
> > not set in both pP and pI.  Dropping a bit from pP or pI drops that
> > bit from pA.  This ensures that existing programs that try to drop
> > capabilities still do so, with a complication.  Because capability
>
> Ok that is a good improvement.
>
> > inheritance is so broken, setting KEEPCAPS, using setresuid to
> > switch to nonroot uids, or calling execve effectively drops
> > capabilities.  Therefore, setresuid from root to nonroot
> > conditionally clears pA unless SECBIT_NO_SETUID_FIXUP is set.
> > Processes that don't like this can re-add bits to pA afterwards.
> >
> > The capability evolution rules are changed:
> >
> >   pA' = (file caps or setuid or setgid ? 0 : pA)
> >   pP' = (X & fP) | (pI & fI) | pA'
> >   pI' = pI
> >   pE' = (fE ? pP' : pA')
>
> Isnt this equal to
>
>     pE' = (fE & pP') | pA'
>
> which does not require conditionals and is symmetric to how pP' is
> calculated. Your formula seems to indicate that pA' bits are not set if
> fE is set. However they are already set unconditionally in pP' regardless.
> This makes it more explicit I think. And I thought we are dealing with
> bitmask arithmetic here?

I think you're right, except that fE is a Boolean, not a bit mask, so
fE | pP' is an odd thing to talk about.

We could say (fE ? pP' : 0) | pA', which could simplify the code a tiny bit.

>
>
> > If you are nonroot but you have a capability, you can add it to pA.
> > If you do so, your children get that capability in pA, pP, and pE.
> > For example, you can set pA = CAP_NET_BIND_SERVICE, and your
> > children can automatically bind low-numbered ports.  Hallelujah!
>
> I love this solution.
>
> > [2] The libcap capability mask parsers and formatters are
> > dangerously misleading and the documentation is flat-out wrong.  fE
> > is *not* a mask; it's a single bit.  This has probably confused
> > every single person who has tried to use file capabilities.
>
> Hmmm... yes lets clean that up as well. Then your formula makes sense.
>

Maybe a follow-up patch to change the docs would be a good idea.

--Andy
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