Re: [RFC] Implement ambient capability set.

[Serge E. Hallyn]

Quoting Serge E. Hallyn (serge@xxxxxxxxxx):
> Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx):
> > On Wed, Feb 4, 2015 at 1:27 PM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> > > Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx):
> > >> On Wed, Feb 4, 2015 at 1:16 PM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> > >> > Quoting Andy Lutomirski (luto@xxxxxxxxxxxxxx):
> > >> >> On Wed, Feb 4, 2015 at 10:49 AM, Christoph Lameter <cl@xxxxxxxxx> wrote:
> > >> >> > +
> > >> >> > +               if (!cap_valid(arg2))
> > >> >> > +                       return -EINVAL;
> > >> >> > +
> > >> >> > +               new =prepare_creds();
> > >> >> > +               if (arg3 == 0)
> > >> >> > +                       cap_lower(new->cap_ambient, arg2);
> > >> >> > +               else
> > >> >> > +                       cap_raise(new->cap_ambient, arg2);
> > >> >> > +               return commit_creds(new);
> > >> >> > +
> > >> >>
> > >> >> This let you add capabilities you don't even have to cap_ambient.  I'm
> > >> >> fine with that as long as the cap evolution rule changes, as above.
> > >> >
> > >> > How about if instead we do restrict it to what's in pP?  I don't
> > >> > want CAP_SETPCAP to become a cheap way to get all caps back.  With
> > >> > or without NNP.
> > >>
> > >> We'd also have to modify everything that can change pP to change pA as
> > >> well if we went this route.  I'd be okay with that, but it would make
> > >> the patch much larger, and I'm not entirely sure I see the benefit.
> > >> It would keep the number of possible states smaller, which could be
> > >> nice.
> > >
> > > Do you mean if we didn't require NNP?  I'm suggesting that even if
> > > we require NNP we should restrict any new bits added to pA to be
> > > in pP at the prctl call.  Then whether or not to drop them from
> > > pA when they are dropped from pP, I'm not yet certain.
> > 
> > I mean regardless of whether we require NNP.
> > 
> > I think that, unless we change the evolution rule, we would need to
> > drop from pA when bits are dropped from pP to preserve the idea that
> > dropping bits from pP drops them for good (as long as ruid != 0 or
> > some securebit is set).
> 
> Ok, so iiuc the rules would be:
> 
> 1. must set nnp and have ns_capable(CAP_SETPCAP) to
> call prctl(PR_SET_AMBIENT_WHATEVER)
> 
> 2. adding bits to pA requires they be in pP at prctl time
> 
> 3. dropping bits from pP drops them also from pA
> 
> 4. at exec, fP |= pA;  pA' = pA

Actually I'm tempted to say that if fP is not empty, then we stick with current
rules for fP/pP and clear out pA.  If fP is empty, then fP = pA

Then, if fP is not empty on the file, we either drop nnp at exec (or
don't use nnp at all), or we refuse exec if fP > pA.

We definately do not want to run a file which has capability X
in fP, when X is not in pA.

> Christoph, would these suffice for your use caes?
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