linux-kernel - Re: [RFC] Implement ambient capability set.

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Date:	Thu, 5 Feb 2015 01:03:09 +0100
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Christoph Lameter <cl@...ux.com>,
	"Andrew G. Morgan" <morgan@...nel.org>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Jonathan Corbet <corbet@....net>,
	Aaron Jones <aaronmdjones@...il.com>, Ted Ts'o <tytso@....edu>,
	LSM List <linux-security-module@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...uxfoundation.org>
Subject: Re: [RFC] Implement ambient capability set.

Quoting Andy Lutomirski (luto@...capital.net):
> On Wed, Feb 4, 2015 at 2:02 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> > Quoting Serge E. Hallyn (serge@...lyn.com):
> >> Quoting Andy Lutomirski (luto@...capital.net):
> >> > On Wed, Feb 4, 2015 at 1:27 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> >> > > Quoting Andy Lutomirski (luto@...capital.net):
> >> > >> On Wed, Feb 4, 2015 at 1:16 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> >> > >> > Quoting Andy Lutomirski (luto@...capital.net):
> >> > >> >> On Wed, Feb 4, 2015 at 10:49 AM, Christoph Lameter <cl@...ux.com> 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
> 
> I'm still unconvinced that 2 and 3 is better than using pP & pA in
> execve, but I could go either way on that.
> 
> >>
> >> 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 can't drop nnp at exec.
> 
> >
> > We definately do not want to run a file which has capability X
> > in fP, when X is not in pA.
> 
> Confused.  This will break everything, including Christoph's use case.
> The status quo for running ping from bash has pA == 0 and fP != 0.

Sorry, I meant only if pA is not empty.

But it sounds like we've in any case not hit on anything that will
actually work for Christoph.
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