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

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Message-ID: <20150204215158.GA21613@mail.hallyn.com>
Date:	Wed, 4 Feb 2015 22:51:58 +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 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

4. at exec, fP |= pA;  pA' = pA

Christoph, would these suffice for your use caes?
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