Re: [RFC] capabilities: Ambient capabilities

[Andy Lutomirski]

On Sat, Mar 14, 2015 at 2:09 PM, Andrew G. Morgan <morgan@xxxxxxxxxx> wrote:
> On Fri, Mar 13, 2015 at 10:57 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Mar 13, 2015 6:24 AM, "Andrew G. Morgan" <morgan@xxxxxxxxxx> wrote:
>>>
>>> > It's to preserve the invariant that pA is always a subset of pI.
>>>
>>> But since a user can always raise a bit in pI if it is present in pP,
>>> what does this invariant add to your model other than inconvenience?
>>
>> The useful part is that dropping a bit from pI also drops it from pA.
>> To keep the model consistent, I also require that you add the bit to
>> pI before adding it to pA.
>
> So you are saying that pA is always a strict subset of pI (and pP)?
> Then why not explicitly implement it as:
>
>   pA' = (file caps or setuid or setgid ? 0 : pA)
>   pP' = (fP & X) | (pI & [fI | (pA' & pP)] )
>
> As it is you have so distributed these constraints that it is hard to
> be sure it will remain that way.

That would be insecure.  If an attacker had pA = CAP_SYS_ADMIN, pI =
0, pP = 0 (i.e. no privs but pA is set somehow) then, unless that's
there's some other protection implemented, they could run some setuid
program, and that program could switch back to non-root, set pI = 0,
and call execve.  Unexpectedly, CAP_SYS_ADMIN would be inherited.

So I made the invariant explicit and added an assertion.

>>
>> If you have a program that deliberately uses PR_CAP_AMBIENT, then
>> setting such a securebit will break the program, so it still doesn't
>> buy you anything.
>
> Not if you make the bit lockable (like the other bits). If you want to
> run with your model in effect, you lock the enable bit on.

I don't see the point.  Again, this should be the default.


>>
>>>
>>> > In the mean time, I don't even believe that there's a legitimate use
>>> > for any of the other secure bits (except keepcaps, and I don't know
>>> > why that's a securebit in the first place).
>>>
>>> Those bits currently make it possible to run a subsystem with no
>>> [set]uid-0 support in its process tree.
>>
>> Not usefully, because even with all the securebits set to their
>> non-legacy modes, caps don't inherit, so it doesn't work.  I've tried.
>
> Not sure I follow. They work for a definition if inheritable that you
> seem to refuse to accept.

I, and everyone I know who's tried to use inheritable capabilities,
has run into the near-complete uselessness of the current model.  I
understand that a defunct POSIX draft specified it, but it's still
nearly useless.

You've objected to changing it, but you've never directly addressed
any of the reasons why Christoph, Google, and I all believe that we
can't usefully use it.

>>> I think it is safe to say that naive privilege inheritance has a fair
>>> track record of being exploited orders of magnitude more frequently
>>> than this. After all, these are the reasons LD_PRELOAD and shell
>>> script setuid bits are suppressed.
>>
>> I don't know what you mean here by naive privilege inheritance.  The
>> examples you're taking about aren't inheritance at all; they're
>> exploring privilege *grants* during execve.  My patch deliberately
>> leaves grants like that alone.
>
> The pI set is inherited through this exec unmolested.

This is flat-out useless.  Having pI = CAP_NET_BIND_SERVICE doesn't
let me bind low-numbered ports, full stop.

> My Nack remains that you are eliminating the explicit enforcement of
> selective inheritance. A lockable secure bit protecting access to your
> prctl() function would address this concern.

Would a sysctl or securebit that *optionally* allows pA to be disabled
satisfy you?

I don't understand why lockable is at all useful.  You'd need
CAP_SETPCAP to flip it regardless.

--Andy
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