linux-kernel - Re: [PATCH] capabilities: Ambient capability set V1

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Date:	Thu, 26 Feb 2015 12:58:33 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Aaron Jones <aaronmdjones@...il.com>,
	"Ted Ts'o" <tytso@....edu>,
	LSM List <linux-security-module@...r.kernel.org>,
	Andrew Morton <akpm@...uxfoundation.org>,
	"Andrew G. Morgan" <morgan@...nel.org>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Markku Savela <msa@...h.iki.fi>,
	Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] capabilities: Ambient capability set V1

On Thu, Feb 26, 2015 at 12:55 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> On Thu, Feb 26, 2015 at 12:51:57PM -0800, Andy Lutomirski wrote:
>> On Thu, Feb 26, 2015 at 12:34 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
>> > On Thu, Feb 26, 2015 at 02:13:00PM -0600, Christoph Lameter wrote:
>> >> On Thu, 26 Feb 2015, Serge E. Hallyn wrote:
>> >>
>> >> > Andrew Morgan was against that.  What if we changed
>> >> >
>> >> > pE' = pP' & (fE | pA)
>> >> >
>> >> > to
>> >> >
>> >> >     if (pA)
>> >> >             pE' = pP' & fE
>> >> >     else
>> >> >             pE' = pP'
>> >> >
>> >>
>> >> Same problem as before. The ambient bits will not be set in pE'.
>> >
>> > And what if I weren't scatterbrained and we did
>> >
>> >         if (pA)
>> >                 pE' = pP'
>> >         else
>> >                 pE' = pP' & fE
>> >
>> > All pP' bits would be set in pE'.
>>
>> That seems reasonable to me, except for my paranoia:
>>
>> What if there's a program with CAP_DAC_OVERRIDE in fP and fE set to
>> the empty set (i.e. the magic effective bit cleared), and the program
>> relies on that.  A malicious user has CAP_NET_BIND and sets pA =
>> CAP_NET_BIND.  Boom!
>>
>> If we changed that to if (pA') and zeroed pA if fP is non-empty then
>> this problem goes away.
>
> Hm, the problem is that then the empty pA is inherited by children.
> I do see that any program with fP set should probably run with only
> what it requested.  Would
>
>          if (pA && is_empty(fP))
>                  pE' = pP'
>          else
>                  pE' = pP' & fE
>
> help?  Or are you worried about a program with fP set which then
> executes other programs?

The particular worry I expressed there was just about pE.

I'm still extremely nervous about allowing nonempty pA to propagate to
setuid or nonzero fP programs.  It's less obviously dangerous if pA is
never a superset of pP, but it could still cause problems with setuid
programs that execute intentionally deprivileged helpers.

--Andy
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