linux-kernel - Re: [PATCH v2 1/2] capabilities: Ambient capabilities

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Date:	Wed, 27 May 2015 15:35:58 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
	"Ted Ts'o" <tytso@....edu>, "Andrew G. Morgan" <morgan@...nel.org>,
	Andrew Morton <akpm@...uxfoundation.org>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	Linux API <linux-api@...r.kernel.org>,
	Austin S Hemmelgarn <ahferroin7@...il.com>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	Aaron Jones <aaronmdjones@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Markku Savela <msa@...h.iki.fi>,
	Kees Cook <keescook@...omium.org>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v2 1/2] capabilities: Ambient capabilities

On Mon, May 18, 2015 at 12:43 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On May 15, 2015 11:31 PM, "Christoph Lameter" <cl@...ux.com> wrote:
>>
>> It would be best to start a complete new thread about this. You
>> replied to earlier posts about ambient capabilities and
>> people may not see it as a new release.
>>
>> > pA obeys the invariant that no bit can ever be set in pA if it is
>> > not set in both pP and pI.  Dropping a bit from pP or pI drops that
>> > bit from pA.  This ensures that existing programs that try to drop
>> > capabilities still do so, with a complication.  Because capability
>>
>> Ok that is a good improvement.
>>
>> > inheritance is so broken, setting KEEPCAPS, using setresuid to
>> > switch to nonroot uids, or calling execve effectively drops
>> > capabilities.  Therefore, setresuid from root to nonroot
>> > conditionally clears pA unless SECBIT_NO_SETUID_FIXUP is set.
>> > Processes that don't like this can re-add bits to pA afterwards.
>> >
>> > The capability evolution rules are changed:
>> >
>> >   pA' = (file caps or setuid or setgid ? 0 : pA)
>> >   pP' = (X & fP) | (pI & fI) | pA'
>> >   pI' = pI
>> >   pE' = (fE ? pP' : pA')
>>
>> Isnt this equal to
>>
>>     pE' = (fE & pP') | pA'
>>
>> which does not require conditionals and is symmetric to how pP' is
>> calculated. Your formula seems to indicate that pA' bits are not set if
>> fE is set. However they are already set unconditionally in pP' regardless.
>> This makes it more explicit I think. And I thought we are dealing with
>> bitmask arithmetic here?
>
> I think you're right, except that fE is a Boolean, not a bit mask, so
> fE | pP' is an odd thing to talk about.
>
> We could say (fE ? pP' : 0) | pA', which could simplify the code a tiny bit.

It turns out that would be almost a pure addition of code, so I'll
leave it as is.

--Andy
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