linux-kernel - Re: [capabilities] Allow normal inheritance for a configurable set of capabilities

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Date:	Wed, 4 Feb 2015 16:56:17 +0100
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	"Andrew G. Morgan" <morgan@...nel.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Andy Lutomirski <luto@...capital.net>,
	Jonathan Corbet <corbet@....net>,
	Aaron Jones <aaronmdjones@...il.com>, Ted Ts'o <tytso@....edu>,
	linux-security-module@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>, akpm@...uxfoundation.org
Subject: Re: [capabilities] Allow normal inheritance for a configurable set
 of capabilities

Quoting Christoph Lameter (cl@...ux.com):
> On Wed, 4 Feb 2015, Andrew G. Morgan wrote:
> 
> > I'm not generally in favor of this. Mostly because this seems to be a
> > mini-root kind of inheritance that propagates privilege to binaries
> > that aren't prepared for privilege. I don't really buy the mmap code
> > concern because the model as it stands says that you trust the binary
> > (and all of the various ways it was programmed to execute code) with
> > specific privileges. If the binary mmap's some code (PAM modules come
> > to mind) then that is part of what it was programmed to/allowed to do.
> >
> > That being said, if you really really want this kind of thing, then
> > make it a single secure bit (with another lock on/off bit) which, when
> > set, makes: fI default to X.
> >
> >    pP' = (X & fP) | (pI & fI)
> >
> > That way the per-process bounding set still works as advertised and
> > you don't need to worry about the existing semantics being violated.
> 
> Ok but then also fI needs to be set to X so that the binary f invokes
> can also inherit.  So if we copy the inheritable flags to fI then we
> wont be needing the bounding set anymore.
> 
> The changes to brpm_caps_from_vfs_cap would then
> be only the following? (substitute capable(CAP_INHERIT_BY_DEFAULT through
> any other means like PRCTL if wanted).
> 
> 
> Index: linux/security/commoncap.c
> ===================================================================
> --- linux.orig/security/commoncap.c	2015-02-04 09:44:25.000000000 -0600
> +++ linux/security/commoncap.c	2015-02-04 09:45:59.381572756 -0600
> @@ -350,6 +350,9 @@ static inline int bprm_caps_from_vfs_cap
>  		__u32 permitted = caps->permitted.cap[i];
>  		__u32 inheritable = caps->inheritable.cap[i];
> 
> +		if (capable(CAP_INHERIT_BY_DEFAULT)
> +			new->cap_inheritable.cap[i] = inheritable;
> +
>  		/*
>  		 * pP' = (X & fP) | (pI & fI)
>  		 */

Not quite - I think more like

	if (secure(SECURE_AMBIENT_PRIVS))
		new->cap_inheritable.cap[i] = inheritable;

Then ns_capable(CAP_INHERIT_BY_DEFAULT), or perhaps rather
ns_capable(CAP_SETPCAP), would be required in order to set
SECURE_AMBIENT_PRIVS, which is off by default.
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