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

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Message-ID: <alpine.DEB.2.11.1502040946080.16341@gentwo.org>
Date:	Wed, 4 Feb 2015 09:50:04 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	"Andrew G. Morgan" <morgan@...nel.org>
cc:	"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

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)
 		 */
--
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