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

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Message-ID: <CALQRfL4wb=hyEkiugAHVE9iCOHz=H7KqhDVB3X1G0Wt5O_XtKg@mail.gmail.com>
Date:	Wed, 4 Feb 2015 08:12:04 -0800
From:	"Andrew G. Morgan" <morgan@...nel.org>
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>,
	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

I was thinking more like this:

    int override = secure(SECURE_AMBIENT_PRIVS) &&
cap_isclear(caps->inheritable.cap);

    CAP_FOR_EACH_U32(i) {
        __u32 permitted = caps->permitted.cap[i];
        __u32 inheritable = override ? new->cap_bset.cap[i] :
caps->inheritable.cap[i];
    [...]

Cheers

Andrew

On Wed, Feb 4, 2015 at 7:56 AM, Serge E. Hallyn <serge@...lyn.com> wrote:
> 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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