Re: RFC: capabilities(7): notes for kernel developers

Michael Kerrisk (man-pages)
2016-12-16T15:04:02Z

On 12/15/2016 09:40 PM, Casey Schaufler wrote:
> On 12/15/2016 11:41 AM, Michael Kerrisk (man-pages) wrote:
>> Hello Casey,
>>
>> On 12/15/2016 05:29 PM, Casey Schaufler wrote:
>>> On 12/15/2016 3:40 AM, Michael Kerrisk (man-pages) wrote:

[...]

>>>> * To determine which existing capability might best be associated
>>>> with your new feature, review the list of capabilities above in
>>>> order to find a "silo" into which your new feature best fits.
>>> One approach to take is to determine if there are other features
>>> requiring capabilities that will always be use along with the
>>> new feature. If the new feature is useless without these other
>>> features, you should use the same capability as the other features.
>> Thanks, I've lifted those words exactly as you gave them into the man page.
>>
>>>> * Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A
>>>> vast proportion of existing capability checks are associated
>>>> with this capability, to the point where it can plausibly be
>>>> called "the new root". Don't make the problem worse. The only
>>>> new features that should be associated with CAP_SYS_ADMIN are
>>>> ones that closely match existing uses in that silo.
>>> I don't agree with this advice. Use CAP_SYS_ADMIN if you are
>>> preforming system administration functions. Odds are very good
>>> that if a program is using one system administration feature
>>> it will be using others.
>> Really? To me, the CAP_SYS_ADMIN situation is a terrible mess. Around a
>> third of all of the capability checks in the kernel are for that
>> capability. Or, to put it another way, it is so broad, that if a process
>> has to have that capability, it may as well be root. And because it is
>> so broad, the number of binaries that might need that file capability is
>> large. (See also https://lwn.net/Articles/486306/)
>
> Back in the days of the POSIX P1003.1e/2c working group
> we struggled with what to do about the things that required
> privilege but that were not related to the enforcement of
> security policy. Everyone involved was looking to use
> capabilities to meet B2* least privilege requirements in
> NSA security evaluations. Because those evaluations where
> of security policy, by far the easiest thing to do was to
> create a single capability for all the things that didn't
> show up in the security policy and declare that the people
> doing the evaluation didn't have to look over there. Since
> then, people have taken a more practical view that includes
> security relevance in addition to security policy.

Ahhh -- thanks for a bit of the history! Helps me to
understand a bit more why things are as they are...

> In retrospect we should have grouped all of the attribute
> changes (chmod, chown, ...) into one capability and broken
> the non-policy actions into a set on 2 or three.
>
> The way that we think of privilege has evolved. We're not
> focused on policy the way we used to be. We'll never get
> everyone to agree on what the *right* granularity and
> grouping is, either.

:-)

> -----
> * The B2 least privilege requirements are amusing.
> If you want more information, look up "TCSEC orange book".

Maybe one day...

[...]

Cheers,

Mcihael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/