RFC: capabilities(7): notes for kernel developers

From: Michael Kerrisk (man-pages)
Date: Thu Dec 15 2016 - 06:41:07 EST

Hello all,

Because the topic every now then comes up "which capability
should I associate with the new feature that I'm adding to
the kernel?", I propose to add the text below to the
capabilities(7) man page [1] with some recommendations
on how to go about choosing. I would be happy
to get feedback, suggestions for improvement and
so on.



[1] http://man7.org/linux/man-pages/man7/capabilities.7.html

Notes to kernel developers
When adding a new kernel feature that should be governed by a
capability, consider the following points.

* The goal of capabilities is divide the power of superuser into
small pieces, such that if a program that has capabilities is
compromised, its power to do damage to the system would be much
less than a similar set-user-ID-root program.

* You have the choice of either creating a new capability for
your new feature, or associating the feature with one of the
existing capabilities. Because the size of capability sets is
currently limited to 64 bits, the latter option is preferable,
unless there are compelling reasons to take the former option.

* 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.

* 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.

* If you have determined that it really is necessary to create a
new capability for your feature, avoid making (and naming) it
as a "single-use" capability. Thus, for example, the addition
of the highly specific CAP_WAKE_ALARM was probably a mistake.
Instead, try to identify and name your new capability as a
broader silo into which other related future use cases might

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