linux-kernel - Re: [PATCH RFC 1/1] Documentation: describe how to add a system call

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Message-ID: <CAGXu5jKqDzapDjhnKHV_-4iLb3qkqh-wZj0d3q6Lg0Pc02FT7A@mail.gmail.com>
Date:	Tue, 28 Jul 2015 10:17:06 -0700
From:	Kees Cook <keescook@...omium.org>
To:	David Drysdale <drysdale@...gle.com>
Cc:	Linux API <linux-api@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	Shuah Khan <shuahkh@....samsung.com>,
	Jonathan Corbet <corbet@....net>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric B Munson <emunson@...mai.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Al Viro <viro@...iv.linux.org.uk>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Peter Zijlstra <peterz@...radead.org>,
	Vivek Goyal <vgoyal@...hat.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	David Herrmann <dh.herrmann@...il.com>,
	"Theodore Ts'o" <tytso@....edu>, Milosz Tanski <milosz@...in.com>,
	Fam Zheng <famz@...hat.com>,
	Josh Triplett <josh@...htriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 1/1] Documentation: describe how to add a system call

On Tue, Jul 28, 2015 at 10:13 AM, David Drysdale <drysdale@...gle.com> wrote:
> On Tue, Jul 28, 2015 at 5:43 PM, Kees Cook <keescook@...omium.org> wrote:
>> On Tue, Jul 28, 2015 at 4:41 AM, David Drysdale <drysdale@...gle.com> wrote:
>>> Add a document describing the process of adding a new system call,
>>> including the need for a flags argument for future compatibility, and
>>> covering 32-bit/64-bit concerns (albeit in an x86-centric way).
>>>
>>> Signed-off-by: David Drysdale <drysdale@...gle.com>
>>> Reviewed-by: Michael Kerrisk <mtk.manpages@...il.com>
>>
>> This is great!
>>
>> Reviewed-by: Kees Cook <keescook@...omium.org>
>>
>> I have a few minor suggestions below...
>
> Thanks, I've applied all bar one -- a query below.
>
>>> ---
>>>  Documentation/adding-syscalls.txt | 454 ++++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 454 insertions(+)
>>>  create mode 100644 Documentation/adding-syscalls.txt
>>>
>>> diff --git a/Documentation/adding-syscalls.txt b/Documentation/adding-syscalls.txt
>>> new file mode 100644
>>> index 000000000000..5f52edda8951
>>> --- /dev/null
>>> +++ b/Documentation/adding-syscalls.txt
>
> [snip]
>
>>> + - If there is an existing capability that governs related functionality, then
>>> +   use that.  However, avoid combining lots of only vaguely related functions
>>> +   together under the same bit, as this goes against capabilities' purpose of
>>> +   splitting the power of root.  In particular, avoid adding new uses of the
>>> +   already overly-general CAP_SYS_ADMIN capability.
>>> + - If there is no related capability, then consider adding a new capability
>>> +   bit -- but bear in mind that the numbering space is limited, and each new
>>> +   bit needs to be understood and administered by sysadmins.
>>
>> Perhaps mention alternative mechanisms for access control when working
>> on file descriptors, like avoiding security issues by looking at fd
>> _opener_ credentials, rather than current's credentials?
>
> I'm struggling to cope up with text about this that doesn't feel either
> too vague or much too detailed / internal, so maybe I'm misunderstanding
> what you're after.  Could you clarify or maybe suggest a sentence or two?

Hm, yes, I think you're right: my suggestion here was too specific.
Please ignore! :)

-Kees

-- 
Kees Cook
Chrome OS Security
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