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

[Kees Cook]

On Tue, Jul 28, 2015 at 10:13 AM, David Drysdale <drysdale@xxxxxxxxxx> wrote:
> On Tue, Jul 28, 2015 at 5:43 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Tue, Jul 28, 2015 at 4:41 AM, David Drysdale <drysdale@xxxxxxxxxx> 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@xxxxxxxxxx>
>>> Reviewed-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
>>
>> This is great!
>>
>> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>>
>> 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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