Re: [PATCH 2/2] proc: Add /proc/<pid>/timerslack_ns interface

[Kees Cook]

On Wed, Jul 13, 2016 at 4:47 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> On Tue, Feb 16, 2016 at 5:06 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
>> This patch provides a proc/PID/timerslack_ns interface which
>> exposes a task's timerslack value in nanoseconds and allows it
>> to be changed.
>>
>> This allows power/performance management software to set timer
>> slack for other threads according to its policy for the thread
>> (such as when the thread is designated foreground vs. background
>> activity)
>>
>> If the value written is non-zero, slack is set to that value.
>> Otherwise sets it to the default for the thread.
>>
>> This interface checks that the calling task has permissions to
>> to use PTRACE_MODE_ATTACH_FSCREDS on the target task, so that we
>> can ensure arbitrary apps do not change the timer slack for other
>> apps.
>
> Sigh.
>
> So I wanted to pull this thread up again, because when I originally
> proposed upstreaming the PR_SET_TIMERSLACK_PID feature from the AOSP
> common.git tree, the first objection from Arjan was that it only
> required CAP_SYS_NICE:
>    http://lkml.iu.edu/hypermail/linux/kernel/1506.3/01491.html
>
> And reasonably, setting timerslack to very large values does have the
> potential to effect applications much further then what a task could
> do previously with CAP_SYS_NICE.
>
> CAP_SYS_PTRACE was suggested instead, as that allows applications to
> manipulate other tasks more drastically.
>
> (At the time, I checked with some of the Android developers, and got
> no objection to changing to use this capability.)
>
> However, after submitting the changes to Android required to support
> the upstreamed /proc/<tid>/timerslack_ns interface, I've gotten some
> objections with adding CAP_SYS_PTRACE to the system_server, as this
> would allow the system_server to be able to inspect and modify memory
> on any task in the system. This gives the system_server privileged to
> effect applications much further then what it could do previously.
>
> So I worry I'm a bit stuck here. For general systems, CAP_SYS_NICE is
> too low a level of privilege  to set a tasks timerslack, but
> apparently CAP_SYS_PTRACE is too high a privilege for Android's
> system_server to require just to set a tasks timerslack value.
>
> So I wanted to ask again if we might consider backing this down to
> CAP_SYS_NICE, or if we can instead introduce a new CAP_SYS_TIMERSLACK
> or something to provide the needed in-between capability level.

Adding new capabilities appears to not really be viable (lots of
threads about this...)

I think the original CAP_SYS_NICE should be fine. A malicious
CAP_SYS_NICE process can do plenty of insane things, I don't feel like
the timer slack adds to any realistic risks.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security