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

[Serge E. Hallyn]

Quoting Kees Cook (keescook@xxxxxxxxxxxx):
> 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...)

Sorry - why is 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.

Can someone give a detailed explanation of what you could do with
the new timerslack feature and compare it to what you can do with
sys_nice?