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    SubjectRe: [PATCH 2/2] proc: Add /proc/<pid>/timerslack_ns interface
    On Wed, Jul 13, 2016 at 4:47 PM, John Stultz <john.stultz@linaro.org> wrote:
    > On Tue, Feb 16, 2016 at 5:06 PM, John Stultz <john.stultz@linaro.org> 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

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