On Wed, Jul 13, 2016 at 4:47 PM, John Stultz <john.stu...@linaro.org> wrote: > On Tue, Feb 16, 2016 at 5:06 PM, John Stultz <john.stu...@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
