On Thu, Jul 14, 2016 at 11:50 AM, John Stultz <[email protected]> wrote: > When an interface to allow a task to change another tasks > timerslack was first proposed, it was suggested that something > greater then CAP_SYS_NICE would be needed, as a task could be > delayed further then what normally could be done with nice > adjustments. > > So CAP_SYS_PTRACE was adopted instead for what became the > /proc/<tid>/timerslack_ns interface. However, for Android (where > this feature originates), giving the system_server > CAP_SYS_PTRACE would allow it to observe and modify all tasks > memory. This is considered too high a privilege level for only > needing to change the timerslack. > > After some discussion, it was realized that a CAP_SYS_NICE > process can set a task as SCHED_FIFO, so they could fork some > spinning processes and set them all SCHED_FIFO 99, in effect > delaying all other tasks for an infinite amount of time. > > So as a CAP_SYS_NICE task can already cause trouble for other > tasks, using it as a required capability for accessing and > modifying /proc/<tid>/timerslack_ns seems sufficient. > > Thus, this patch loosens the capability requirements to > CAP_SYS_NICE. > > For ABI preservation, it still allows CAP_SYS_PTRACE tasks to > access/modify timerslack values, but I'm fine with removing > this if others agree.
Is anything actually using this ABI yet? (Regardless, I'm fine allowing both caps.) Reviewed-by: Kees Cook <[email protected]> -Kees > > Cc: Kees Cook <[email protected]> > Cc: "Serge E. Hallyn" <[email protected]> > Cc: Andrew Morton <[email protected]> > Cc: Thomas Gleixner <[email protected]> > CC: Arjan van de Ven <[email protected]> > Cc: Oren Laadan <[email protected]> > Cc: Ruchi Kandoi <[email protected]> > Cc: Rom Lemarchand <[email protected]> > Cc: Todd Kjos <[email protected]> > Cc: Colin Cross <[email protected]> > Cc: Nick Kralevich <[email protected]> > Cc: Dmitry Shmidt <[email protected]> > Cc: Elliott Hughes <[email protected]> > Cc: Android Kernel Team <[email protected]> > Signed-off-by: John Stultz <[email protected]> > --- > fs/proc/base.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/fs/proc/base.c b/fs/proc/base.c > index a11eb71..d32033e 100644 > --- a/fs/proc/base.c > +++ b/fs/proc/base.c > @@ -2281,7 +2281,8 @@ static ssize_t timerslack_ns_write(struct file *file, > const char __user *buf, > if (!p) > return -ESRCH; > > - if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) { > + if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS) || > + capable(CAP_SYS_NICE)) { > task_lock(p); > if (slack_ns == 0) > p->timer_slack_ns = p->default_timer_slack_ns; > @@ -2306,7 +2307,8 @@ static int timerslack_ns_show(struct seq_file *m, void > *v) > if (!p) > return -ESRCH; > > - if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) { > + if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS) || > + capable(CAP_SYS_NICE)) { > task_lock(p); > seq_printf(m, "%llu\n", p->timer_slack_ns); > task_unlock(p); > -- > 1.9.1 > -- Kees Cook Chrome OS & Brillo Security
