On Wed, Aug 10, 2016 at 2:12 PM, John Stultz <[email protected]> wrote: > On Wed, Aug 10, 2016 at 2:02 PM, Kees Cook <[email protected]> wrote: >> On Wed, Aug 10, 2016 at 11:36 AM, Kees Cook <[email protected]> wrote: >>> On Tue, Aug 9, 2016 at 4:54 PM, John Stultz <[email protected]> wrote: >>>> In changing from checking ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS) >>>> to capable(CAP_SYS_NICE), I missed that ptrace_my_access succeeds >>>> when p == current, but the CAP_SYS_NICE doesn't. >>>> >>>> Thus while the previous commit was intended to loosen the needed >>>> privledges to modify a processes timerslack, it needlessly restricted >>>> a task modifying its own timerslack via the proc/<tid>/timerslack_ns >>>> (which is permitted also via the PR_SET_TIMERSLACK method). >>>> >>>> This patch corrects this by checking if p == current before checking >>>> the CAP_SYS_NICE value. >>>> >>>> This patch applies on top of my two previous patches currently in -mm >>>> >>>> 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 | 34 +++++++++++++++++++--------------- >>>> 1 file changed, 19 insertions(+), 15 deletions(-) >>>> >>>> diff --git a/fs/proc/base.c b/fs/proc/base.c >>>> index 02f8389..01c3c2d 100644 >>>> --- a/fs/proc/base.c >>>> +++ b/fs/proc/base.c >>>> @@ -2281,15 +2281,17 @@ static ssize_t timerslack_ns_write(struct file >>>> *file, const char __user *buf, >>>> if (!p) >>>> return -ESRCH; >>>> >>>> - if (!capable(CAP_SYS_NICE)) { >>>> - count = -EPERM; >>>> - goto out; >>>> - } >>>> + if (p != current) { >>>> + if (!capable(CAP_SYS_NICE)) { >>>> + count = -EPERM; >>>> + goto out; >>>> + } >>>> >>>> - err = security_task_setscheduler(p); >>>> - if (err) { >>>> - count = err; >>>> - goto out; >>>> + err = security_task_setscheduler(p); >>>> + if (err) { >>>> + count = err; >>>> + goto out; >>>> + } >>>> } >>> >>> This entirely bypasses LSM when p == current. Is that intended? >> >> I take back my concern. :) I think this is correct (as you mention in >> the thread: the prctl LSM hook already fired), so until there is a > > But did it? The prctrl hook is just for the prctrl interface. The > proc/<tid>/timerslack_ns is separate.
Oh, hrm, well, I think I'm still fine with it as-is: if we end up needing to tighten this to block current, we can change it. Nick, would this be something you'd want to be able to do with the hook? -Kees > This is part of my confusion here, mostly in that I'm not really sure > I have a good sense of philosophy for LSM hooks. > Are these just interface guards/hooks, or are we trying to map the > hook to the underlying action being taken? > > As with the prctrl interface, it seems like its just an interface > guard, but the /proc/<tid>/timerslack_ns interface checking > security_task_setscheduler() seems to be more connected to the > underlying action being done by changing the timerslack value. -- Kees Cook Nexus Security
