Re: [PATCH] proc: Fix timerslack_ns CAP_SYS_NICE check when adjusting self

[John Stultz]

On Wed, Aug 10, 2016 at 2:02 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Wed, Aug 10, 2016 at 11:36 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Tue, Aug 9, 2016 at 4:54 PM, John Stultz <john.stultz@xxxxxxxxxx> 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 <keescook@xxxxxxxxxxxx>
>>> Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
>>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>>> CC: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
>>> Cc: Oren Laadan <orenl@xxxxxxxxxxx>
>>> Cc: Ruchi Kandoi <kandoiruchi@xxxxxxxxxx>
>>> Cc: Rom Lemarchand <romlem@xxxxxxxxxxx>
>>> Cc: Todd Kjos <tkjos@xxxxxxxxxx>
>>> Cc: Colin Cross <ccross@xxxxxxxxxxx>
>>> Cc: Nick Kralevich <nnk@xxxxxxxxxx>
>>> Cc: Dmitry Shmidt <dimitrysh@xxxxxxxxxx>
>>> Cc: Elliott Hughes <enh@xxxxxxxxxx>
>>> Cc: Android Kernel Team <kernel-team@xxxxxxxxxxx>
>>> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
>>> ---
>>>  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.

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.

thanks
-john