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

[Kees Cook]

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
specific use-case that wants to block current from these actions, we
can adjust the logic then.

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

>
> -Kees
>
>>
>>         task_lock(p);
>> @@ -2315,14 +2317,16 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
>>         if (!p)
>>                 return -ESRCH;
>>
>> -       if (!capable(CAP_SYS_NICE)) {
>> -               err = -EPERM;
>> -               goto out;
>> -       }
>> +       if (p != current) {
>>
>> -       err = security_task_getscheduler(p);
>> -       if (err)
>> -               goto out;
>> +               if (!capable(CAP_SYS_NICE)) {
>> +                       err = -EPERM;
>> +                       goto out;
>> +               }
>> +               err = security_task_getscheduler(p);
>> +               if (err)
>> +                       goto out;
>> +       }
>>
>>         task_lock(p);
>>         seq_printf(m, "%llu\n", p->timer_slack_ns);
>> --
>> 1.9.1
>>
>
>
>
> --
> Kees Cook
> Nexus Security



-- 
Kees Cook
Nexus Security