Re: [RFC][PATCH 1/2 v2] proc: Relax /proc/<tid>/timerslack_ns capability requirements

[Nick Kralevich]

On Fri, Jul 15, 2016 at 10:24 AM, John Stultz <john.stultz@xxxxxxxxxx> 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 and removes CAP_SYS_PTRACE, simplifying some
> of the code flow as well.
>
> This is technically an ABI change, but as the feature just
> landed in 4.6, I suspect no one is yet using it.
>
> 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>
> ---
> v2: Removed CAP_SYS_PTRACE check and simplified code flow
>
>  fs/proc/base.c | 33 ++++++++++++++++-----------------
>  1 file changed, 16 insertions(+), 17 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index a11eb71..8f4f8d7 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2277,19 +2277,19 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
>         if (err < 0)
>                 return err;
>
> +       if (!capable(CAP_SYS_NICE))
> +               return -EPERM;
> +
>         p = get_proc_task(inode);
>         if (!p)
>                 return -ESRCH;

The capable(CAP_SYS_NICE) permission check should be moved to this
point, since it doesn't make sense to return EPERM if the task
structure doesn't exist.

>
> -       if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
> -               task_lock(p);
> -               if (slack_ns == 0)
> -                       p->timer_slack_ns = p->default_timer_slack_ns;
> -               else
> -                       p->timer_slack_ns = slack_ns;
> -               task_unlock(p);
> -       } else
> -               count = -EPERM;
> +       task_lock(p);
> +       if (slack_ns == 0)
> +               p->timer_slack_ns = p->default_timer_slack_ns;
> +       else
> +               p->timer_slack_ns = slack_ns;
> +       task_unlock(p);
>
>         put_task_struct(p);
>
> @@ -2300,22 +2300,21 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
>  {
>         struct inode *inode = m->private;
>         struct task_struct *p;
> -       int err =  0;
> +
> +       if (!capable(CAP_SYS_NICE))
> +               return -EPERM;

This should also have a similar LSM check for reads. For the SELinux
implementation, this can map to the PROCESS__GETSCHED permission.

  security/selinux/hooks.c:

  static int selinux_task_gettimerslack(struct task_struct *p)
  {
        return current_has_perm(p, PROCESS__GETSCHED);
  }

>
>         p = get_proc_task(inode);
>         if (!p)
>                 return -ESRCH;

As above, recommend moving the capable(CAP_SYS_NICE) check to this point.

>
> -       if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
> -               task_lock(p);
> -               seq_printf(m, "%llu\n", p->timer_slack_ns);
> -               task_unlock(p);
> -       } else
> -               err = -EPERM;
> +       task_lock(p);
> +       seq_printf(m, "%llu\n", p->timer_slack_ns);
> +       task_unlock(p);
>
>         put_task_struct(p);
>
> -       return err;
> +       return 0;
>  }
>
>  static int timerslack_ns_open(struct inode *inode, struct file *filp)
> --
> 1.9.1
>



-- 
Nick Kralevich | Android Security | nnk@xxxxxxxxxx | 650.214.4037