linux-kernel - Re: [RFC][PATCH 1/2 v3] proc: Relax /proc/<tid>/timerslack

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Date:	Mon, 18 Jul 2016 13:41:45 -0700
From:	Kees Cook <keescook@...omium.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Oren Laadan <orenl@...lrox.com>,
	Ruchi Kandoi <kandoiruchi@...gle.com>,
	Rom Lemarchand <romlem@...roid.com>,
	Todd Kjos <tkjos@...gle.com>, Colin Cross <ccross@...roid.com>,
	Nick Kralevich <nnk@...gle.com>,
	Dmitry Shmidt <dimitrysh@...gle.com>,
	Elliott Hughes <enh@...gle.com>,
	Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [RFC][PATCH 1/2 v3] proc: Relax /proc/<tid>/timerslack_ns
 capability requirements

On Mon, Jul 18, 2016 at 1:11 PM, John Stultz <john.stultz@...aro.org> 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@...omium.org>
> Cc: "Serge E. Hallyn" <serge@...lyn.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> CC: Arjan van de Ven <arjan@...ux.intel.com>
> Cc: Oren Laadan <orenl@...lrox.com>
> Cc: Ruchi Kandoi <kandoiruchi@...gle.com>
> Cc: Rom Lemarchand <romlem@...roid.com>
> Cc: Todd Kjos <tkjos@...gle.com>
> Cc: Colin Cross <ccross@...roid.com>
> Cc: Nick Kralevich <nnk@...gle.com>
> Cc: Dmitry Shmidt <dimitrysh@...gle.com>
> Cc: Elliott Hughes <enh@...gle.com>
> Cc: Android Kernel Team <kernel-team@...roid.com>
> Signed-off-by: John Stultz <john.stultz@...aro.org>

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
> v2: Removed CAP_SYS_PTRACE check and simplified code flow
> v3: Tweaked where CAP_SYS_NICE check is made, suggeded by NickK
>
>  fs/proc/base.c | 34 ++++++++++++++++++++--------------
>  1 file changed, 20 insertions(+), 14 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index a11eb71..c94abae 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2281,16 +2281,19 @@ 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)) {
> -               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
> +       if (!capable(CAP_SYS_NICE)) {
>                 count = -EPERM;
> +               goto out;
> +       }
> +
> +       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);
>
> +out:
>         put_task_struct(p);
>
>         return count;
> @@ -2300,19 +2303,22 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
>  {
>         struct inode *inode = m->private;
>         struct task_struct *p;
> -       int err =  0;
> +       int err = 0;
>
>         p = get_proc_task(inode);
>         if (!p)
>                 return -ESRCH;
>
> -       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
> +       if (!capable(CAP_SYS_NICE)) {
>                 err = -EPERM;
> +               goto out;
> +       }
>
> +       task_lock(p);
> +       seq_printf(m, "%llu\n", p->timer_slack_ns);
> +       task_unlock(p);
> +
> +out:
>         put_task_struct(p);
>
>         return err;
> --
> 1.9.1
>



-- 
Kees Cook
Chrome OS & Brillo Security