Re: [RFC][PATCH 1/2 v3] proc: Relax /proc/<tid>/timerslack_ns capability requirements

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting John Stultz (john.stultz@linaro.org):
> 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@chromium.org>
> Cc: "Serge E. Hallyn" <serge@hallyn.com>

Acked-by: Serge Hallyn <serge@hallyn.com>

> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> CC: Arjan van de Ven <arjan@linux.intel.com>
> Cc: Oren Laadan <orenl@cellrox.com>
> Cc: Ruchi Kandoi <kandoiruchi@google.com>
> Cc: Rom Lemarchand <romlem@android.com>
> Cc: Todd Kjos <tkjos@google.com>
> Cc: Colin Cross <ccross@android.com>
> Cc: Nick Kralevich <nnk@google.com>
> Cc: Dmitry Shmidt <dimitrysh@google.com>
> Cc: Elliott Hughes <enh@google.com>
> Cc: Android Kernel Team <kernel-team@android.com>
> Signed-off-by: John Stultz <john.stultz@linaro.org>
> ---
> 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