proc: relax /proc/<tid>/timerslack_ns capability requirements

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.

Link: http://lkml.kernel.org/r/1469132667-17377-1-git-send-email-john.stultz@linaro.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Reviewed-by: Nick Kralevich <nnk@google.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -2280,16 +2280,19 @@ 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;
@@ -2299,19 +2302,22 @@ 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;