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

[John Stultz]

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
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