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

[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.

For ABI preservation, it still allows CAP_SYS_PTRACE tasks to
access/modify timerslack values, but I'm fine with removing
this if others agree.

Cc: Kees Cook <keescook@chromium.org>
Cc: "Serge E. 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>
---
 fs/proc/base.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index a11eb71..d32033e 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2281,7 +2281,8 @@ 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)) {
+	if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS) ||
+	    capable(CAP_SYS_NICE)) {
 		task_lock(p);
 		if (slack_ns == 0)
 			p->timer_slack_ns = p->default_timer_slack_ns;
@@ -2306,7 +2307,8 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
 	if (!p)
 		return -ESRCH;
 
-	if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)) {
+	if (ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS) ||
+	    capable(CAP_SYS_NICE)) {
 		task_lock(p);
 		seq_printf(m, "%llu\n", p->timer_slack_ns);
 		task_unlock(p);
-- 
1.9.1