FROMLIST: 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.

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>
Reviewed-by: Nick Kralevich <nnk@google.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
FROMLIST URL: https://lkml.org/lkml/2016/7/21/522
Change-Id: Ia75481402e3948165a1b7c1551c539530cb25509
(Cherry picked against common/android-4.4)
Signed-off-by: John Stultz <john.stultz@linaro.org>

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 67b6d7e2313f3d3b3dd9e19a032affcb93433df3..f9600828c2e013af0615727b7d319769687a564b 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2261,16 +2261,19 @@ 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;
@@ -2280,19 +2283,22 @@ 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;