proc: fix timerslack_ns CAP_SYS_NICE check when adjusting self

In changing from checking ptrace_may_access(p, PTRACE_MODE_ATTACH_FSCREDS)
to capable(CAP_SYS_NICE), I missed that ptrace_my_access succeeds when p
== current, but the CAP_SYS_NICE doesn't.

Thus while the previous commit was intended to loosen the needed
privileges to modify a processes timerslack, it needlessly restricted a
task modifying its own timerslack via the proc/<tid>/timerslack_ns
(which is permitted also via the PR_SET_TIMERSLACK method).

This patch corrects this by checking if p == current before checking the
CAP_SYS_NICE value.

This patch applies on top of my two previous patches currently in -mm

Link: http://lkml.kernel.org/r/1471906870-28624-1-git-send-email-john.stultz@linaro.orgSigned-off-by: John Stultz <john.stultz@linaro.org>
Acked-by: 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

@@ -2280,6 +2280,7 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
 	if (!p)
 		return -ESRCH;
 
+	if (p != current) {
 		if (!capable(CAP_SYS_NICE)) {
 			count = -EPERM;
 			goto out;
@@ -2290,6 +2291,7 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
 			count = err;
 			goto out;
 		}
+	}
 
 	task_lock(p);
 	if (slack_ns == 0)
@@ -2314,14 +2316,16 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
 	if (!p)
 		return -ESRCH;
 
+	if (p != current) {
+
 		if (!capable(CAP_SYS_NICE)) {
 			err = -EPERM;
 			goto out;
 		}
-
 		err = security_task_getscheduler(p);
 		if (err)
 			goto out;
+	}
 
 	task_lock(p);
 	seq_printf(m, "%llu\n", p->timer_slack_ns);