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.org
Signed-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>

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ebccdc1..dc7fe5f 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2280,15 +2280,17 @@ static ssize_t timerslack_ns_write(struct file *file, const char __user *buf,
        if (!p)
                return -ESRCH;
 
-       if (!capable(CAP_SYS_NICE)) {
-               count = -EPERM;
-               goto out;
-       }
+       if (p != current) {
+               if (!capable(CAP_SYS_NICE)) {
+                       count = -EPERM;
+                       goto out;
+               }
 
-       err = security_task_setscheduler(p);
-       if (err) {
-               count = err;
-               goto out;
+               err = security_task_setscheduler(p);
+               if (err) {
+                       count = err;
+                       goto out;
+               }
        }
 
        task_lock(p);
@@ -2314,14 +2316,16 @@ static int timerslack_ns_show(struct seq_file *m, void *v)
        if (!p)
                return -ESRCH;
 
-       if (!capable(CAP_SYS_NICE)) {
-               err = -EPERM;
-               goto out;
-       }
+       if (p != current) {
 
-       err = security_task_getscheduler(p);
-       if (err)
-               goto out;
+               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);