linux-kernel - [RFC][PATCH] proc: Fix timerslack_ns CAP_SYS

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Message-Id: <1470185661-15655-1-git-send-email-john.stultz@linaro.org>
Date:	Tue,  2 Aug 2016 17:54:21 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	lkml <linux-kernel@...r.kernel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	Kees Cook <keescook@...omium.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Oren Laadan <orenl@...lrox.com>,
	Ruchi Kandoi <kandoiruchi@...gle.com>,
	Rom Lemarchand <romlem@...roid.com>,
	Todd Kjos <tkjos@...gle.com>, Colin Cross <ccross@...roid.com>,
	Nick Kralevich <nnk@...gle.com>,
	Dmitry Shmidt <dimitrysh@...gle.com>,
	Elliott Hughes <enh@...gle.com>,
	Android Kernel Team <kernel-team@...roid.com>
Subject: [RFC][PATCH] 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
privledges 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

Cc: Kees Cook <keescook@...omium.org>
Cc: "Serge E. Hallyn" <serge@...lyn.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
CC: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Oren Laadan <orenl@...lrox.com>
Cc: Ruchi Kandoi <kandoiruchi@...gle.com>
Cc: Rom Lemarchand <romlem@...roid.com>
Cc: Todd Kjos <tkjos@...gle.com>
Cc: Colin Cross <ccross@...roid.com>
Cc: Nick Kralevich <nnk@...gle.com>
Cc: Dmitry Shmidt <dimitrysh@...gle.com>
Cc: Elliott Hughes <enh@...gle.com>
Cc: Android Kernel Team <kernel-team@...roid.com>
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
 fs/proc/base.c | 34 +++++++++++++++++++---------------
 1 file changed, 19 insertions(+), 15 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 02f8389..01c3c2d 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2281,15 +2281,17 @@ 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);
@@ -2315,14 +2317,16 @@ 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);
-- 
1.9.1