Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Julia Cartwright <julia@ni.com>
To:     linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Carsten Emde <C.Emde@osadl.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        John Kacur <jkacur@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Daniel Wagner <daniel.wagner@siemens.com>,
        tom.zanussi@linux.intel.com, Peter Zijlstra <peterz@infradead.org>,
        Gratian Crisan <gratian.crisan@ni.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        dvhart@infradead.org,
        syzbot 
        <bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75765@syzkaller.appspotmail.com>,
        syzkaller-bugs@googlegroups.com, stable@vger.kernel.org,
        Ingo Molnar <mingo@kernel.org>
Subject: [PATCH RT 02/22] futex: Fix more put_pi_state() vs. exit_pi_state_list() races
Date:   Mon,  6 Aug 2018 04:17:18 -0500
Message-Id: <31e791651f8d58a70490d8a6bb0809521f64ca6e.1533540554.git.julia@ni.com>
In-Reply-To: <cover.1533540554.git.julia@ni.com>
References: <cover.1533540554.git.julia@ni.com>
MIME-Version: 1.0
Content-Type: text/plain
Received-SPF: None (protection.outlook.com: ni.com does not designate
 permitted sender hosts)
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 06 Aug 2018 09:17:42.1041 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: f000c027-8b01-4a3c-27c4-08d5fb7d7691
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 87ba1f9a-44cd-43a6-b008-6fdb45a5204e
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY4PR04MB0966
X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-08-06_05:,,
 signatures=0
X-Proofpoint-Spam-Details: rule=inbound_policy_notspam policy=inbound_policy score=30
 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0
 spamscore=0 clxscore=1011 lowpriorityscore=0 mlxscore=0 impostorscore=0
 mlxlogscore=999 adultscore=0 classifier=spam adjust=30 reason=mlx
 scancount=1 engine=8.0.1-1807170000 definitions=main-1808060100
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

From: Peter Zijlstra <peterz@infradead.org>

4.9.115-rt94-rc1 stable review patch.
If you have any objection to the inclusion of this patch, let me know.

--- 8< --- 8< --- 8< ---
[ Upstream commit 51d00899f7e6ded15c89cb4e2cb11a35283bac81 ]

Dmitry (through syzbot) reported being able to trigger the WARN in
get_pi_state() and a use-after-free on:

	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

Both are due to this race:

  exit_pi_state_list()				put_pi_state()

  lock(&curr->pi_lock)
  while() {
	pi_state = list_first_entry(head);
	hb = hash_futex(&pi_state->key);
	unlock(&curr->pi_lock);

						dec_and_test(&pi_state->refcount);

	lock(&hb->lock)
	lock(&pi_state->pi_mutex.wait_lock)	// uaf if pi_state free'd
	lock(&curr->pi_lock);

	....

	unlock(&curr->pi_lock);
	get_pi_state();				// WARN; refcount==0

The problem is we take the reference count too late, and don't allow it
being 0. Fix it by using inc_not_zero() and simply retrying the loop
when we fail to get a refcount. In that case put_pi_state() should
remove the entry from the list.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Gratian Crisan <gratian.crisan@ni.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: dvhart@infradead.org
Cc: syzbot <bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75765@syzkaller.appspotmail.com>
Cc: syzkaller-bugs@googlegroups.com
Cc: <stable@vger.kernel.org>
Fixes: c74aef2d06a9 ("futex: Fix pi_state->owner serialization")
Link: http://lkml.kernel.org/r/20171031101853.xpfh72y643kdfhjs@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Julia Cartwright <julia@ni.com>
---
 kernel/futex.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 47e42faad6c5..270148be5647 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -899,11 +899,27 @@ void exit_pi_state_list(struct task_struct *curr)
 	 */
 	raw_spin_lock_irq(&curr->pi_lock);
 	while (!list_empty(head)) {
-
 		next = head->next;
 		pi_state = list_entry(next, struct futex_pi_state, list);
 		key = pi_state->key;
 		hb = hash_futex(&key);
+
+		/*
+		 * We can race against put_pi_state() removing itself from the
+		 * list (a waiter going away). put_pi_state() will first
+		 * decrement the reference count and then modify the list, so
+		 * its possible to see the list entry but fail this reference
+		 * acquire.
+		 *
+		 * In that case; drop the locks to let put_pi_state() make
+		 * progress and retry the loop.
+		 */
+		if (!atomic_inc_not_zero(&pi_state->refcount)) {
+			raw_spin_unlock_irq(&curr->pi_lock);
+			cpu_relax();
+			raw_spin_lock_irq(&curr->pi_lock);
+			continue;
+		}
 		raw_spin_unlock_irq(&curr->pi_lock);
 
 		spin_lock(&hb->lock);
@@ -914,10 +930,12 @@ void exit_pi_state_list(struct task_struct *curr)
 		 * task still owns the PI-state:
 		 */
 		if (head->next != next) {
+			/* retain curr->pi_lock for the loop invariant */
 			raw_spin_unlock(&curr->pi_lock);
 			raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
 			spin_unlock(&hb->lock);
 			raw_spin_lock_irq(&curr->pi_lock);
+			put_pi_state(pi_state);
 			continue;
 		}
 
@@ -925,9 +943,8 @@ void exit_pi_state_list(struct task_struct *curr)
 		WARN_ON(list_empty(&pi_state->list));
 		list_del_init(&pi_state->list);
 		pi_state->owner = NULL;
-		raw_spin_unlock(&curr->pi_lock);
 
-		get_pi_state(pi_state);
+		raw_spin_unlock(&curr->pi_lock);
 		raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
 		spin_unlock(&hb->lock);
 
-- 
2.18.0