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From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Waiman Long <longman@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Davidlohr Bueso <dave@stgolabs.net>
Subject: [patch 1/2] locking/rtmutex: Dont dereference waiter lockless
Date: Wed, 25 Aug 2021 12:33:12 +0200 (CEST)	[thread overview]
Message-ID: <20210825102453.981720644@linutronix.de> (raw)
In-Reply-To: <20210825101857.420032248@linutronix.de>

The new rt_mutex_spin_on_onwer() loop checks whether the spinning waiter is
still the top waiter on the lock by utilizing rt_mutex_top_waiter(), which
is broken because that function contains a sanity check which dereferences
the top waiter pointer to check whether the waiter belongs to the
lock. That's wrong in the lockless spinwait case:

 CPU 0							CPU 1
 rt_mutex_lock(lock)					rt_mutex_lock(lock);
   queue(waiter0)
   waiter0 == rt_mutex_top_waiter(lock)
   rt_mutex_spin_on_onwer(lock, waiter0) {		queue(waiter1)
   					 		waiter1 == rt_mutex_top_waiter(lock)
   							...
     top_waiter = rt_mutex_top_waiter(lock)
       leftmost = rb_first_cached(&lock->waiters);
							-> signal
							dequeue(waiter1)
							destroy(waiter1)
       w = rb_entry(leftmost, ....)
       BUG_ON(w->lock != lock)	 <- UAF

The BUG_ON() is correct for the case where the caller holds lock->wait_lock
which guarantees that the leftmost waiter entry cannot vanish. For the
lockless spinwait case it's broken.

Create a new helper function which avoids the pointer dereference and just
compares the leftmost entry pointer with current's waiter pointer to
validate that currrent is still elegible for spinning.

Fixes: 992caf7f1724 ("locking/rtmutex: Add adaptive spinwait mechanism")
Reported-by: Sebastian Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/locking/rtmutex.c        |    5 +++--
 kernel/locking/rtmutex_common.h |   13 +++++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1329,8 +1329,9 @@ static bool rtmutex_spin_on_owner(struct
 		 *    for CONFIG_PREEMPT_RCU=y)
 		 *  - the VCPU on which owner runs is preempted
 		 */
-		if (!owner->on_cpu || waiter != rt_mutex_top_waiter(lock) ||
-		    need_resched() || vcpu_is_preempted(task_cpu(owner))) {
+		if (!owner->on_cpu || need_resched() ||
+		    rt_mutex_waiter_is_top_waiter(lock, waiter) ||
+		    vcpu_is_preempted(task_cpu(owner))) {
 			res = false;
 			break;
 		}
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -95,6 +95,19 @@ static inline int rt_mutex_has_waiters(s
 	return !RB_EMPTY_ROOT(&lock->waiters.rb_root);
 }
 
+/*
+ * Lockless speculative check whether @waiter is still the top waiter on
+ * @lock. This is solely comparing pointers and not derefencing the
+ * leftmost entry which might be about to vanish.
+ */
+static inline bool rt_mutex_waiter_is_top_waiter(struct rt_mutex_base *lock,
+						 struct rt_mutex_waiter *waiter)
+{
+	struct rb_node *leftmost = rb_first_cached(&lock->waiters);
+
+	return rb_entry(leftmost, struct rt_mutex_waiter, tree_entry) == waiter;
+}
+
 static inline struct rt_mutex_waiter *rt_mutex_top_waiter(struct rt_mutex_base *lock)
 {
 	struct rb_node *leftmost = rb_first_cached(&lock->waiters);


  reply	other threads:[~2021-08-25 10:33 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-25 10:33 [patch 0/2] locking/rtmutex: Cure two subtle bugs Thomas Gleixner
2021-08-25 10:33 ` Thomas Gleixner [this message]
2021-08-25 14:17   ` [tip: locking/core] locking/rtmutex: Dont dereference waiter lockless tip-bot2 for Thomas Gleixner
2021-08-25 10:33 ` [patch 2/2] locking/rtmutex: Dequeue waiter on ww_mutex deadlock Thomas Gleixner
2021-08-25 14:17   ` [tip: locking/core] " tip-bot2 for Thomas Gleixner
2021-08-25 11:40 ` [patch 0/2] locking/rtmutex: Cure two subtle bugs Peter Zijlstra
2021-08-26 13:26 ` Peter Zijlstra
2021-08-27  7:56   ` Sebastian Andrzej Siewior
2021-08-27 12:31   ` [tip: locking/core] locking/rtmutex: Return success on deadlock for ww_mutex waiters tip-bot2 for Peter Zijlstra
2021-08-27 12:31   ` [tip: locking/core] locking/rtmutex: Prevent spurious EDEADLK return caused by ww_mutexes tip-bot2 for Peter Zijlstra

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