From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 84BC1C4338F for ; Wed, 25 Aug 2021 10:33:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6501261181 for ; Wed, 25 Aug 2021 10:33:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239770AbhHYKeA (ORCPT ); Wed, 25 Aug 2021 06:34:00 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:53402 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237180AbhHYKd7 (ORCPT ); Wed, 25 Aug 2021 06:33:59 -0400 Message-ID: <20210825102453.981720644@linutronix.de> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1629887593; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: references:references; bh=xZVC8GIfMUdVYAKKo0GJV44oAzqzVytz7bZY0+FiN58=; b=UZCp1w0n3uxCKkUzoNRYlx/9FWn7XheOaQoOf3pPBLj7rxTAtbnWZAxieuWtrdP+Q6G6+z 34pZ6QToYyLZBur430osrEJjuLRHi4uHFzGoEAOghWUbku4QMb2OimqiDdxn3ILzTn4IEM fV5+ne4PR6ehwtpKZoXn2RuPJroGtkfliRkfqUbJVgjP02jjYe5sbp+fEkGgcmY6f8xUUd pAQ+kPOdPQDN7OBPQPbKhmAzyvEgoGLmMbnVyoP1doPBbsH7Rfevv+0QCAxtB5s1yreTm5 HF/xHywVklSw48tfgIuw42Ckk03kb4hsG0AMo7HW3hFI6PxraY8HRB9tLSohrw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1629887593; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: references:references; bh=xZVC8GIfMUdVYAKKo0GJV44oAzqzVytz7bZY0+FiN58=; b=+yWURWkcwi1rQ0orcO2p1JpEc9pTS/sWsXAUalR3k7E/SWsVx/TgqIotrGjorfu/iHZv04 b7XBiDtv5Jhz9kCw== From: Thomas Gleixner To: LKML Cc: Peter Zijlstra , Ingo Molnar , Steven Rostedt , Waiman Long , Sebastian Andrzej Siewior , Davidlohr Bueso Subject: [patch 1/2] locking/rtmutex: Dont dereference waiter lockless References: <20210825101857.420032248@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-transfer-encoding: 8-bit Date: Wed, 25 Aug 2021 12:33:12 +0200 (CEST) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Signed-off-by: Thomas Gleixner --- 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);