From patchwork Mon Mar 1 17:31:55 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ben Hutchings X-Patchwork-Id: 389904 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=-13.8 required=3.0 tests=BAYES_00, 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 17444C433DB for ; Mon, 1 Mar 2021 17:41:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D4538650BB for ; Mon, 1 Mar 2021 17:41:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232318AbhCARlA (ORCPT ); Mon, 1 Mar 2021 12:41:00 -0500 Received: from maynard.decadent.org.uk ([95.217.213.242]:32858 "EHLO maynard.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237892AbhCARco (ORCPT ); Mon, 1 Mar 2021 12:32:44 -0500 Received: from [2a02:1811:d34:3700:3b8d:b310:d327:e418] (helo=deadeye) by maynard with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1lGmOi-0002ae-Sy; Mon, 01 Mar 2021 18:31:56 +0100 Received: from ben by deadeye with local (Exim 4.94) (envelope-from ) id 1lGmOh-004iYH-Rj; Mon, 01 Mar 2021 18:31:55 +0100 Date: Mon, 1 Mar 2021 18:31:55 +0100 From: Ben Hutchings To: stable@vger.kernel.org Cc: Lee Jones , "Luis Claudio R. Goncalves" Subject: [PATCH 4.9 6/7] futex: Fix more put_pi_state() vs. exit_pi_state_list() races Message-ID: References: MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-SA-Exim-Connect-IP: 2a02:1811:d34:3700:3b8d:b310:d327:e418 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on maynard); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Peter Zijlstra commit 153fbd1226fb30b8630802aa5047b8af5ef53c9f upstream. 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 Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Thomas Gleixner Cc: Gratian Crisan Cc: Linus Torvalds Cc: Peter Zijlstra Cc: dvhart@infradead.org Cc: syzbot Cc: syzkaller-bugs@googlegroups.com Cc: 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 Signed-off-by: Ben Hutchings --- kernel/futex.c | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index a07f6080c8b0..855dae277f83 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -941,11 +941,27 @@ static 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); @@ -956,8 +972,10 @@ static 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(&pi_state->pi_mutex.wait_lock); spin_unlock(&hb->lock); + put_pi_state(pi_state); continue; } @@ -965,9 +983,8 @@ static 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);