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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 20C57EB64DA for ; Sun, 2 Jul 2023 19:43:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231778AbjGBTnV (ORCPT ); Sun, 2 Jul 2023 15:43:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52358 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231358AbjGBTmf (ORCPT ); Sun, 2 Jul 2023 15:42:35 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D92C7294D; Sun, 2 Jul 2023 12:41:44 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id B3EFB60C8C; Sun, 2 Jul 2023 19:40:47 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 58952C433C7; Sun, 2 Jul 2023 19:40:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1688326847; bh=TBuX7D5DT9ayEsecwGU5wss8qZIQldBK8gN7a6H8FTA=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=NQFXiVYaQVBgOFZSaPiXDhK6ZYrpAo6a9YXXzcAf6yzYYidJPsxDQ0InA6tSVf9jM 6C4IDitKBNIidqgDJgg1mYJpEIGXxcDSaac+Yi3NsjZxLSO37GAJMzv1B3rmtF6CIU 0jvA5tq980QHSplbpYxwxQEnXVcXF0B1QqHHPk0iv3Mg9nQnqwnPRFbdOW9spe6nzk TYE9k3Wiq856EhntBZsKHnMB7PWnx3U1aqeHykEssfgQYPKERuRKBAzGYnOhbZ6/i2 zbFgHJ/w2cZSv9KH4aaGOLdJy5h4AYOqTy1Hb/ujNv07rAI3lGIZ8zvvDAqs9oX0Gj g0Z01xKSfKTlg== From: Sasha Levin To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Thomas Gleixner , syzbot+5c54bd3eb218bb595aa9@syzkaller.appspotmail.com, Dmitry Vyukov , Frederic Weisbecker , Sasha Levin , ebiederm@xmission.com Subject: [PATCH AUTOSEL 6.4 11/15] posix-timers: Ensure timer ID search-loop limit is valid Date: Sun, 2 Jul 2023 15:40:16 -0400 Message-Id: <20230702194020.1776895-11-sashal@kernel.org> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230702194020.1776895-1-sashal@kernel.org> References: <20230702194020.1776895-1-sashal@kernel.org> MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore X-stable-base: Linux 6.4.1 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Thomas Gleixner [ Upstream commit 8ce8849dd1e78dadcee0ec9acbd259d239b7069f ] posix_timer_add() tries to allocate a posix timer ID by starting from the cached ID which was stored by the last successful allocation. This is done in a loop searching the ID space for a free slot one by one. The loop has to terminate when the search wrapped around to the starting point. But that's racy vs. establishing the starting point. That is read out lockless, which leads to the following problem: CPU0 CPU1 posix_timer_add() start = sig->posix_timer_id; lock(hash_lock); ... posix_timer_add() if (++sig->posix_timer_id < 0) start = sig->posix_timer_id; sig->posix_timer_id = 0; So CPU1 can observe a negative start value, i.e. -1, and the loop break never happens because the condition can never be true: if (sig->posix_timer_id == start) break; While this is unlikely to ever turn into an endless loop as the ID space is huge (INT_MAX), the racy read of the start value caught the attention of KCSAN and Dmitry unearthed that incorrectness. Rewrite it so that all id operations are under the hash lock. Reported-by: syzbot+5c54bd3eb218bb595aa9@syzkaller.appspotmail.com Reported-by: Dmitry Vyukov Signed-off-by: Thomas Gleixner Reviewed-by: Frederic Weisbecker Link: https://lore.kernel.org/r/87bkhzdn6g.ffs@tglx Signed-off-by: Sasha Levin --- include/linux/sched/signal.h | 2 +- kernel/time/posix-timers.c | 31 ++++++++++++++++++------------- 2 files changed, 19 insertions(+), 14 deletions(-) diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index 20099268fa257..669e8cff40c74 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -135,7 +135,7 @@ struct signal_struct { #ifdef CONFIG_POSIX_TIMERS /* POSIX.1b Interval Timers */ - int posix_timer_id; + unsigned int next_posix_timer_id; struct list_head posix_timers; /* ITIMER_REAL timer for the process */ diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c index 808a247205a9a..4431aecb8b12c 100644 --- a/kernel/time/posix-timers.c +++ b/kernel/time/posix-timers.c @@ -140,25 +140,30 @@ static struct k_itimer *posix_timer_by_id(timer_t id) static int posix_timer_add(struct k_itimer *timer) { struct signal_struct *sig = current->signal; - int first_free_id = sig->posix_timer_id; struct hlist_head *head; - int ret = -ENOENT; + unsigned int cnt, id; - do { + /* + * FIXME: Replace this by a per signal struct xarray once there is + * a plan to handle the resulting CRIU regression gracefully. + */ + for (cnt = 0; cnt <= INT_MAX; cnt++) { spin_lock(&hash_lock); - head = &posix_timers_hashtable[hash(sig, sig->posix_timer_id)]; - if (!__posix_timers_find(head, sig, sig->posix_timer_id)) { + id = sig->next_posix_timer_id; + + /* Write the next ID back. Clamp it to the positive space */ + sig->next_posix_timer_id = (id + 1) & INT_MAX; + + head = &posix_timers_hashtable[hash(sig, id)]; + if (!__posix_timers_find(head, sig, id)) { hlist_add_head_rcu(&timer->t_hash, head); - ret = sig->posix_timer_id; + spin_unlock(&hash_lock); + return id; } - if (++sig->posix_timer_id < 0) - sig->posix_timer_id = 0; - if ((sig->posix_timer_id == first_free_id) && (ret == -ENOENT)) - /* Loop over all possible ids completed */ - ret = -EAGAIN; spin_unlock(&hash_lock); - } while (ret == -ENOENT); - return ret; + } + /* POSIX return code when no timer ID could be allocated */ + return -EAGAIN; } static inline void unlock_timer(struct k_itimer *timr, unsigned long flags) -- 2.39.2