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 1AD4EEB64D9 for ; Sun, 2 Jul 2023 19:46:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232948AbjGBTqy (ORCPT ); Sun, 2 Jul 2023 15:46:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59156 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232686AbjGBTpT (ORCPT ); Sun, 2 Jul 2023 15:45:19 -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 BAC5E3A89; Sun, 2 Jul 2023 12:42:31 -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 740AB60CF9; Sun, 2 Jul 2023 19:42:30 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0475EC433C7; Sun, 2 Jul 2023 19:42:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1688326949; bh=Ia4fmT+Si/PFwr2soXt58gSzgoBhlcuIetIqaM4LkSk=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=AOuwYpAKnlb6QBSgLG0+QZbtYIi0YAqvJRrBoiGUvMLARQ4uw1Nk4ioD8v2ORDXuc aGoz17FvsF1xMOA8pB5fKulZcigEoRAFj8mxBamil7FBullW8OD/H5MjwQ3hWmvKq9 Wxi5SBgNA9wl4Ed1syGpoKZ1mpLvHnFFz1AOINehXsACIP5EdutTFs3ottLvrp09l9 uNFvCA/vtRCa8khHQ+c8IrAZJJ6Xa7/EWzgMWQR8t/TmvYFR6CW21z5/zj2jsneLpi V+faKVQX0+m8UHTjX0GYfLTwltFgKZWSFeQTFfM8gSiTT8DhinyF2NAQCcY35Tm0/F IkDxy5ju7pq/w== 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 4.19 5/5] posix-timers: Ensure timer ID search-loop limit is valid Date: Sun, 2 Jul 2023 15:42:19 -0400 Message-Id: <20230702194219.1779408-5-sashal@kernel.org> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230702194219.1779408-1-sashal@kernel.org> References: <20230702194219.1779408-1-sashal@kernel.org> MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore X-stable-base: Linux 4.19.288 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 660d78c9af6c8..6a55b30ae742b 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -127,7 +127,7 @@ 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 1234868b3b03e..8768ce2c4bf52 100644 --- a/kernel/time/posix-timers.c +++ b/kernel/time/posix-timers.c @@ -159,25 +159,30 @@ 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