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 C3557C4321E for ; Tue, 29 Nov 2022 14:50:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235772AbiK2OuS (ORCPT ); Tue, 29 Nov 2022 09:50:18 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52700 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235809AbiK2Otx (ORCPT ); Tue, 29 Nov 2022 09:49:53 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E40C2FCA; Tue, 29 Nov 2022 06:48:35 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 8339061769; Tue, 29 Nov 2022 14:48:35 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C6380C433C1; Tue, 29 Nov 2022 14:48:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1669733314; bh=fCPTOuDQLVzzoeN7L8h0yP7y2/WG+kDgXWk9SkG16OE=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=WJS5xOLAJ8HrucLZMjrHmPPhEKfC05q6EnqK2V0A7jtDrml2YZf6wzEfYyXhqXVrY vd2YBRqRwxOFXQxvqLQgFiimWjiY3yZ+OVG95mf5ZaAOLhDxGtilpQXNAWkSS5Cyxg QA3Bv4XYigIqr/nHZdAkaKjy4i+n0fzF/L9wdpOn2LNd9quZNGMXeR/LsNd51rX3Pt u8cSi3vfheMcdsbCFdftOsCDAEU2tYxfSxgdjW6x0D1Op+cGvrL6Th7WZX9CnUiOzg 6nxtWheYL2nCEeiZqUXTtMm4nHky7iCtbe5eMMbXhMQosMh8qnCF2IL2IEQdOLPYTN 1c8rmqzZbtmcA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 69D105C03A4; Tue, 29 Nov 2022 06:48:34 -0800 (PST) Date: Tue, 29 Nov 2022 06:48:34 -0800 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: LKML , "Eric W . Biederman" , Neeraj Upadhyay , Oleg Nesterov , Pengfei Xu , Boqun Feng , Lai Jiangshan , rcu@vger.kernel.org Subject: Re: [PATCH 0/3] rcu-tasks: Fix race against exiting pid_ns Message-ID: <20221129144834.GW4001@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20221125135500.1653800-1-frederic@kernel.org> <20221129002240.GP4001@paulmck-ThinkPad-P17-Gen-1> <20221129095500.GA1706373@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221129095500.GA1706373@lothringen> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 29, 2022 at 10:55:00AM +0100, Frederic Weisbecker wrote: > On Mon, Nov 28, 2022 at 04:22:40PM -0800, Paul E. McKenney wrote: > > On Fri, Nov 25, 2022 at 02:54:57PM +0100, Frederic Weisbecker wrote: > > > Pengfei Xu has reported a deadlock involving calls to unshare(), > > > perf_event_open() and clone3() calls. It requires CAP_SYS_ADMIN > > > to reproduce (at least I don't see a way for a non privilege process to > > > reproduce). > > > > > > See this thread for details: https://lore.kernel.org/all/Y3sOgrOmMQqPMItu@xpf.sh.intel.com/ > > > And this document for the collaborative analysis with Boqun, Paul and Neeraj: > > > https://docs.google.com/document/d/1hJxgiZ5TMZ4YJkdJPLAkRvq7sYQ-A7svgA8no6i-v8k > > > > > > The two first patches are small improvements. The fix is in the last patch. > > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git > > > rcu/dev > > > > > > HEAD: 45ef5a0a4be4e0db9eadcc86e8f346d34c62e744 > > > > Hearing no objections, queued for further review and testing. > > > > And thank you very much! That race between synchronize_rcu_tasks() and > > zap_pid_ns_processes() certainly was more than a bit on the non-trivial > > side. Good show!!! > > Thanks! > > Also please replace the last patch with the following to fix > a !CONFIG_RCU_TASKS issue: Like this? ;-) a0c355bbdfee ("squash! rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes()") Thanx, Paul > --- > From: Frederic Weisbecker > Date: Thu, 24 Nov 2022 18:15:46 +0100 > Subject: [PATCH] rcu-tasks: Fix synchronize_rcu_tasks() VS > zap_pid_ns_processes() > > RCU Tasks and PID-namespace unshare can interact in do_exit() in a > complicated circular dependency: > > 1) TASK A calls unshare(CLONE_NEWPID), this creates a new PID namespace > that every subsequent child of TASK A will belong to. But TASK A > doesn't itself belong to that new PID namespace. > > 2) TASK A forks() and creates TASK B. TASK A stays attached to its PID > namespace (let's say PID_NS1) and TASK B is the first task belonging > to the new PID namespace created by unshare() (let's call it PID_NS2). > > 3) Since TASK B is the first task attached to PID_NS2, it becomes the > PID_NS2 child reaper. > > 4) TASK A forks() again and creates TASK C which get attached to PID_NS2. > Note how TASK C has TASK A as a parent (belonging to PID_NS1) but has > TASK B (belonging to PID_NS2) as a pid_namespace child_reaper. > > 5) TASK B exits and since it is the child reaper for PID_NS2, it has to > kill all other tasks attached to PID_NS2, and wait for all of them to > die before getting reaped itself (zap_pid_ns_process()). > > 6) TASK A calls synchronize_rcu_tasks() which leads to > synchronize_srcu(&tasks_rcu_exit_srcu). > > 7) TASK B is waiting for TASK C to get reaped. But TASK B is under a > tasks_rcu_exit_srcu SRCU critical section (exit_notify() is between > exit_tasks_rcu_start() and exit_tasks_rcu_finish()), blocking TASK A. > > 8) TASK C exits and since TASK A is its parent, it waits for it to reap > TASK C, but it can't because TASK A waits for TASK B that waits for > TASK C. > > Pid_namespace semantics can hardly be changed at this point. But the > coverage of tasks_rcu_exit_srcu can be reduced instead. > > The current task is assumed not to be concurrently reapable at this > stage of exit_notify() and therefore tasks_rcu_exit_srcu can be > temporarily relaxed without breaking its constraints, providing a way > out of the deadlock scenario. > > Fixes: 3f95aa81d265 ("rcu: Make TASKS_RCU handle tasks that are almost done exiting") > Reported-by: Pengfei Xu > Suggested-by: Boqun Feng > Suggested-by: Neeraj Upadhyay > Suggested-by: Paul E. McKenney > Cc: Oleg Nesterov > Cc: Lai Jiangshan > Cc: Eric W . Biederman > Signed-off-by: Frederic Weisbecker > --- > include/linux/rcupdate.h | 2 ++ > kernel/pid_namespace.c | 17 +++++++++++++++++ > kernel/rcu/tasks.h | 15 +++++++++++++-- > 3 files changed, 32 insertions(+), 2 deletions(-) > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index 89b3036746d2..a19d91d5461c 100644 > --- a/include/linux/rcupdate.h > +++ b/include/linux/rcupdate.h > @@ -238,6 +238,7 @@ void synchronize_rcu_tasks_rude(void); > > #define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t, false) > void exit_tasks_rcu_start(void); > +void exit_tasks_rcu_stop(void); > void exit_tasks_rcu_finish(void); > #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */ > #define rcu_tasks_classic_qs(t, preempt) do { } while (0) > @@ -246,6 +247,7 @@ void exit_tasks_rcu_finish(void); > #define call_rcu_tasks call_rcu > #define synchronize_rcu_tasks synchronize_rcu > static inline void exit_tasks_rcu_start(void) { } > +static inline void exit_tasks_rcu_stop(void) { } > static inline void exit_tasks_rcu_finish(void) { } > #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */ > > diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c > index f4f8cb0435b4..fc21c5d5fd5d 100644 > --- a/kernel/pid_namespace.c > +++ b/kernel/pid_namespace.c > @@ -244,7 +244,24 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns) > set_current_state(TASK_INTERRUPTIBLE); > if (pid_ns->pid_allocated == init_pids) > break; > + /* > + * Release tasks_rcu_exit_srcu to avoid following deadlock: > + * > + * 1) TASK A unshare(CLONE_NEWPID) > + * 2) TASK A fork() twice -> TASK B (child reaper for new ns) > + * and TASK C > + * 3) TASK B exits, kills TASK C, waits for TASK A to reap it > + * 4) TASK A calls synchronize_rcu_tasks() > + * -> synchronize_srcu(tasks_rcu_exit_srcu) > + * 5) *DEADLOCK* > + * > + * It is considered safe to release tasks_rcu_exit_srcu here > + * because we assume the current task can not be concurrently > + * reaped at this point. > + */ > + exit_tasks_rcu_stop(); > schedule(); > + exit_tasks_rcu_start(); > } > __set_current_state(TASK_RUNNING); > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h > index 9a8114114b48..4dda8e6e5707 100644 > --- a/kernel/rcu/tasks.h > +++ b/kernel/rcu/tasks.h > @@ -1016,16 +1016,27 @@ void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu) > * task is exiting and may be removed from the tasklist. See > * corresponding synchronize_srcu() for further details. > */ > -void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu) > +void exit_tasks_rcu_stop(void) __releases(&tasks_rcu_exit_srcu) > { > struct task_struct *t = current; > > __srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx); > - exit_tasks_rcu_finish_trace(t); > +} > + > +/* > + * Contribute to protect against tasklist scan blind spot while the > + * task is exiting and may be removed from the tasklist. See > + * corresponding synchronize_srcu() for further details. > + */ > +void exit_tasks_rcu_finish(void) > +{ > + exit_tasks_rcu_stop(); > + exit_tasks_rcu_finish_trace(current); > } > > #else /* #ifdef CONFIG_TASKS_RCU */ > void exit_tasks_rcu_start(void) { } > +void exit_tasks_rcu_stop(void) { } > void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); } > #endif /* #else #ifdef CONFIG_TASKS_RCU */ > > -- > 2.25.1 >