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 61746C433FE for ; Tue, 29 Nov 2022 16:06:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235715AbiK2QGp (ORCPT ); Tue, 29 Nov 2022 11:06:45 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60178 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235850AbiK2QGH (ORCPT ); Tue, 29 Nov 2022 11:06:07 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 25A1266C99 for ; Tue, 29 Nov 2022 08:03:44 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1669737823; 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: in-reply-to:in-reply-to:references:references; bh=0Gt3AIWb2ri6G3KKDanOT6IHhzcEIhTnTK93LHPt92E=; b=ajHvdcUcrhZ1ONMyVeO/iKIk3Lu2A/AYfjsISLLqGz5WPWgPTjHFzXQUbOeHsrmrrVczvy dhWBmMby5zqDCBHvOEjJODME9dVfoLZCEfBODLZnfga83AIemM/Vi92CIpzGD3T7uDZ5A/ PxLoQGaq/mxLBt4NtbWLLHvwYFVVIsM= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-212-ryLL5bSvPnmYfwHwl01TRA-1; Tue, 29 Nov 2022 11:03:39 -0500 X-MC-Unique: ryLL5bSvPnmYfwHwl01TRA-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 9E14A811E87; Tue, 29 Nov 2022 16:03:38 +0000 (UTC) Received: from [10.22.17.30] (unknown [10.22.17.30]) by smtp.corp.redhat.com (Postfix) with ESMTP id CC51C492B07; Tue, 29 Nov 2022 16:03:37 +0000 (UTC) Message-ID: <4e93019d-4b19-14f9-14d7-da43456d3546@redhat.com> Date: Tue, 29 Nov 2022 11:03:35 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.0 Subject: Re: [PATCH-tip v4] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr() Content-Language: en-US To: Will Deacon Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Phil Auld , Wenjie Li , =?UTF-8?B?RGF2aWQgV2FuZyDnjovmoIc=?= , linux-kernel@vger.kernel.org References: <20221125023943.1118603-1-longman@redhat.com> <92b99a5e-1588-4e08-a652-72e9c51421cf@redhat.com> <20221128120008.GA25090@willie-the-truck> <20221129140759.GA26437@willie-the-truck> <20221129155757.GC26561@willie-the-truck> From: Waiman Long In-Reply-To: <20221129155757.GC26561@willie-the-truck> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/29/22 10:57, Will Deacon wrote: > On Tue, Nov 29, 2022 at 10:32:49AM -0500, Waiman Long wrote: >> On 11/29/22 09:07, Will Deacon wrote: >>> On Mon, Nov 28, 2022 at 10:11:52AM -0500, Waiman Long wrote: >>>> On 11/28/22 07:00, Will Deacon wrote: >>>>> On Sun, Nov 27, 2022 at 08:43:27PM -0500, Waiman Long wrote: >>>>>> On 11/24/22 21:39, Waiman Long wrote: >>>>>>> In general, a non-null user_cpus_ptr will remain set until the task dies. >>>>>>> A possible exception to this is the fact that do_set_cpus_allowed() >>>>>>> will clear a non-null user_cpus_ptr. To allow this possible racing >>>>>>> condition, we need to check for NULL user_cpus_ptr under the pi_lock >>>>>>> before duping the user mask. >>>>>>> >>>>>>> Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()") >>>>>>> Signed-off-by: Waiman Long >>>>>> This is actually a pre-existing use-after-free bug since commit 07ec77a1d4e8 >>>>>> ("sched: Allow task CPU affinity to be restricted on asymmetric systems"). >>>>>> So it needs to be fixed in the stable release as well. Will resend the patch >>>>>> with an additional fixes tag and updated commit log. >>>>> Please can you elaborate on the use-after-free here? Looking at >>>>> 07ec77a1d4e8, the mask is only freed in free_task() when the usage refcount >>>>> has dropped to zero and I can't see how that can race with fork(). >>>>> >>>>> What am I missing? >>>> I missed that at first. The current task cloning process copies the content >>>> of the task structure over to the newly cloned/forked task. IOW, if >>>> user_cpus_ptr had been set up previously, it will be copied over to the >>>> cloned task. Now if user_cpus_ptr of the source task is cleared right after >>>> that and before dup_user_cpus_ptr() is called. The obsolete user_cpus_ptr >>>> value in the cloned task will remain and get used even if it has been freed. >>>> That is what I call as use-after-free and double-free. >>> If the parent task can be modified concurrently with dup_task_struct() then >>> surely we'd have bigger issues because that's not going to be atomic? At the >>> very least we'd have a data race, but it also feels like we could end up >>> with inconsistent task state in the child. In fact, couldn't the normal >>> 'cpus_mask' be corrupted by a concurrent set_cpus_allowed_common()? >>> >>> Or am I still failing to understand the race? >>> >> A major difference between cpus_mask and user_cpus_ptr is that for >> cpus_mask, the bitmap is embedded into task_struct whereas user_cpus_ptr is >> a pointer to an external bitmap. So there is no issue of use-after-free wrt >> cpus_mask. That is not the case where the memory of the user_cpus_ptr of the >> parent task is freed, but then a reference to that memory is still available >> in the child's task struct and may be used. > Sure, I'm not saying there's a UAF on cpus_mask, but I'm concerned that we > could corrupt the data and end up with an affinity mask that doesn't correspond > to anything meaningful. Do you agree that's possible? That is certainly possible. So we have to be careful about it. > >> Note that the problematic concurrence is not between the copying of task >> struct and changing of the task struct. It is what will happen after the >> task struct copying has already been done with an extra reference present in >> the child's task struct. > Well, sort of, but the child only has the extra reference _because_ the parent > pointer was concurrently cleared to NULL, otherwise dup_user_cpus_ptr() would > have allocated a new copy and we'd be ok, no? Yes, that is exactly where the problem is and this is what my patch is trying to fix. > > Overall, I'm just very wary that we seem to be saying that copy_process() > can run concurrently with changes to the parent. Maybe it's all been written > with that in mindi (including all the arch callbacks), but I'd be astonished > if this is the only problem in there. It seems like that, at least in some cases, the clearing of a task's user_cpus_ptr can be done by another task. So the parent may be unaware of it and so is not its fault. Cheers, Longman