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 7D5EBC433FE for ; Mon, 28 Nov 2022 15:12:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231223AbiK1PM4 (ORCPT ); Mon, 28 Nov 2022 10:12:56 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54910 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229670AbiK1PMv (ORCPT ); Mon, 28 Nov 2022 10:12:51 -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 E360620986 for ; Mon, 28 Nov 2022 07:12:00 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1669648319; 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=i+TN9hP6pZ4DRKDCz+V1dUYAB9I5oUrteAvF0crrcJI=; b=alzunQAhskjQiS5PphfPxaf1jc5p4R/T7WY/ApzVORF2djwju53dC52XfTCZ2r+3fN/MLs NMcl8IMBR4dYQXG2MFbaPe2xQPKGUiodETpm3qW6OJqLm5WBtjRuOhM3Y5XZZa3obBDKmX WnGwrmkqQ9oYlreoVhzd08KWof4NU8U= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-558-XLRQ24AjN--Pt_9xiqlJYg-1; Mon, 28 Nov 2022 10:11:56 -0500 X-MC-Unique: XLRQ24AjN--Pt_9xiqlJYg-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id BEFB33C0DDA5; Mon, 28 Nov 2022 15:11:55 +0000 (UTC) Received: from [10.22.10.34] (unknown [10.22.10.34]) by smtp.corp.redhat.com (Postfix) with ESMTP id CA8611121314; Mon, 28 Nov 2022 15:11:54 +0000 (UTC) Message-ID: Date: Mon, 28 Nov 2022 10:11:52 -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> From: Waiman Long In-Reply-To: <20221128120008.GA25090@willie-the-truck> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Cheers, Longman