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 9566EC433F5 for ; Wed, 16 Mar 2022 06:39:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244317AbiCPGk6 (ORCPT ); Wed, 16 Mar 2022 02:40:58 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39924 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231577AbiCPGky (ORCPT ); Wed, 16 Mar 2022 02:40:54 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id AE99B57B10 for ; Tue, 15 Mar 2022 23:39:39 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.54]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4KJLFF60v9zfYqd; Wed, 16 Mar 2022 14:38:09 +0800 (CST) Received: from [10.174.177.76] (10.174.177.76) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2308.21; Wed, 16 Mar 2022 14:39:37 +0800 Subject: Re: [PATCH] mm/mempolicy: fix potential mpol_new leak in shared_policy_replace To: Michal Hocko CC: , , , , References: <20220311093624.39546-1-linmiaohe@huawei.com> <26577566-ae1e-801c-8c64-89c2c89a487d@huawei.com> From: Miaohe Lin Message-ID: <24b2a9ef-eea0-09bd-6842-121d8436e56a@huawei.com> Date: Wed, 16 Mar 2022 14:39:37 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems701-chm.china.huawei.com (10.3.19.178) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/3/15 23:27, Michal Hocko wrote: > On Tue 15-03-22 21:42:29, Miaohe Lin wrote: >> On 2022/3/15 0:44, Michal Hocko wrote: >>> On Fri 11-03-22 17:36:24, Miaohe Lin wrote: >>>> If mpol_new is allocated but not used in restart loop, mpol_new will be >>>> freed via mpol_put before returning to the caller. But refcnt is not >>>> initialized yet, so mpol_put could not do the right things and might >>>> leak the unused mpol_new. >>> >>> The code is really hideous but is there really any bug there? AFAICS the >>> new policy is only allocated in if (n->end > end) branch and that one >>> will set the reference count on the retry. Or am I missing something? >>> >> >> Many thanks for your comment. >> IIUC, new policy is allocated via the below code: >> >> shared_policy_replace: >> alloc_new: >> write_unlock(&sp->lock); >> ret = -ENOMEM; >> n_new = kmem_cache_alloc(sn_cache, GFP_KERNEL); >> if (!n_new) >> goto err_out; >> mpol_new = kmem_cache_alloc(policy_cache, GFP_KERNEL); >> if (!mpol_new) >> goto err_out; >> goto restart; >> >> And mpol_new' reference count will be set before used in n->end > end case. But >> if that is "not" the case, i.e. mpol_new is not inserted into the rb_tree, mpol_new >> will be freed via mpol_put before return: > > One thing I have missed previously is that the lock is dropped during > the allocation so I guess the memory policy could have been changed > during that time. Is this possible? Have you explored this possibility? > Is this a theoretical problem or it can be triggered intentionally. > This is found via code investigation. I think this could be triggered if there are many concurrent mpol_set_shared_policy in place. But the user-visible effect might be obscure as only sizeof(struct mempolicy) bytes leaks possiblely every time. > These details would be really interesting for the changelog so that we > can judge how important this would be. This might not be that important as this issue should have been well-concealed for almost ten years (since commit 42288fe366c4 ("mm: mempolicy: Convert shared_policy mutex to spinlock")). > Thanks.