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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6AB64C433FE for ; Wed, 16 Mar 2022 06:55:23 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id A9B8B8D0002; Wed, 16 Mar 2022 02:55:22 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id A4AE68D0001; Wed, 16 Mar 2022 02:55:22 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 912398D0002; Wed, 16 Mar 2022 02:55:22 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0025.hostedemail.com [216.40.44.25]) by kanga.kvack.org (Postfix) with ESMTP id 8237F8D0001 for ; Wed, 16 Mar 2022 02:55:22 -0400 (EDT) Received: from smtpin20.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 22480A2AED for ; Wed, 16 Mar 2022 06:55:22 +0000 (UTC) X-FDA: 79249338084.20.584C694 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by imf08.hostedemail.com (Postfix) with ESMTP id A871216000C for ; Wed, 16 Mar 2022 06:55:20 +0000 (UTC) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.53]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4KJLXc092Jz9sgQ; Wed, 16 Mar 2022 14:51:28 +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 14:55:16 +0800 Subject: Re: [PATCH v2] mm/mlock: fix potential imbalanced rlimit ucounts adjustment To: "Eric W. Biederman" CC: , , , , Alexey Gladkov References: <20220314064039.62972-1-linmiaohe@huawei.com> <87h78036hl.fsf@email.froward.int.ebiederm.org> <82cf5aa8-a721-3ff3-7b09-54a66da0d506@huawei.com> <87lexbyslf.fsf@email.froward.int.ebiederm.org> From: Miaohe Lin Message-ID: <4803adf1-ba98-badc-6820-0948871b0742@huawei.com> Date: Wed, 16 Mar 2022 14:55:15 +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: <87lexbyslf.fsf@email.froward.int.ebiederm.org> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected X-Rspamd-Queue-Id: A871216000C X-Stat-Signature: jqfser145g6hayxaybjj8yht5j5ruw46 X-Rspam-User: Authentication-Results: imf08.hostedemail.com; dkim=none; dmarc=pass (policy=quarantine) header.from=huawei.com; spf=pass (imf08.hostedemail.com: domain of linmiaohe@huawei.com designates 45.249.212.189 as permitted sender) smtp.mailfrom=linmiaohe@huawei.com X-Rspamd-Server: rspam02 X-HE-Tag: 1647413720-520645 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2022/3/16 2:32, Eric W. Biederman wrote: > Miaohe Lin writes: > >> On 2022/3/14 23:21, Eric W. Biederman wrote: >>> Miaohe Lin writes: >>> >>>> user_shm_lock forgets to set allowed to 0 when get_ucounts fails. So >>>> the later user_shm_unlock might do the extra dec_rlimit_ucounts. Fix >>>> this by resetting allowed to 0. >>> >>> This fix looks correct. But the ability for people to follow and read >>> the code seems questionable. I saw in v1 of this patch Hugh originally >>> misread the logic. >>> >>> Could we instead change the code to leave lock_limit at ULONG_MAX aka >>> RLIM_INFINITY, leave initialized to 0, and not even need a special case >>> of RLIM_INFINITY as nothing can be greater that ULONG_MAX? >>> >> >> Many thanks for your advice. This looks good but it seems this results in different >> behavior: When (memlock == LONG_MAX) && !capable(CAP_IPC_LOCK), we would fail now >> while it will always success without this change. We should avoid this difference. >> Or am I miss something? Maybe the origin patch is more suitable and >> simple? > > Interesting. I think that is an unintended and necessary bug fix. > > When memlock == LONG_MAX that means inc_rlimit_ucounts failed. > > It either failed because at another level the limit was exceeded or > because the counter wrapped. In either case it is not appropriate to > succeed if inc_rlimit_ucounts detects a failure. > > Which is a long way of saying I think we really want the simplification > because it found and fixed another bug as well. > > Without the simplification I don't think I will be confident the code is > correct. Agree with you. This is a potential bug and you just catch it with the code simplification. :) Am I supposed to do this altogether or will you do this simplification part? Many thanks. > > Eric > > >> Thanks. >> >>> Something like this? >>> >>> diff --git a/mm/mlock.c b/mm/mlock.c >>> index 8f584eddd305..e7eabf5193ab 100644 >>> --- a/mm/mlock.c >>> +++ b/mm/mlock.c >>> @@ -827,13 +827,12 @@ int user_shm_lock(size_t size, struct ucounts *ucounts) >>> >>> locked = (size + PAGE_SIZE - 1) >> PAGE_SHIFT; >>> lock_limit = rlimit(RLIMIT_MEMLOCK); >>> - if (lock_limit == RLIM_INFINITY) >>> - allowed = 1; >>> - lock_limit >>= PAGE_SHIFT; >>> + if (lock_limit != RLIM_INFINITY) >>> + lock_limit >>= PAGE_SHIFT; >>> spin_lock(&shmlock_user_lock); >>> memlock = inc_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked); >>> >>> - if (!allowed && (memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) { >>> + if ((memlock == LONG_MAX || memlock > lock_limit) && !capable(CAP_IPC_LOCK)) { >>> dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked); >>> goto out; >>> } >>> >>>> >>>> Fixes: d7c9e99aee48 ("Reimplement RLIMIT_MEMLOCK on top of ucounts") >>>> Signed-off-by: Miaohe Lin >>>> Acked-by: Hugh Dickins >>>> --- >>>> v1->v2: >>>> correct Fixes tag and collect Acked-by tag >>>> Thanks Hugh for review! >>>> --- >>>> mm/mlock.c | 1 + >>>> 1 file changed, 1 insertion(+) >>>> >>>> diff --git a/mm/mlock.c b/mm/mlock.c >>>> index 29372c0eebe5..efd2dd2943de 100644 >>>> --- a/mm/mlock.c >>>> +++ b/mm/mlock.c >>>> @@ -733,6 +733,7 @@ int user_shm_lock(size_t size, struct ucounts *ucounts) >>>> } >>>> if (!get_ucounts(ucounts)) { >>>> dec_rlimit_ucounts(ucounts, UCOUNT_RLIMIT_MEMLOCK, locked); >>>> + allowed = 0; >>>> goto out; >>>> } >>>> allowed = 1; >>> >>> Eric >>> . >>> > . >