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 X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id ACEC6C32770 for ; Sat, 4 Jan 2020 09:16:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8E8D424649 for ; Sat, 4 Jan 2020 09:16:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726103AbgADJQo (ORCPT ); Sat, 4 Jan 2020 04:16:44 -0500 Received: from szxga04-in.huawei.com ([45.249.212.190]:8678 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726133AbgADJQn (ORCPT ); Sat, 4 Jan 2020 04:16:43 -0500 Received: from DGGEMS410-HUB.china.huawei.com (unknown [172.30.72.59]) by Forcepoint Email with ESMTP id D4EE1D14FE2955177214; Sat, 4 Jan 2020 17:16:39 +0800 (CST) Received: from [127.0.0.1] (10.133.219.224) by DGGEMS410-HUB.china.huawei.com (10.3.19.210) with Microsoft SMTP Server id 14.3.439.0; Sat, 4 Jan 2020 17:16:34 +0800 Subject: Re: [PATCH] block: make sure last_lookup set as NULL after part deleted To: Joel Fernandes , "Paul E. McKenney" CC: Yufen Yu , , , , , , , References: <20191231110945.10857-1-yuyufen@huawei.com> <20191231231158.GW13449@paulmck-ThinkPad-P72> <20200103234521.GG189259@google.com> From: Hou Tao Message-ID: <27b4c66e-316a-b261-364d-0b246d48e6bd@huawei.com> Date: Sat, 4 Jan 2020 17:16:33 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: <20200103234521.GG189259@google.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.133.219.224] X-CFilter-Loop: Reflected Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Hi Joel, On 2020/1/4 7:45, Joel Fernandes wrote: > On Tue, Dec 31, 2019 at 03:11:58PM -0800, Paul E. McKenney wrote: >> On Tue, Dec 31, 2019 at 10:55:47PM +0800, Hou Tao wrote: >>> Hi, >>> >>> On 2019/12/31 19:09, Yufen Yu wrote: >>>> When delete partition executes concurrently with IOs issue, >>>> it may cause use-after-free on part in disk_map_sector_rcu() >>>> as following: >>> snip >>> >>>> >>>> diff --git a/block/genhd.c b/block/genhd.c >>>> index ff6268970ddc..39fa8999905f 100644 >>>> --- a/block/genhd.c >>>> +++ b/block/genhd.c >>>> @@ -293,7 +293,23 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector) >>>> part = rcu_dereference(ptbl->part[i]); >>>> >>>> if (part && sector_in_part(part, sector)) { >>> snip >>> >>>> rcu_assign_pointer(ptbl->last_lookup, part); >>>> + part = rcu_dereference(ptbl->part[i]); >>>> + if (part == NULL) { >>>> + rcu_assign_pointer(ptbl->last_lookup, NULL); >>>> + break; >>>> + } >>>> return part; >>>> } >>>> } >>> >>> Not ensure whether the re-read can handle the following case or not: >>> >>> process A process B process C >>> >>> disk_map_sector_rcu(): delete_partition(): disk_map_sector_rcu(): >>> >>> rcu_read_lock >>> >>> // need to iterate partition table >>> part[i] != NULL (1) part[i] = NULL (2) >>> smp_mb() >>> last_lookup = NULL (3) >>> call_rcu() (4) >>> last_lookup = part[i] (5) >>> >>> >>> rcu_read_lock() >>> read last_lookup return part[i] (6) >>> sector_in_part() is OK (7) >>> return part[i] (8) >> >> Just for the record... >> >> Use of RCU needs to ensure that readers cannot access the to-be-freed >> structure -before- invoking call_rcu(). Which does look to happen here >> with the "last_lookup = NULL". But in addition, the callback needs to >> get access to the to-be-freed structure via some sideband (usually the >> structure passed to call_rcu()), not from the reader-accessible structure. >> >> Or am I misinterpreting this sequence of events? > > If I understand correctly, the issue described above is there are 2 threads > setting last_lookup pointer simultaneously, one of them is NULLing it and > waiting for a GP before freeing it (process B above), while the other is > assigning to it concurrently after it was just NULLed (process A). Meanwhile > process C starts a reader section *after* the GP by process B already started > and accesses the reassigned pointer causing use-after-free. > > Did I miss something? > No. It's exactly the same as you have summarized. And thanks for that. > I believe the fix is what Tao already posted which is to use refcounts so > that the destructor does not free it while references are already held. Is > that what the final fix is going to be? That other thread is pretty long so I > lost track a bit.. > We are just trying to find a better solution (e.g. more readable or understandable). Regards, Tao > thanks, > > - Joel > > > >> Thanx, Paul >> >>> part[i] == NULL (9) >>> last_lookup = NULL (10) >>> rcu_read_unlock() (11) >>> one RCU grace period completes >>> __delete_partition() (12) >>> free hd_partition (13) >>> // use-after-free >>> hd_struct_try_get(part[i]) (14) >>> >>> * the number in the parenthesis is the sequence of events. >>> >>> Maybe RCU experts can shed some light on this problem, so cc +paulmck@kernel.org, +joel@joelfernandes.org and +RCU maillist. >>> >>> If the above case is possible, maybe we can fix the problem by pinning last_lookup through increasing its ref-count >>> (the following patch is only compile tested): >>> >>> diff --git a/block/genhd.c b/block/genhd.c >>> index 6e8543ca6912..179e0056fae1 100644 >>> --- a/block/genhd.c >>> +++ b/block/genhd.c >>> @@ -279,7 +279,14 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector) >>> part = rcu_dereference(ptbl->part[i]); >>> >>> if (part && sector_in_part(part, sector)) { >>> - rcu_assign_pointer(ptbl->last_lookup, part); >>> + struct hd_struct *old; >>> + >>> + if (!hd_struct_try_get(part)) >>> + break; >>> + >>> + old = xchg(&ptbl->last_lookup, part); >>> + if (old) >>> + hd_struct_put(old); >>> return part; >>> } >>> } >>> @@ -1231,7 +1238,11 @@ static void disk_replace_part_tbl(struct gendisk *disk, >>> rcu_assign_pointer(disk->part_tbl, new_ptbl); >>> >>> if (old_ptbl) { >>> - rcu_assign_pointer(old_ptbl->last_lookup, NULL); >>> + struct hd_struct *part; >>> + >>> + part = xchg(&old_ptbl->last_lookup, NULL); >>> + if (part) >>> + hd_struct_put(part); >>> kfree_rcu(old_ptbl, rcu_head); >>> } >>> } >>> diff --git a/block/partition-generic.c b/block/partition-generic.c >>> index 98d60a59b843..441c1c591c04 100644 >>> --- a/block/partition-generic.c >>> +++ b/block/partition-generic.c >>> @@ -285,7 +285,8 @@ void delete_partition(struct gendisk *disk, int partno) >>> return; >>> >>> rcu_assign_pointer(ptbl->part[partno], NULL); >>> - rcu_assign_pointer(ptbl->last_lookup, NULL); >>> + if (cmpxchg(&ptbl->last_lookup, part, NULL) == part) >>> + hd_struct_put(part); >>> kobject_put(part->holder_dir); >>> device_del(part_to_dev(part)); >>> >>> -- >>> 2.22.0 >>> >>> Regards, >>> Tao >>> >>> >>>> diff --git a/block/partition-generic.c b/block/partition-generic.c >>>> index 1d20c9cf213f..1e0065ed6f02 100644 >>>> --- a/block/partition-generic.c >>>> +++ b/block/partition-generic.c >>>> @@ -284,6 +284,13 @@ void delete_partition(struct gendisk *disk, int partno) >>>> return; >>>> >>>> rcu_assign_pointer(ptbl->part[partno], NULL); >>>> + /* >>>> + * Without the memory barrier, disk_map_sector_rcu() >>>> + * may read the old value after overwriting the >>>> + * last_lookup. Then it can not clear last_lookup, >>>> + * which may cause use-after-free. >>>> + */ >>>> + smp_mb(); >>>> rcu_assign_pointer(ptbl->last_lookup, NULL); >>>> kobject_put(part->holder_dir); >>>> device_del(part_to_dev(part)); >>>> >>> > > . >