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=-5.3 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 D388BC32771 for ; Fri, 3 Jan 2020 03:06:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B24AF21D7D for ; Fri, 3 Jan 2020 03:06:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727331AbgACDGe (ORCPT ); Thu, 2 Jan 2020 22:06:34 -0500 Received: from szxga06-in.huawei.com ([45.249.212.32]:47644 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727325AbgACDGe (ORCPT ); Thu, 2 Jan 2020 22:06:34 -0500 Received: from DGGEMS413-HUB.china.huawei.com (unknown [172.30.72.59]) by Forcepoint Email with ESMTP id 7E5ED8A828111D5EE3C1; Fri, 3 Jan 2020 11:06:31 +0800 (CST) Received: from [127.0.0.1] (10.133.219.224) by DGGEMS413-HUB.china.huawei.com (10.3.19.213) with Microsoft SMTP Server id 14.3.439.0; Fri, 3 Jan 2020 11:06:26 +0800 Subject: Re: [PATCH] block: make sure last_lookup set as NULL after part deleted To: Ming Lei CC: Yufen Yu , , , , , , , , References: <20191231110945.10857-1-yuyufen@huawei.com> <20200102012314.GB16719@ming.t460p> From: Hou Tao Message-ID: Date: Fri, 3 Jan 2020 11:06:25 +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: <20200102012314.GB16719@ming.t460p> 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, On 2020/1/2 9:23, Ming Lei 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: >> We have written a similar test case for the following case and found out that process C still may got the freed hd_struct pointer from process A. So the re-read will not resolve the problem. >> 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) >> >> 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)); > > IMO this approach looks good. > Not sure about the overhead when there are concurrent IOs on different partitions, we will measure that. We have got a seemingly better solution: caching the index of last_lookup in tbl->part[] instead of caching the pointer itself, so we can ensure the validity of returned pointer by ensuring it's not NULL in tbl->part[] as does when last_lookup is NULL or 0. > Given partition is actually protected by percpu-refcount now, I guess the > RCU annotation for referencing ->part[partno] and ->last_lookup may not > be necessary, together with the part->rcu_work. > So we will depends on the invocation of of call_rcu() on __percpu_ref_switch_mode() to ensure the RCU readers will find part[i] is NULL before trying to increasing the atomic ref-counter of part[i], right ? Regards, Tao > > Thanks, > Ming > > > . >