From patchwork Wed Jun 13 14:46:06 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jan Kara X-Patchwork-Id: 10462365 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id B314260348 for ; Wed, 13 Jun 2018 14:46:13 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 9D86627CF9 for ; Wed, 13 Jun 2018 14:46:13 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 91C4928173; Wed, 13 Jun 2018 14:46:13 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.9 required=2.0 tests=BAYES_00, MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id AB9AC27CF9 for ; Wed, 13 Jun 2018 14:46:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935734AbeFMOqL (ORCPT ); Wed, 13 Jun 2018 10:46:11 -0400 Received: from mx2.suse.de ([195.135.220.15]:53014 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935703AbeFMOqK (ORCPT ); Wed, 13 Jun 2018 10:46:10 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (charybdis-ext-too.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 0F8D0ABF4; Wed, 13 Jun 2018 14:46:08 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 7E24B1E0D63; Wed, 13 Jun 2018 16:46:06 +0200 (CEST) Date: Wed, 13 Jun 2018 16:46:06 +0200 From: Jan Kara To: Tetsuo Handa Cc: Jan Kara , Tejun Heo , Dmitry Vyukov , Jens Axboe , syzbot , syzkaller-bugs , linux-fsdevel , LKML , Al Viro , Dave Chinner , linux-block@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH] bdi: Fix another oops in wb_workfn() Message-ID: <20180613144606.nvbcyg2rdjpxhf7s@quack2.suse.cz> References: <2b437c6f-3e10-3d83-bdf3-82075d3eaa1a@i-love.sakura.ne.jp> <3cf4b0e3-31b6-8cdc-7c1e-15ba575a7879@i-love.sakura.ne.jp> <20180611091248.2i6nt27h5mxrodm2@quack2.suse.cz> <20180611160131.GQ1351649@devbig577.frc2.facebook.com> <20180611162920.mwapvuqotvhkntt3@quack2.suse.cz> <20180611172053.GR1351649@devbig577.frc2.facebook.com> <20180612155754.x5k2yndh5t6wlmpy@quack2.suse.cz> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170421 (1.8.2) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP On Wed 13-06-18 19:43:47, Tetsuo Handa wrote: > Can't we utilize RCU grace period (like shown below) ? Honestly, the variant 1 looks too ugly to me. However variant 2 looks mostly OK. We can also avoid the schedule_timeout_uninterruptible(HZ / 10) from your patch by careful handling of the bit waitqueues. Also I'd avoid the addition argument to wb_writeback() and split the function instead. The patch resulting from your and mine ideas is attached. Thoughts? Honza From d0fbfad964ae5b0a331f0d814d527150bd0c305c Mon Sep 17 00:00:00 2001 From: Tetsuo Handa Date: Wed, 13 Jun 2018 16:39:00 +0200 Subject: [PATCH] bdi: Fix another oops in wb_workfn() syzbot is reporting NULL pointer dereference at wb_workfn() [1] due to wb->bdi->dev being NULL. And Dmitry confirmed that wb->state was WB_shutting_down after wb->bdi->dev became NULL. This indicates that unregister_bdi() failed to call wb_shutdown() on one of wb objects. The problem is in cgwb_bdi_unregister() which does cgwb_kill() and thus drops bdi's reference to wb structures before going through the list of wbs again and calling wb_shutdown() on each of them. This way the loop iterating through all wbs can easily miss a wb if that wb has already passed through cgwb_remove_from_bdi_list() called from wb_shutdown() from cgwb_release_workfn() and as a result fully shutdown bdi although wb_workfn() for this wb structure is still running. In fact there are also other ways cgwb_bdi_unregister() can race with cgwb_release_workfn() leading e.g. to use-after-free issues: CPU1 CPU2 cgwb_bdi_unregister() cgwb_kill(*slot); cgwb_release() queue_work(cgwb_release_wq, &wb->release_work); cgwb_release_workfn() wb = list_first_entry(&bdi->wb_list, ...) spin_unlock_irq(&cgwb_lock); wb_shutdown(wb); ... kfree_rcu(wb, rcu); wb_shutdown(wb); -> oops use-after-free We solve these issues by making wb_shutdown() remove the writeback structure from the bdi_list only after all the shutdown is done and by making sure cgwb_bdi_unregister() properly synchronizes with concurrent shutdowns using WB_shutting_down bit. Signed-off-by: Tetsuo Handa Signed-off-by: Jan Kara --- mm/backing-dev.c | 69 ++++++++++++++++++++++++++++++++++++++++++++------------ 1 file changed, 55 insertions(+), 14 deletions(-) diff --git a/mm/backing-dev.c b/mm/backing-dev.c index 347cc834c04a..433807ae364e 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -350,27 +350,21 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi, static void cgwb_remove_from_bdi_list(struct bdi_writeback *wb); -/* - * Remove bdi from the global list and shutdown any threads we have running - */ -static void wb_shutdown(struct bdi_writeback *wb) +static bool wb_start_shutdown(struct bdi_writeback *wb) { /* Make sure nobody queues further work */ spin_lock_bh(&wb->work_lock); if (!test_and_clear_bit(WB_registered, &wb->state)) { spin_unlock_bh(&wb->work_lock); - /* - * Wait for wb shutdown to finish if someone else is just - * running wb_shutdown(). Otherwise we could proceed to wb / - * bdi destruction before wb_shutdown() is finished. - */ - wait_on_bit(&wb->state, WB_shutting_down, TASK_UNINTERRUPTIBLE); - return; + return false; } set_bit(WB_shutting_down, &wb->state); spin_unlock_bh(&wb->work_lock); + return true; +} - cgwb_remove_from_bdi_list(wb); +static void __wb_shutdown(struct bdi_writeback *wb) +{ /* * Drain work list and shutdown the delayed_work. !WB_registered * tells wb_workfn() that @wb is dying and its work_list needs to @@ -379,6 +373,7 @@ static void wb_shutdown(struct bdi_writeback *wb) mod_delayed_work(bdi_wq, &wb->dwork, 0); flush_delayed_work(&wb->dwork); WARN_ON(!list_empty(&wb->work_list)); + cgwb_remove_from_bdi_list(wb); /* * Make sure bit gets cleared after shutdown is finished. Matches with * the barrier provided by test_and_clear_bit() above. @@ -387,6 +382,23 @@ static void wb_shutdown(struct bdi_writeback *wb) clear_and_wake_up_bit(WB_shutting_down, &wb->state); } +/* + * Remove bdi from the global list and shutdown any threads we have running + */ +static void wb_shutdown(struct bdi_writeback *wb) +{ + if (!wb_start_shutdown(wb)) { + /* + * Wait for wb shutdown to finish if someone else is just + * running wb_shutdown(). Otherwise we could proceed to wb / + * bdi destruction before wb_shutdown() is finished. + */ + wait_on_bit(&wb->state, WB_shutting_down, TASK_UNINTERRUPTIBLE); + return; + } + __wb_shutdown(wb); +} + static void wb_exit(struct bdi_writeback *wb) { int i; @@ -706,6 +718,35 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi) return ret; } +/* + * Mark writeback structure as shutting down. The function returns true if + * we successfully marked the structure, false if shutdown was already in + * progress (in which case we also wait for it to finish). + * + * The function requires cgwb_lock to be held and releases it. Note that the + * caller need not hold reference to the writeback structure and thus once + * cgwb_lock is released, the writeback structure can be freed. + */ +static bool cgwb_start_shutdown(struct bdi_writeback *wb) + __releases(cgwb_lock) +{ + if (!wb_start_shutdown(wb)) { + DEFINE_WAIT(wait); + wait_queue_head_t *wqh = bit_waitqueue(&wb->state, + WB_shutting_down); + bool sleep; + + prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE); + sleep = test_bit(WB_shutting_down, &wb->state); + spin_unlock_irq(&cgwb_lock); + if (sleep) + schedule(); + return false; + } + spin_unlock_irq(&cgwb_lock); + return true; +} + static void cgwb_bdi_unregister(struct backing_dev_info *bdi) { struct radix_tree_iter iter; @@ -721,8 +762,8 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi) while (!list_empty(&bdi->wb_list)) { wb = list_first_entry(&bdi->wb_list, struct bdi_writeback, bdi_node); - spin_unlock_irq(&cgwb_lock); - wb_shutdown(wb); + if (cgwb_start_shutdown(wb)) + __wb_shutdown(wb); spin_lock_irq(&cgwb_lock); } spin_unlock_irq(&cgwb_lock); -- 2.13.7