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Tue, 29 Nov 2022 15:34:25 -0500 X-MC-Unique: SZGhVrRyO_-myQQoKno4yA-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 60622802314; Tue, 29 Nov 2022 20:34:23 +0000 (UTC) Received: from llong.com (unknown [10.22.17.30]) by smtp.corp.redhat.com (Postfix) with ESMTP id 696AE140EBF5; Tue, 29 Nov 2022 20:34:22 +0000 (UTC) From: Waiman Long To: Jens Axboe , Tejun Heo Cc: cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Ming Lei , Andy Shevchenko , Andrew Morton , =?utf-8?q?Michal_Koutn=C3=BD?= , Hillf Danton , Chaitanya Kulkarni , Bart Van Assche , Josef Bacik , Waiman Long , Yi Zhang Subject: [PATCH-block v2] bdi, blk-cgroup: Fix potential UAF of blkcg Date: Tue, 29 Nov 2022 15:34:00 -0500 Message-Id: <20221129203400.1456100-1-longman@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 ARC-Message-Signature: i=1; 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dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=AmmWTYNq; spf=pass (imf04.hostedemail.com: domain of longman@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=longman@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-HE-Tag: 1669754074-813358 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: Commit 59b57717fff8 ("blkcg: delay blkg destruction until after writeback has finished") delayed call to blkcg_destroy_blkgs() to cgwb_release_workfn(). However, it is done after a css_put() of blkcg which may be the final put that causes the blkcg to be freed as RCU read lock isn't held. By adding a css_tryget() into blkcg_destroy_blkgs() and warning its failure, the following stack trace was produced in a test system on bootup. [ 34.254240] RIP: 0010:blkcg_destroy_blkgs+0x16a/0x1a0 : [ 34.339943] Call Trace: [ 34.342395] [ 34.344510] blkcg_unpin_online+0x38/0x60 [ 34.348523] cgwb_release_workfn+0x6a/0x200 [ 34.352708] process_one_work+0x1e5/0x3b0 [ 34.356742] ? rescuer_thread+0x390/0x390 [ 34.360758] worker_thread+0x50/0x3a0 [ 34.364425] ? rescuer_thread+0x390/0x390 [ 34.368447] kthread+0xd9/0x100 [ 34.371592] ? kthread_complete_and_exit+0x20/0x20 [ 34.376386] ret_from_fork+0x22/0x30 [ 34.379982] This confirms that a potential UAF situation can happen. Fix that by delaying the css_put() until after the blkcg_unpin_online() call. Also use css_tryget() in blkcg_destroy_blkgs() and issue a warning if css_tryget() fails with no RCU read lock held. The reproducing system can no longer produce a warning with this patch. All the runnable block/0* tests including block/027 were run successfully without failure. Fixes: 59b57717fff8 ("blkcg: delay blkg destruction until after writeback has finished") Suggested-by: Michal Koutný Reported-by: Yi Zhang Signed-off-by: Waiman Long --- block/blk-cgroup.c | 10 +++++++++- mm/backing-dev.c | 8 ++++++-- 2 files changed, 15 insertions(+), 3 deletions(-) diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c index 57941d2a8ba3..904372bb96f1 100644 --- a/block/blk-cgroup.c +++ b/block/blk-cgroup.c @@ -1088,7 +1088,15 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg) might_sleep(); - css_get(&blkcg->css); + /* + * blkcg_destroy_blkgs() shouldn't be called with all the blkcg + * references gone and rcu_read_lock not held. + */ + if (!css_tryget(&blkcg->css)) { + WARN_ON_ONCE(!rcu_read_lock_held()); + return; + } + spin_lock_irq(&blkcg->lock); while (!hlist_empty(&blkcg->blkg_list)) { struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first, diff --git a/mm/backing-dev.c b/mm/backing-dev.c index c30419a5e119..36f75b072325 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -390,11 +390,15 @@ static void cgwb_release_workfn(struct work_struct *work) wb_shutdown(wb); css_put(wb->memcg_css); - css_put(wb->blkcg_css); mutex_unlock(&wb->bdi->cgwb_release_mutex); - /* triggers blkg destruction if no online users left */ + /* + * Triggers blkg destruction if no online users left + * The final blkcg css_put() has to be done after blkcg_unpin_online() + * to avoid use-after-free. + */ blkcg_unpin_online(wb->blkcg_css); + css_put(wb->blkcg_css); fprop_local_destroy_percpu(&wb->memcg_completions);