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Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1 Subject: Re: [PATCH] mm: fix pgdat->kswap accessed concurrently Content-Language: en-US To: Andrew Morton , Muchun Song CC: Linux MM , Qian Cai , References: <20220820032506.126860-1-wangkefeng.wang@huawei.com> <1E87F09C-4904-49E2-B45C-C408DD5F6F62@linux.dev> <20220820135955.1520aa480fe04ab31d4fce1f@linux-foundation.org> From: Kefeng Wang In-Reply-To: <20220820135955.1520aa480fe04ab31d4fce1f@linux-foundation.org> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.243] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To dggpemm500001.china.huawei.com (7.185.36.107) X-CFilter-Loop: Reflected ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1661216881; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; 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dmarc=pass (policy=quarantine) header.from=huawei.com X-Stat-Signature: hpjkdcdqx4cf1sqjot3qe9hn4awepyxd X-Rspamd-Queue-Id: 0EF834002B X-Rspamd-Server: rspam08 X-HE-Tag: 1661216880-852276 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 2022/8/21 4:59, Andrew Morton wrote: > On Sat, 20 Aug 2022 15:33:04 +0800 Muchun Song wrote: > >> >>> + if (IS_ERR(t)) { >>> /* failure at boot is fatal */ >>> BUG_ON(system_state < SYSTEM_RUNNING); >>> pr_err("Failed to start kswapd on node %d\n", nid); >>> - pgdat->kswapd = NULL; >>> + WRITE_ONCE(pgdat->kswapd, NULL); >>> + } else { >>> + WRITE_ONCE(pgdat->kswapd, t); >>> } >>> } >> IIUC, the race is like the followings: >> >> CPU 0: CPU 1: >> >> kswapd_run() >> pgdat->kswapd = kthread_run() >> if (IS_ERR(pgdat->kswapd)) >> kswapd_is_running >> // load pgdat->kswapd and it is NOT NULL. >> pgdat->kswapd = NULL >> task_is_running(pgdat->kswapd); // NULL pointer dereference >> > But don't we still have a bug? Sure, kswapd_is_running() will no > longer deref a null pointer. But it now runs kswapd_is_running() > against a task which has exited - a use-after-free? we could add get/put_task_struct() to avoid the UAF， will update， thanks. > .