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Thu, 05 Mar 2020 10:13:38 +0000 Received: from pps.filterd (aserp3020.oracle.com [127.0.0.1]) by aserp3020.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 025ACuAd064867; Thu, 5 Mar 2020 10:13:38 GMT Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by aserp3020.oracle.com with ESMTP id 2yg1rus3dv-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 05 Mar 2020 10:13:37 +0000 Received: from abhmp0004.oracle.com (abhmp0004.oracle.com [141.146.116.10]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id 025ADYBw000924; Thu, 5 Mar 2020 10:13:34 GMT Received: from [192.168.1.14] (/114.88.246.185) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Thu, 05 Mar 2020 02:13:34 -0800 Subject: Re: [PATCH v2] blktrace: fix dereference after null check To: Cengiz Can , Jens Axboe , Steven Rostedt , Ingo Molnar Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <20200304105818.11781-1-cengiz@kernel.wtf> From: Bob Liu Message-ID: Date: Thu, 5 Mar 2020 18:13:52 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: <20200304105818.11781-1-cengiz@kernel.wtf> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9550 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxscore=0 mlxlogscore=999 suspectscore=2 malwarescore=0 adultscore=0 spamscore=0 phishscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2003050064 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9550 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 phishscore=0 mlxscore=0 bulkscore=0 adultscore=0 suspectscore=2 spamscore=0 malwarescore=0 impostorscore=0 priorityscore=1501 mlxlogscore=999 lowpriorityscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2003050063 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 3/4/20 6:58 PM, Cengiz Can wrote: > There was a recent change in blktrace.c that added a RCU protection to > `q->blk_trace` in order to fix a use-after-free issue during access. > > However the change missed an edge case that can lead to dereferencing of > `bt` pointer even when it's NULL: > > Coverity static analyzer marked this as a FORWARD_NULL issue with CID > 1460458. > > ``` > /kernel/trace/blktrace.c: 1904 in sysfs_blk_trace_attr_store() > 1898 ret = 0; > 1899 if (bt == NULL) > 1900 ret = blk_trace_setup_queue(q, bdev); > 1901 > 1902 if (ret == 0) { > 1903 if (attr == &dev_attr_act_mask) >>>> CID 1460458: Null pointer dereferences (FORWARD_NULL) >>>> Dereferencing null pointer "bt". > 1904 bt->act_mask = value; > 1905 else if (attr == &dev_attr_pid) > 1906 bt->pid = value; > 1907 else if (attr == &dev_attr_start_lba) > 1908 bt->start_lba = value; > 1909 else if (attr == &dev_attr_end_lba) > ``` > > Added a reassignment with RCU annotation to fix the issue. > > Fixes: c780e86dd48 ("blktrace: Protect q->blk_trace with RCU") > > Signed-off-by: Cengiz Can This version looks good to me, thanks. Reviewed-by: Bob Liu > --- > > Patch Changelog > * v2: Added RCU annotation to assignment > > kernel/trace/blktrace.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c > index 4560878f0bac..ca39dc3230cb 100644 > --- a/kernel/trace/blktrace.c > +++ b/kernel/trace/blktrace.c > @@ -1896,8 +1896,11 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev, > } > > ret = 0; > - if (bt == NULL) > + if (bt == NULL) { > ret = blk_trace_setup_queue(q, bdev); > + bt = rcu_dereference_protected(q->blk_trace, > + lockdep_is_held(&q->blk_trace_mutex)); > + } > > if (ret == 0) { > if (attr == &dev_attr_act_mask) > -- > 2.25.1 >