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Tue, 03 Mar 2020 13:29:01 +0000 Received: from pps.filterd (aserp3030.oracle.com [127.0.0.1]) by aserp3030.oracle.com (8.16.0.42/8.16.0.42) with SMTP id 023DR2RR127560; Tue, 3 Mar 2020 13:29:00 GMT Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by aserp3030.oracle.com with ESMTP id 2yg1gxeuuk-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 03 Mar 2020 13:29:00 +0000 Received: from abhmp0005.oracle.com (abhmp0005.oracle.com [141.146.116.11]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id 023DSvk6001632; Tue, 3 Mar 2020 13:28:58 GMT Received: from [192.168.1.14] (/114.88.246.185) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Tue, 03 Mar 2020 05:28:57 -0800 Subject: Re: [PATCH] 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: <20200303073358.57799-1-cengiz@kernel.wtf> From: Bob Liu Message-ID: Date: Tue, 3 Mar 2020 21:29:08 +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: <20200303073358.57799-1-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=9548 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 adultscore=0 phishscore=0 suspectscore=2 malwarescore=0 mlxlogscore=999 mlxscore=0 spamscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2003030103 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9548 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 malwarescore=0 phishscore=0 spamscore=0 impostorscore=0 mlxscore=0 adultscore=0 mlxlogscore=999 lowpriorityscore=0 priorityscore=1501 bulkscore=0 clxscore=1011 suspectscore=2 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2003030103 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 3/3/20 3:33 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: > > ``` > bt->act_mask = value; // bt can still be NULL here > ``` > > Added a reassignment into the NULL check block to fix the issue. > > Fixes: c780e86dd48 ("blktrace: Protect q->blk_trace with RCU") > > Signed-off-by: Cengiz Can > --- > Huge thanks goes to Steven Rostedt for his assistance. > > kernel/trace/blktrace.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c > index 4560878f0bac..29ea88f10b87 100644 > --- a/kernel/trace/blktrace.c > +++ b/kernel/trace/blktrace.c > @@ -1896,8 +1896,10 @@ 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 = q->blk_trace; The return value 'ret' should be judged, it's wrong to set 'bt' if blk_trace_setup_queue() return failure. > + } > > if (ret == 0) { > if (attr == &dev_attr_act_mask) > -- > 2.25.1 >