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Fri, 06 Dec 2019 20:16:21 +0000 Received: from pps.filterd (userp3030.oracle.com [127.0.0.1]) by userp3030.oracle.com (8.16.0.27/8.16.0.27) with SMTP id xB6K9O0n145755; Fri, 6 Dec 2019 20:16:21 GMT Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by userp3030.oracle.com with ESMTP id 2wqt45atmg-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 06 Dec 2019 20:16:20 +0000 Received: from abhmp0002.oracle.com (abhmp0002.oracle.com [141.146.116.8]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id xB6KGJPs030735; Fri, 6 Dec 2019 20:16:19 GMT Received: from bostrovs-us.us.oracle.com (/10.152.32.65) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Fri, 06 Dec 2019 12:16:19 -0800 Subject: Re: [PATCH] xen/pciback: Prevent NULL pointer dereference in quirks_show To: "Nuernberger, Stefan" , "linux-kernel@vger.kernel.org" Cc: "xen-devel@lists.xenproject.org" , "Seidel, Conny" , "jgross@suse.com" , "ross.lagerwall@citrix.com" , "Dannowski, Uwe" , "stable@vger.kernel.org" References: <20191206134804.4537-1-snu@amazon.com> <9917a357-12f6-107f-e08d-33e464036317@oracle.com> <1575655787.7257.42.camel@amazon.de> From: Boris Ostrovsky Autocrypt: addr=boris.ostrovsky@oracle.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <1575655787.7257.42.camel@amazon.de> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9463 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1911140001 definitions=main-1912060161 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9463 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1011 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1911140001 definitions=main-1912060162 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/6/19 1:09 PM, Nuernberger, Stefan wrote: > On Fri, 2019-12-06 at 10:11 -0500, Boris Ostrovsky wrote: >> On 12/6/19 8:48 AM, Stefan Nuernberger wrote: >>> From: Uwe Dannowski >>> >>> Reading /sys/bus/pci/drivers/pciback/quirks while unbinding can >>> result >>> in dereferencing a NULL pointer. Instead, skip printing information >>> about the dangling quirk. >>> >>> Reported-by: Conny Seidel >>> Signed-off-by: Uwe Dannowski >>> Signed-off-by: Stefan Nuernberger >>> >>> Cc: xen-devel@lists.xenproject.org >>> Cc: stable@vger.kernel.org >>> --- >>>  drivers/xen/xen-pciback/pci_stub.c | 2 ++ >>>  1 file changed, 2 insertions(+) >>> >>> diff --git a/drivers/xen/xen-pciback/pci_stub.c b/drivers/xen/xen- >>> pciback/pci_stub.c >>> index 097410a7cdb7..da725e474294 100644 >>> --- a/drivers/xen/xen-pciback/pci_stub.c >>> +++ b/drivers/xen/xen-pciback/pci_stub.c >>> @@ -1346,6 +1346,8 @@ static ssize_t quirks_show(struct >>> device_driver *drv, char *buf) >>>      quirk->devid.subdevice); >>>   >>>   dev_data = pci_get_drvdata(quirk->pdev); >>> + if (!dev_data) >>> + continue; >>>   >>>   list_for_each_entry(cfg_entry, &dev_data- >>>> config_fields, list) { >> Couldn't you have the same race here? > Not quite the same, but it might not be entirely safe yet. The > 'quirks_show' takes the 'device_ids_lock' and races with unbind / > 'pcistub_device_release' "which takes device_lock mutex". So this might > now be a UAF read access instead of a NULL pointer dereference. Yes, that's what I meant (although I don't see much difference in this context). > We have > not observed adversarial effects in our testing (compared to the > obvious issues with NULL pointer) but that's not a guarantee of course. > > So should quirks_show actually be protected by pcistub_devices_lock > instead as are other functions that access dev_data? Does it need both > locks in that case? device_ids_lock protects device_ids list, which is not what you are trying to access, so that doesn't look like right lock to hold. And AFAICT pcistub_devices_lock is not held when device data is cleared in pcistub_device_release() (which I think is where we are racing). -boris