linux-kernel - Re: [PATCH -next V3] ubi: fix race condition between ctrl_cdev_ioctl and ubi_cdev

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Date:   Thu, 23 Dec 2021 22:06:08 +0100 (CET)
From:   Richard Weinberger <richard@....at>
To:     "libaokun (A)" <libaokun1@...wei.com>
Cc:     Miquel Raynal <miquel.raynal@...tlin.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, yukuai3@...wei.com,
        Hulk Robot <hulkci@...wei.com>
Subject: Re: [PATCH -next V3] ubi: fix race condition between
 ctrl_cdev_ioctl and ubi_cdev_ioctl

----- Ursprüngliche Mail -----
> Von: "libaokun (A)" <libaokun1@...wei.com>
> 在 2021/11/5 17:30, Baokun Li 写道:
> 
> ping

Thanks for your patience.
 
>> Hulk Robot reported a KASAN report about use-after-free:

[...]

>> The cause of this problem is that commit 714fb87e8bc0 make device
>> "available" before it becomes accessible via sysfs. Therefore, we
>> roll back the modification. We will fix the race condition between
>> ubi device creation and udev by removing ubi_get_device in
>> vol_attribute_show and dev_attribute_show.This avoids accessing
>> uninitialized ubi_devices[ubi_num].
>>
>> ubi_get_device is used to prevent devices from being deleted during
>> sysfs execution. However, now kernfs ensures that devices will not
>> be deleted before all reference counting are released.
>> The key process is shown in the following stack.

ubi_get_device() in dev_attribute_show() is used to detect whether
the ubi device got detached while the sysfs file is open.

Hmm. I thought for sysfs this is not the case since sysfs does not implement
a release() method. So kernfs_drain_open_files() will return early.
But there is a good chance that I don't got all kernfs/sysfs details.

Thanks,
//richard