linux-kernel - Re: [PATCH v2 1/4] vfio-mdev: Fix remove race

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Date:   Mon, 26 Dec 2016 08:58:51 +0530
From:   Kirti Wankhede <kwankhede@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     <jike.song@...el.com>, <linux-kernel@...r.kernel.org>,
        <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] vfio-mdev: Fix remove race



On 12/26/2016 1:10 AM, Alex Williamson wrote:
> On Sun, 25 Dec 2016 22:39:47 +0530
> Kirti Wankhede <kwankhede@...dia.com> wrote:
> 
>> On 12/23/2016 1:51 AM, Alex Williamson wrote:
>>> Using the mtty mdev sample driver we can generate a remove race by
>>> starting one shell that continuously creates mtty devices and several
>>> other shells all attempting to remove devices, in my case four remove
>>> shells.  The fault occurs in mdev_remove_sysfs_files() where the
>>> passed type arg is NULL, which suggests we've received a struct device
>>> in mdev_device_remove() but it's in some sort of teardown state.  The
>>> solution here is to make use of the accidentally unused list_head on
>>> the mdev_device such that the mdev core keeps a list of all the mdev
>>> devices.  This allows us to validate that we have a valid mdev before
>>> we start removal, remove it from the list to prevent others from
>>> working on it, and if the vendor driver refuses to remove, we can
>>> re-add it to the list.
>>>   
>>
>> Alex,
>>
>> Writing 1 on 'remove' first removes itself, i.e. calls
>> device_remove_file_self(dev, attr). So if the file is removed then
>> device_remove_file_self() should return false, isn't that returns false?
>> kernfs_remove_self() hold the mutex that should handle this condition.
> 
> In theory, I agree.  In practice I was able to generate the race
> described.  We're getting through to call mdev_device_remove with
> a struct device that resolves to an mdev where the type_kobj is
> NULL, presumably it's been freed.  Maybe there's a better fix
> within kernfs, but this sanitizes the mdev on our end to resolve
> it.  To see the issue, simply run 'while true; do uuidgen >
> create; done', then from a few other shells loop finding mdev
> devices and remove any that are found.  Set dmesg to only print
> critical messages or else it'll slow create and delete to the
> point where it'll be difficult to get the race.  Thanks,
> 

I see. pci-sysfs too uses mutex around its remove function even after
device_remove_file_self() returned true. Yes, probably kernfs might have
better fix.
This change looks good to me.

Thanks,
Kirti