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Date:   Sun, 25 Dec 2016 12:40:58 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Kirti Wankhede <kwankhede@...dia.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 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,

Alex
 
> > Cc: Kirti Wankhede <kwankhede@...dia.com>
> > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > ---
> >  drivers/vfio/mdev/mdev_core.c |   36 ++++++++++++++++++++++++++++++++++--
> >  1 file changed, 34 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> > index be1ee89..6bb4d4c 100644
> > --- a/drivers/vfio/mdev/mdev_core.c
> > +++ b/drivers/vfio/mdev/mdev_core.c
> > @@ -27,6 +27,9 @@
> >  static DEFINE_MUTEX(parent_list_lock);
> >  static struct class_compat *mdev_bus_compat_class;
> >  
> > +static LIST_HEAD(mdev_list);
> > +static DEFINE_MUTEX(mdev_list_lock);
> > +
> >  static int _find_mdev_device(struct device *dev, void *data)
> >  {
> >  	struct mdev_device *mdev;
> > @@ -316,6 +319,11 @@ int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
> >  	dev_dbg(&mdev->dev, "MDEV: created\n");
> >  
> >  	mutex_unlock(&parent->lock);
> > +
> > +	mutex_lock(&mdev_list_lock);
> > +	list_add(&mdev->next, &mdev_list);
> > +	mutex_unlock(&mdev_list_lock);
> > +
> >  	return ret;
> >  
> >  create_failed:
> > @@ -329,12 +337,30 @@ int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
> >  
> >  int mdev_device_remove(struct device *dev, bool force_remove)
> >  {
> > -	struct mdev_device *mdev;
> > +	struct mdev_device *mdev, *tmp;
> >  	struct parent_device *parent;
> >  	struct mdev_type *type;
> >  	int ret;
> > +	bool found = false;
> >  
> >  	mdev = to_mdev_device(dev);
> > +
> > +	mutex_lock(&mdev_list_lock);
> > +	list_for_each_entry(tmp, &mdev_list, next) {
> > +		if (tmp == mdev) {
> > +			found = true;
> > +			break;
> > +		}
> > +	}
> > +
> > +	if (found)
> > +		list_del(&mdev->next);
> > +
> > +	mutex_unlock(&mdev_list_lock);
> > +
> > +	if (!found)
> > +		return -ENODEV;
> > +
> >  	type = to_mdev_type(mdev->type_kobj);
> >  	parent = mdev->parent;
> >  	mutex_lock(&parent->lock);
> > @@ -342,6 +368,11 @@ int mdev_device_remove(struct device *dev, bool force_remove)
> >  	ret = mdev_device_remove_ops(mdev, force_remove);
> >  	if (ret) {
> >  		mutex_unlock(&parent->lock);
> > +
> > +		mutex_lock(&mdev_list_lock);
> > +		list_add(&mdev->next, &mdev_list);
> > +		mutex_unlock(&mdev_list_lock);
> > +
> >  		return ret;
> >  	}
> >  
> > @@ -349,7 +380,8 @@ int mdev_device_remove(struct device *dev, bool force_remove)
> >  	device_unregister(dev);
> >  	mutex_unlock(&parent->lock);
> >  	mdev_put_parent(parent);
> > -	return ret;
> > +
> > +	return 0;
> >  }
> >  
> >  static int __init mdev_init(void)
> >   

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