Re: [PATCH] vfio/mdev: fix possible memory leak in module init funcs

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 1 Dec 2022 02:00:57 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Alex Williamson <alex.williamson@redhat.com>
> > Sent: Thursday, December 1, 2022 7:06 AM
> > 
> > [Cc +vfio-ap, vfio-ccw]
> > 
> > On Fri, 18 Nov 2022 11:28:27 +0800
> > ruanjinjie <ruanjinjie@huawei.com> wrote:
> >   
> > > Inject fault while probing module, if device_register() fails,
> > > but the refcount of kobject is not decreased to 0, the name
> > > allocated in dev_set_name() is leaked. Fix this by calling
> > > put_device(), so that name can be freed in callback function
> > > kobject_cleanup().  
> 
> It's not just about the name. The problem of kboject not being
> released is a bigger one.
> 
> put_device() is always required no matter device_register()
> succeeds or not:
> 
> * NOTE: _Never_ directly free @dev after calling this function, even
>  * if it returned an error! Always use put_device() to give up the
>  * reference initialized in this function instead.
>  */
> int device_register(struct device *dev)
> 
> > > @@ -1430,8 +1430,10 @@ static int __init mbochs_dev_init(void)
> > >  	dev_set_name(&mbochs_dev, "%s", MBOCHS_NAME);
> > >
> > >  	ret = device_register(&mbochs_dev);
> > > -	if (ret)
> > > +	if (ret) {
> > > +		put_device(&mbochs_dev);
> > >  		goto err_class;
> > > +	}
> > >
> > >  	ret = mdev_register_parent(&mbochs_parent, &mbochs_dev,  
> > &mbochs_driver,  
> > >  				   mbochs_mdev_types,  
> > 
> > 
> > vfio-ap has a similar unwind as the sample drivers, but actually makes
> > an attempt to catch this ex:
> > 
> > 	...
> >         ret = device_register(&matrix_dev->device);
> >         if (ret)
> >                 goto matrix_reg_err;
> > 
> >         ret = driver_register(&matrix_driver);
> >         if (ret)
> >                 goto matrix_drv_err;
> > 
> >         return 0;
> > 
> > matrix_drv_err:
> >         device_unregister(&matrix_dev->device);
> > matrix_reg_err:
> >         put_device(&matrix_dev->device);
> > 	...
> > 
> > So of the vfio drivers calling device_register(), vfio-ap is the only
> > one that does a put_device() if device_register() fails, but it also
> > seems sketchy to call both device_unregister() and put_device() in the
> > case that we exit via matrix_drv_err.
> > 
> > I wonder if all of these shouldn't adopt a flow like:
> > 
> > 	ret = device_register(&dev);
> > 	if (ret)
> > 		goto err1;
> > 
> > 	....
> > 
> > 	return 0;
> > 
> > err2:
> > 	device_del(&dev);
> > err1:
> > 	put_device(&dev);
> >   
> 
> It's kind of a mixed model.
> 
> With above unwind it's clearer to use device_initialize() and device_add() instead.

That would go against the comment for device_register() recommending
that device_initialize() and device_add() should only be called
separately if we have a clearly defined need.  I can only imagine a
patch bot would quickly come along to rectify the situation if we
simply open code device_register() for aesthetics.  I don't see that
splitting device_unregister() for the purpose of having a common unwind
path necessitates any changes relative to device_register().

> Otherwise what this patch does looks better IMHO:
> 
> 	ret = device_register(&dev);
> 	if (ret) {
> 		put_device(&dev);
> 		goto err1;
> 	}
> 
> 	...
> 
> 	return 0;
> 
> err2:
> 	device_unregister(&dev);
> err1:
> 	earlier_unwind();
> 

This is essentially what was originally proposed.  It could also be
called a "mixed model", implementing part of the unwind in the error
branch before jumping to the common unwind.  As demonstrated below,
every current vfio driver calling device_register() follows a similar
goto unwind stack as found in the sample drivers, which makes it
trivially easy to split the device_unregister() call and add a goto
target in between.

Either way, they're equivalent and I'll take whichever version
addresses all the vfio related use cases and gets acks from their
maintainers.  Thanks,

Alex

diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
index c2a65808605a..54aba7cceb33 100644
--- a/drivers/s390/cio/vfio_ccw_drv.c
+++ b/drivers/s390/cio/vfio_ccw_drv.c
@@ -199,8 +199,9 @@ static int vfio_ccw_sch_probe(struct subchannel *sch)
 	return 0;
 
 out_unreg:
-	device_unregister(&parent->dev);
+	device_del(&parent->dev);
 out_free:
+	put_device(&parent->dev);
 	dev_set_drvdata(&sch->dev, NULL);
 	return ret;
 }
diff --git a/drivers/s390/crypto/vfio_ap_drv.c b/drivers/s390/crypto/vfio_ap_drv.c
index f43cfeabd2cc..997b524bdd2b 100644
--- a/drivers/s390/crypto/vfio_ap_drv.c
+++ b/drivers/s390/crypto/vfio_ap_drv.c
@@ -122,7 +122,7 @@ static int vfio_ap_matrix_dev_create(void)
 	return 0;
 
 matrix_drv_err:
-	device_unregister(&matrix_dev->device);
+	device_del(&matrix_dev->device);
 matrix_reg_err:
 	put_device(&matrix_dev->device);
 matrix_alloc_err:
diff --git a/samples/vfio-mdev/mbochs.c b/samples/vfio-mdev/mbochs.c
index 8b5a3a778a25..e54eb752e1ba 100644
--- a/samples/vfio-mdev/mbochs.c
+++ b/samples/vfio-mdev/mbochs.c
@@ -1430,7 +1430,7 @@ static int __init mbochs_dev_init(void)
 
 	ret = device_register(&mbochs_dev);
 	if (ret)
-		goto err_class;
+		goto err_put;
 
 	ret = mdev_register_parent(&mbochs_parent, &mbochs_dev, &mbochs_driver,
 				   mbochs_mdev_types,
@@ -1441,8 +1441,9 @@ static int __init mbochs_dev_init(void)
 	return 0;
 
 err_device:
-	device_unregister(&mbochs_dev);
-err_class:
+	device_del(&mbochs_dev);
+err_put:
+	put_device(&mbochs_dev);
 	class_destroy(mbochs_class);
 err_driver:
 	mdev_unregister_driver(&mbochs_driver);
diff --git a/samples/vfio-mdev/mdpy.c b/samples/vfio-mdev/mdpy.c
index 721fb06c6413..e8400fdab71d 100644
--- a/samples/vfio-mdev/mdpy.c
+++ b/samples/vfio-mdev/mdpy.c
@@ -717,7 +717,7 @@ static int __init mdpy_dev_init(void)
 
 	ret = device_register(&mdpy_dev);
 	if (ret)
-		goto err_class;
+		goto err_put;
 
 	ret = mdev_register_parent(&mdpy_parent, &mdpy_dev, &mdpy_driver,
 				   mdpy_mdev_types,
@@ -728,8 +728,9 @@ static int __init mdpy_dev_init(void)
 	return 0;
 
 err_device:
-	device_unregister(&mdpy_dev);
-err_class:
+	device_del(&mdpy_dev);
+err_put:
+	put_device(&mdpy_dev);
 	class_destroy(mdpy_class);
 err_driver:
 	mdev_unregister_driver(&mdpy_driver);
diff --git a/samples/vfio-mdev/mtty.c b/samples/vfio-mdev/mtty.c
index 3c2a421b9b69..e887de672c52 100644
--- a/samples/vfio-mdev/mtty.c
+++ b/samples/vfio-mdev/mtty.c
@@ -1330,7 +1330,7 @@ static int __init mtty_dev_init(void)
 
 	ret = device_register(&mtty_dev.dev);
 	if (ret)
-		goto err_class;
+		goto err_put;
 
 	ret = mdev_register_parent(&mtty_dev.parent, &mtty_dev.dev,
 				   &mtty_driver, mtty_mdev_types,
@@ -1340,8 +1340,9 @@ static int __init mtty_dev_init(void)
 	return 0;
 
 err_device:
-	device_unregister(&mtty_dev.dev);
-err_class:
+	device_del(&mtty_dev.dev);
+err_put:
+	put_device(&mtty_dev.dev);
 	class_destroy(mtty_dev.vd_class);
 err_driver:
 	mdev_unregister_driver(&mtty_driver);