Re: [PATCH v2] virtio-blk: handle block_device_operations callbacks after hot unplug

[Stefan Hajnoczi]

On Tue, Apr 28, 2020 at 11:25:07AM -0400, Michael S. Tsirkin wrote:
> On Tue, Apr 28, 2020 at 03:30:09PM +0100, Stefan Hajnoczi wrote:
> > A userspace process holding a file descriptor to a virtio_blk device can
> > still invoke block_device_operations after hot unplug.  For example, a
> > program that has /dev/vdb open can call ioctl(HDIO_GETGEO) after hot
> > unplug to invoke virtblk_getgeo().
> 
> 
> which causes what? a use after free?

Yes, use after free.  I will include the kernel panic in the next
revision.

virtio_check_driver_offered_feature() accesses vdev->dev.driver, which
doesn't work after virtblk_remove() has freed vdev :).

> > 
> > Introduce a reference count in struct virtio_blk so that its lifetime
> > covers both virtio_driver probe/remove and block_device_operations
> > open/release users.  This ensures that block_device_operations functions
> > like virtblk_getgeo() can safely access struct virtio_blk.
> > 
> > Add remove_mutex to prevent block_device_operations functions from
> > accessing vblk->vdev during virtblk_remove() and let the safely check
> 
> let the -> let them?

Thanks, will fix.

> 
> > for !vblk->vdev after virtblk_remove() returns.
> > 
> > Switching to a reference count also solves the vd_index_ida leak where
> > vda, vdb, vdc, etc indices were lost when the device was hot unplugged
> > while the block device was still open.
> 
> Can you move this statement up so we list both issues (use after free
> and leak) upfront, then discuss the fix?

Thanks, will fix.

> 
> > 
> > Reported-by: Lance Digby <ldigby@xxxxxxxxxx>
> > Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> > ---
> > If someone has a simpler solution please let me know.  I looked at
> > various approaches including reusing device_lock(&vblk->vdev.dev) but
> > they were more complex and extending the lifetime of virtio_device after
> > remove() has been called seems questionable.
> > ---
> >  drivers/block/virtio_blk.c | 85 ++++++++++++++++++++++++++++++++++----
> >  1 file changed, 77 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index 93468b7c6701..3dd53b445cc1 100644
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c
> > @@ -44,6 +44,13 @@ struct virtio_blk {
> >  	/* Process context for config space updates */
> >  	struct work_struct config_work;
> >  
> > +	/*
> > +	 * Tracks references from block_device_operations open/release and
> > +	 * virtio_driver probe/remove so this object can be freed once no
> > +	 * longer in use.
> > +	 */
> > +	refcount_t refs;
> > +
> >  	/* What host tells us, plus 2 for header & tailer. */
> >  	unsigned int sg_elems;
> >  
> > @@ -53,6 +60,9 @@ struct virtio_blk {
> >  	/* num of vqs */
> >  	int num_vqs;
> >  	struct virtio_blk_vq *vqs;
> > +
> > +	/* Provides mutual exclusion with virtblk_remove(). */
> 
> This is not the best way to document access rules.
> Which fields does this protect, exactly?
> I think it's just vdev. Right?
> Pls add to the comment.

Yes, vblk->vdev cannot be dereferenced after virtblk_remove() is
entered.

I'll document this mutex as protecting vdev.

> 
> > +	struct mutex remove_mutex;
> >  };
> >  
> >  struct virtblk_req {
> > @@ -295,10 +305,54 @@ static int virtblk_get_id(struct gendisk *disk, char *id_str)
> >  	return err;
> >  }
> >  
> > +static void virtblk_get(struct virtio_blk *vblk)
> > +{
> > +	refcount_inc(&vblk->refs);
> > +}
> > +
> > +static void virtblk_put(struct virtio_blk *vblk)
> > +{
> > +	if (refcount_dec_and_test(&vblk->refs)) {
> > +		ida_simple_remove(&vd_index_ida, vblk->index);
> > +		mutex_destroy(&vblk->remove_mutex);
> > +		kfree(vblk);
> > +	}
> > +}
> > +
> > +static int virtblk_open(struct block_device *bd, fmode_t mode)
> > +{
> > +	struct virtio_blk *vblk = bd->bd_disk->private_data;
> > +	int ret = -ENXIO;
> 
> 
> It's more common to do
> 
> 	int ret = 0;
> 
> and on error:
> 	ret = -ENXIO;
> 
> 
> let's do this.

Will fix.

> > +
> > +	mutex_lock(&vblk->remove_mutex);
> > +
> > +	if (vblk->vdev) {
> > +		virtblk_get(vblk);
> > +		ret = 0;
> > +	}
> 
> I prefer
> 	else
> 		ret = -ENXIO
> 
> here.

Got it.

> > +
> > +	mutex_unlock(&vblk->remove_mutex);
> > +	return ret;
> > +}
> > +
> > +static void virtblk_release(struct gendisk *disk, fmode_t mode)
> > +{
> > +	struct virtio_blk *vblk = disk->private_data;
> > +
> > +	virtblk_put(vblk);
> > +}
> > +
> >  /* We provide getgeo only to please some old bootloader/partitioning tools */
> >  static int virtblk_getgeo(struct block_device *bd, struct hd_geometry *geo)
> >  {
> >  	struct virtio_blk *vblk = bd->bd_disk->private_data;
> > +	int ret = -ENXIO;
> 
> It's more common to do
> 
> 	int ret = 0;
> 
> and on error:
> 	ret = -ENXIO;
> 
> 
> let's do this.

Will fix.

> 
> > +
> > +	mutex_lock(&vblk->remove_mutex);
> > +
> > +	if (!vblk->vdev) {
> > +		goto out;
> > +	}
> 
> 
> single lines are not supposed to use {}.
> if you add ret = -ENXIO here then it won't be a single line anymore
> though.

Oops, a QEMU coding style habit :).

> >  
> >  	/* see if the host passed in geometry config */
> >  	if (virtio_has_feature(vblk->vdev, VIRTIO_BLK_F_GEOMETRY)) {
> > @@ -314,11 +368,17 @@ static int virtblk_getgeo(struct block_device *bd, struct hd_geometry *geo)
> >  		geo->sectors = 1 << 5;
> >  		geo->cylinders = get_capacity(bd->bd_disk) >> 11;
> >  	}
> > -	return 0;
> > +
> > +	ret = 0;
> > +out:
> > +	mutex_unlock(&vblk->remove_mutex);
> > +	return ret;
> >  }
> >  
> >  static const struct block_device_operations virtblk_fops = {
> >  	.owner  = THIS_MODULE,
> > +	.open = virtblk_open,
> > +	.release = virtblk_release,
> >  	.getgeo = virtblk_getgeo,
> >  };
> >  
> > @@ -655,6 +715,10 @@ static int virtblk_probe(struct virtio_device *vdev)
> >  		goto out_free_index;
> >  	}
> >  
> > +	/* This reference is dropped in virtblk_remove(). */
> > +	refcount_set(&vblk->refs, 1);
> > +	mutex_init(&vblk->remove_mutex);
> > +
> >  	vblk->vdev = vdev;
> >  	vblk->sg_elems = sg_elems;
> >  
> > @@ -820,8 +884,12 @@ static int virtblk_probe(struct virtio_device *vdev)
> >  static void virtblk_remove(struct virtio_device *vdev)
> >  {
> >  	struct virtio_blk *vblk = vdev->priv;
> > -	int index = vblk->index;
> > -	int refc;
> > +
> > +	/*
> > +	 * Virtqueue processing is stopped safely here but mutual exclusion is
> > +	 * needed for block_device_operations.
> > +	 */
> > +	mutex_lock(&vblk->remove_mutex);
> >  
> >  	/* Make sure no work handler is accessing the device. */
> >  	flush_work(&vblk->config_work);
> > @@ -834,15 +902,16 @@ static void virtblk_remove(struct virtio_device *vdev)
> >  	/* Stop all the virtqueues. */
> >  	vdev->config->reset(vdev);
> >  
> > -	refc = kref_read(&disk_to_dev(vblk->disk)->kobj.kref);
> > +	/* Virtqueue are stopped, nothing can use vblk->vdev anymore. */
> 
> Virtqueues?

Thanks, will fix.

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