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?
>
> 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?
> 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?
>
> 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.
> + 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.
> +
> + mutex_lock(&vblk->remove_mutex);
> +
> + if (vblk->vdev) {
> + virtblk_get(vblk);
> + ret = 0;
> + }
I prefer
else
ret = -ENXIO
here.
> +
> + 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.
> +
> + 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.
>
> /* 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?
> + vblk->vdev = NULL;
> +
> put_disk(vblk->disk);
> vdev->config->del_vqs(vdev);
> kfree(vblk->vqs);
> - kfree(vblk);
>
> - /* Only free device id if we don't have any users */
> - if (refc == 1)
> - ida_simple_remove(&vd_index_ida, index);
> + mutex_unlock(&vblk->remove_mutex);
> +
> + virtblk_put(vblk);
> }
>
> #ifdef CONFIG_PM_SLEEP
> --
> 2.25.3
>
