Re: [PATCH v3] virtio-blk: handle block_device_operations callbacks after hot unplug

[Stefano Garzarella <sgarzare@redhat.com>]

On Wed, Apr 29, 2020 at 05:53:45PM +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.  This leads to a
> use-after-free accessing vblk->vdev in virtblk_getgeo() when
> ioctl(HDIO_GETGEO) is invoked:
> 
>   BUG: unable to handle kernel NULL pointer dereference at 0000000000000090
>   IP: [<ffffffffc00e5450>] virtio_check_driver_offered_feature+0x10/0x90 [virtio]
>   PGD 800000003a92f067 PUD 3a930067 PMD 0
>   Oops: 0000 [#1] SMP
>   CPU: 0 PID: 1310 Comm: hdio-getgeo Tainted: G           OE  ------------   3.10.0-1062.el7.x86_64 #1
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
>   task: ffff9be5fbfb8000 ti: ffff9be5fa890000 task.ti: ffff9be5fa890000
>   RIP: 0010:[<ffffffffc00e5450>]  [<ffffffffc00e5450>] virtio_check_driver_offered_feature+0x10/0x90 [virtio]
>   RSP: 0018:ffff9be5fa893dc8  EFLAGS: 00010246
>   RAX: ffff9be5fc3f3400 RBX: ffff9be5fa893e30 RCX: 0000000000000000
>   RDX: 0000000000000000 RSI: 0000000000000004 RDI: ffff9be5fbc10b40
>   RBP: ffff9be5fa893dc8 R08: 0000000000000301 R09: 0000000000000301
>   R10: 0000000000000000 R11: 0000000000000000 R12: ffff9be5fdc24680
>   R13: ffff9be5fbc10b40 R14: ffff9be5fbc10480 R15: 0000000000000000
>   FS:  00007f1bfb968740(0000) GS:ffff9be5ffc00000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 0000000000000090 CR3: 000000003a894000 CR4: 0000000000360ff0
>   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>   Call Trace:
>    [<ffffffffc016ac37>] virtblk_getgeo+0x47/0x110 [virtio_blk]
>    [<ffffffff8d3f200d>] ? handle_mm_fault+0x39d/0x9b0
>    [<ffffffff8d561265>] blkdev_ioctl+0x1f5/0xa20
>    [<ffffffff8d488771>] block_ioctl+0x41/0x50
>    [<ffffffff8d45d9e0>] do_vfs_ioctl+0x3a0/0x5a0
>    [<ffffffff8d45dc81>] SyS_ioctl+0xa1/0xc0
> 
> A related problem is that virtblk_remove() leaks the vd_index_ida index
> when something still holds a reference to vblk->disk during hot unplug.
> This causes virtio-blk device names to be lost (vda, vdb, etc).
> 
> Fix these issues by protecting vblk->vdev with a mutex and reference
> counting vblk so the vd_index_ida index can be removed in all cases.
> 
> Fixes: 48e4043d4529523cbc7fa8dd745bd8e2c45ce1d3
>        ("virtio: add virtio disk geometry feature")
> Reported-by: Lance Digby <ldigby@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  drivers/block/virtio_blk.c | 87 ++++++++++++++++++++++++++++++++++----
>  1 file changed, 79 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 93468b7c6701..6f7f277495f4 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -33,6 +33,16 @@ struct virtio_blk_vq {
>  } ____cacheline_aligned_in_smp;
>  
>  struct virtio_blk {
> +	/*
> +	 * vdev may be accessed without taking this mutex in blk-mq and
> +	 * virtqueue code paths because virtblk_remove() stops them before vdev
> +	 * is freed.
> +	 *
> +	 * Everything else must hold this mutex when accessing vdev and must
> +	 * handle the case where vdev is NULL after virtblk_remove() has been
> +	 * called.
> +	 */
> +	struct mutex vdev_mutex;

The patch LGTM, I'm just worried about cache_type_store() and
cache_type_show() because IIUC they aren't in blk-mq and virtqueue code
paths, but they use vdev.

Do we have to take the mutex there too?

Thanks,
Stefano

>  	struct virtio_device *vdev;
>  
>  	/* The disk structure for the kernel. */
> @@ -44,6 +54,13 @@ 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;
>  
> @@ -295,10 +312,55 @@ 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->vdev_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 = 0;
> +
> +	mutex_lock(&vblk->vdev_mutex);
> +
> +	if (vblk->vdev)
> +		virtblk_get(vblk);
> +	else
> +		ret = -ENXIO;
> +
> +	mutex_unlock(&vblk->vdev_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 = 0;
> +
> +	mutex_lock(&vblk->vdev_mutex);
> +
> +	if (!vblk->vdev) {
> +		ret = -ENXIO;
> +		goto out;
> +	}
>  
>  	/* see if the host passed in geometry config */
>  	if (virtio_has_feature(vblk->vdev, VIRTIO_BLK_F_GEOMETRY)) {
> @@ -314,11 +376,15 @@ 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;
> +out:
> +	mutex_unlock(&vblk->vdev_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 +721,10 @@ 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->vdev_mutex);
> +
>  	vblk->vdev = vdev;
>  	vblk->sg_elems = sg_elems;
>  
> @@ -820,8 +890,6 @@ 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;
>  
>  	/* Make sure no work handler is accessing the device. */
>  	flush_work(&vblk->config_work);
> @@ -831,18 +899,21 @@ static void virtblk_remove(struct virtio_device *vdev)
>  
>  	blk_mq_free_tag_set(&vblk->tag_set);
>  
> +	mutex_lock(&vblk->vdev_mutex);
> +
>  	/* Stop all the virtqueues. */
>  	vdev->config->reset(vdev);
>  
> -	refc = kref_read(&disk_to_dev(vblk->disk)->kobj.kref);
> +	/* Virtqueues are stopped, nothing can use vblk->vdev anymore. */
> +	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->vdev_mutex);
> +
> +	virtblk_put(vblk);
>  }
>  
>  #ifdef CONFIG_PM_SLEEP
> -- 
> 2.25.3
>