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@xxxxxxxxxx>
> Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> ---
> 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
>
