Re: [PATCH v2] block-backend: prevent dangling BDS pointers across aio_poll()

[Kevin Wolf <kwolf@redhat.com>]

Am 14.12.2021 um 15:35 hat Stefan Hajnoczi geschrieben:
> The BlockBackend root child can change when aio_poll() is invoked. This
> happens when a temporary filter node is removed upon blockjob
> completion, for example.
> 
> Functions in block/block-backend.c must be aware of this when using a
> blk_bs() pointer across aio_poll() because the BlockDriverState refcnt
> may reach 0, resulting in a stale pointer.
> 
> One example is scsi_device_purge_requests(), which calls blk_drain() to
> wait for in-flight requests to cancel. If the backup blockjob is active,
> then the BlockBackend root child is a temporary filter BDS owned by the
> blockjob. The blockjob can complete during bdrv_drained_begin() and the
> last reference to the BDS is released when the temporary filter node is
> removed. This results in a use-after-free when blk_drain() calls
> bdrv_drained_end(bs) on the dangling pointer.
> 
> Explicitly hold a reference to bs across block APIs that invoke
> aio_poll().
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> v2:
> - Audit block/block-backend.c and fix additional cases
> ---
>  block/block-backend.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/block/block-backend.c b/block/block-backend.c
> index 12ef80ea17..a40ad7fa92 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -828,10 +828,12 @@ void blk_remove_bs(BlockBackend *blk)
>      notifier_list_notify(&blk->remove_bs_notifiers, blk);
>      if (tgm->throttle_state) {
>          bs = blk_bs(blk);
> +        bdrv_ref(bs);
>          bdrv_drained_begin(bs);
>          throttle_group_detach_aio_context(tgm);
>          throttle_group_attach_aio_context(tgm, qemu_get_aio_context());
>          bdrv_drained_end(bs);
> +        bdrv_unref(bs);
>      }
>  
>      blk_update_root_state(blk);

This hunk is unnecessary, we still hold a reference that is only given
up a few lines down with bdrv_root_unref_child(root).

The rest looks good to me, so without this hunk:

Reviewed-by: Kevin Wolf <kwolf@redhat.com>