Re: [PATCH for-7.2] block-backend: avoid bdrv_unregister_buf() NULL pointer deref

[Kevin Wolf <kwolf@redhat.com>]

Am 21.11.2022 um 22:19 hat Stefan Hajnoczi geschrieben:
> bdrv_*() APIs expect a valid BlockDriverState. Calling them with bs=NULL
> leads to undefined behavior.
> 
> Jonathan Cameron reported this following NULL pointer dereference when a
> VM with a virtio-blk device and a memory-backend-file object is
> terminated:
> 1. qemu_cleanup() closes all drives, setting blk->root to NULL
> 2. qemu_cleanup() calls user_creatable_cleanup(), which results in a RAM
>    block notifier callback because the memory-backend-file is destroyed.
> 3. blk_unregister_buf() is called by virtio-blk's BlockRamRegistrar
>    notifier callback and undefined behavior occurs.
> 
> Fixes: baf422684d73 ("virtio-blk: use BDRV_REQ_REGISTERED_BUF optimization hint")
> Co-authored-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

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

This raises some questions, though. What happens if the graph isn't
static between creation and deletion of the device? Do we need to do
something with registered buffers when a node is attached to or detached
from an existing device?

Kevin