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>
---
 block/block-backend.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index b48c91f4e1..d98a96ff37 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2576,14 +2576,25 @@ static void blk_root_drained_end(BdrvChild *child, int *drained_end_counter)
 
 bool blk_register_buf(BlockBackend *blk, void *host, size_t size, Error **errp)
 {
+    BlockDriverState *bs = blk_bs(blk);
+
     GLOBAL_STATE_CODE();
-    return bdrv_register_buf(blk_bs(blk), host, size, errp);
+
+    if (bs) {
+        return bdrv_register_buf(bs, host, size, errp);
+    }
+    return true;
 }
 
 void blk_unregister_buf(BlockBackend *blk, void *host, size_t size)
 {
+    BlockDriverState *bs = blk_bs(blk);
+
     GLOBAL_STATE_CODE();
-    bdrv_unregister_buf(blk_bs(blk), host, size);
+
+    if (bs) {
+        bdrv_unregister_buf(bs, host, size);
+    }
 }
 
 int coroutine_fn blk_co_copy_range(BlockBackend *blk_in, int64_t off_in,
-- 
2.38.1

Merged. I will work on supporting graph changes.

Stefan

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

On Tue, 22 Nov 2022 at 03:22, Kevin Wolf <kwolf@redhat.com> wrote:
>
> 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?

I think you are right. Graph changes need to be handled. Right now they aren't.

Stefan

On 21/11/22 22:19, Stefan Hajnoczi wrote:
> 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>
> ---
>   block/block-backend.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>