Re: [Qemu-block] [RHEL-7.2.z qemu-kvm-rhev PATCH] virtio-blk: Fix double completion for werror=stop

[Fam Zheng]

On Wed, 11/18 11:19, Fam Zheng wrote:
> BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1277922
> Brew: http://brewweb.devel.redhat.com/brew/taskinfo?taskID=10113752
> 
> When a request R is absorbed by request M, it is appended to the
> "mr_next" queue led by M, and is completed together with the completion
> of M, in virtio_blk_rw_complete.
> 
> During DMA restart in virtio_blk_dma_restart_bh, requests in s->rq are
> parsed and submitted again, possibly with a stale req->mr_next. It could
> be a problem if the request merging in virtio_blk_handle_request hasn't
> refreshed every mr_next pointer, in which case, virtio_blk_rw_complete
> could walk through unexpected requests following the stale pointers.
> 
> Fix this by unsetting the pointer in virtio_blk_rw_complete. It is safe
> because this req is either completed and freed right away, or it will be
> restarted and parsed from scratch out of the vq later.
> 
> Signed-off-by: Fam Zheng <address@hidden>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> (cherry picked from commit 10f5a72f70862d299ddbdf226d6dc71fa4ae34dd)
> Signed-off-by: Fam Zheng <address@hidden>
> ---
>  hw/block/virtio-blk.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> index f9301ae..fe146ad 100644
> --- a/hw/block/virtio-blk.c
> +++ b/hw/block/virtio-blk.c
> @@ -112,6 +112,10 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
>               * happen on the other side of the migration).
>               */
>              if (virtio_blk_handle_rw_error(req, -ret, is_read)) {
> +                /* Break the link in case the next request is added to the
> +                 * restart queue and is going to be parsed from the ring 
> again.
> +                 */
> +                req->mr_next = NULL;

Paolo pointed out this is a write-after-free with error=report, will need to
respin after fixing upstream.

NACK

>                  continue;
>              }
>          }
> -- 
> 2.4.3
>