Re: [PATCH for 3.4] virtio-scsi: fix TMF use-after-free

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Wed, 2012-04-18 at 15:46 +0200, Paolo Bonzini wrote:
> Fix a race in TMF path, where cmd may have been already freed
> by virtscsi_complete_free after waking up from the completion.

There's no may about this; the command will be freed long before the
completion waiter is awoken.  The description could be clearer.

The problem is a use after free in virtscsi_tmf because the
virtio_scsi_command is freed before the completion returns.

The fix is to make callers specifying a completion responsible for
freeing the command in all cases.

James

> Cc: James Bottomley <JBottomley@parallels.com>
> Cc: linux-scsi@vger.kernel.org
> Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  drivers/scsi/virtio_scsi.c |   24 +++++++++++++-----------
>  1 file changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
> index efccd72..1b38431 100644
> --- a/drivers/scsi/virtio_scsi.c
> +++ b/drivers/scsi/virtio_scsi.c
> @@ -175,7 +175,8 @@ static void virtscsi_complete_free(void *buf)
>  
>  	if (cmd->comp)
>  		complete_all(cmd->comp);
> -	mempool_free(cmd, virtscsi_cmd_pool);
> +	else
> +		mempool_free(cmd, virtscsi_cmd_pool);
>  }
>  
>  static void virtscsi_ctrl_done(struct virtqueue *vq)
> @@ -311,21 +312,22 @@ out:
>  static int virtscsi_tmf(struct virtio_scsi *vscsi, struct virtio_scsi_cmd *cmd)
>  {
>  	DECLARE_COMPLETION_ONSTACK(comp);
> -	int ret;
> +	int ret = FAILED;
>  
>  	cmd->comp = &comp;
> -	ret = virtscsi_kick_cmd(vscsi, vscsi->ctrl_vq, cmd,
> -			       sizeof cmd->req.tmf, sizeof cmd->resp.tmf,
> -			       GFP_NOIO);
> -	if (ret < 0)
> -		return FAILED;
> +	if (virtscsi_kick_cmd(vscsi, vscsi->ctrl_vq, cmd,
> +			      sizeof cmd->req.tmf, sizeof cmd->resp.tmf,
> +			      GFP_NOIO) < 0)
> +		goto out;
>  
>  	wait_for_completion(&comp);
> -	if (cmd->resp.tmf.response != VIRTIO_SCSI_S_OK &&
> -	    cmd->resp.tmf.response != VIRTIO_SCSI_S_FUNCTION_SUCCEEDED)
> -		return FAILED;
> +	if (cmd->resp.tmf.response == VIRTIO_SCSI_S_OK ||
> +	    cmd->resp.tmf.response == VIRTIO_SCSI_S_FUNCTION_SUCCEEDED)
> +		ret = SUCCESS;
>  
> -	return SUCCESS;
> +out:
> +	mempool_free(cmd, virtscsi_cmd_pool);
> +	return ret;
>  }
>  
>  static int virtscsi_device_reset(struct scsi_cmnd *sc)