Re: [Qemu-devel] [PATCH] vhost: fix crash on virtio_error while device stop

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Dec 06, 2017 at 04:06:18PM +0300, Ilya Maximets wrote:
> In case virtio error occured after vhost_dev_close(), qemu will crash
> in nested cleanup while checking IOMMU flag because dev->vdev already
> set to zero and resources are already freed.
> 
> Example:
> 
> Program received signal SIGSEGV, Segmentation fault.
> vhost_virtqueue_stop at hw/virtio/vhost.c:1155
> 
>     #0  vhost_virtqueue_stop at hw/virtio/vhost.c:1155
>     #1  vhost_dev_stop at hw/virtio/vhost.c:1594
>     #2  vhost_net_stop_one at hw/net/vhost_net.c:289
>     #3  vhost_net_stop at hw/net/vhost_net.c:368
> 
>             Nested call to vhost_net_stop(). First time was at #14.
> 
>     #4  virtio_net_vhost_status at hw/net/virtio-net.c:180
>     #5  virtio_net_set_status (status=79) at hw/net/virtio-net.c:254
>     #6  virtio_set_status at hw/virtio/virtio.c:1146
>     #7  virtio_error at hw/virtio/virtio.c:2455
> 
>         virtqueue_get_head() failed here.
> 
>     #8  virtqueue_get_head at hw/virtio/virtio.c:543
>     #9  virtqueue_drop_all at hw/virtio/virtio.c:984
>     #10 virtio_net_drop_tx_queue_data at hw/net/virtio-net.c:240
>     #11 virtio_bus_set_host_notifier at hw/virtio/virtio-bus.c:297
>     #12 vhost_dev_disable_notifiers at hw/virtio/vhost.c:1431
> 
>         vhost_dev_stop() was executed here. dev->vdev == NULL now.
> 
>     #13 vhost_net_stop_one at hw/net/vhost_net.c:290
>     #14 vhost_net_stop at hw/net/vhost_net.c:368
>     #15 virtio_net_vhost_status at hw/net/virtio-net.c:180
>     #16 virtio_net_set_status (status=15) at hw/net/virtio-net.c:254
>     #17 qmp_set_link ("hostnet0", up=false) at net/net.c:1430
>     #18 chr_closed_bh at net/vhost-user.c:214
>     #19 aio_bh_call at util/async.c:90
>     #20 aio_bh_poll at util/async.c:118
>     #21 aio_dispatch at util/aio-posix.c:429
>     #22 aio_ctx_dispatch at util/async.c:261
>     #23 g_main_context_dispatch
>     #24 glib_pollfds_poll at util/main-loop.c:213
>     #25 os_host_main_loop_wait at util/main-loop.c:261
>     #26 main_loop_wait at util/main-loop.c:515
>     #27 main_loop () at vl.c:1917
>     #28 main at vl.c:4795
> 
> Above backtrace captured from qemu crash on vhost disconnect while
> virtio driver in guest was in failed state.
> 
> We can just add checking for 'vdev' in 'vhost_dev_has_iommu()' but
> it will assert further trying to free already freed ioeventfds. The
> real problem is that we're allowing nested calls to 'vhost_net_stop'.
> 
> This patch is aimed to forbid nested calls to 'vhost_net_stop' to avoid
> any possible double frees and segmentation faults doue to using of
> already freed resources by setting 'vhost_started' flag to zero prior
> to 'vhost_net_stop' call.
> 
> Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
> ---
> 
> This issue was already addressed more than a year ago by the following
> patch: https://lists.gnu.org/archive/html/qemu-devel/2016-03/msg06732.html
> but it was dropped without review due to not yet implemented re-connection
> of vhost-user. Re-connection implementation lately fixed most of the
> nested calls, but few of them still exists. For example, above backtrace
> captured after 'virtqueue_get_head()' failure on vhost-user disconnection.
> 
>  hw/net/virtio-net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 38674b0..4d95a18 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -177,8 +177,8 @@ static void virtio_net_vhost_status(VirtIONet *n, uint8_t status)
>              n->vhost_started = 0;
>          }
>      } else {
> -        vhost_net_stop(vdev, n->nic->ncs, queues);
>          n->vhost_started = 0;
> +        vhost_net_stop(vdev, n->nic->ncs, queues);
>      }
>  }

Well the wider context is


        n->vhost_started = 1;
        r = vhost_net_start(vdev, n->nic->ncs, queues);
        if (r < 0) {
            error_report("unable to start vhost net: %d: "
                         "falling back on userspace virtio", -r);
            n->vhost_started = 0;
        }
    } else {
        vhost_net_stop(vdev, n->nic->ncs, queues);
        n->vhost_started = 0;

So we set it to 1 before start, we should clear after stop.


> -- 
> 2.7.4