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

[Michael S. Tsirkin]

On Thu, Dec 07, 2017 at 09:39:36AM +0300, Ilya Maximets wrote:
> On 06.12.2017 19:45, Michael S. Tsirkin wrote:
> > 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 <address@hidden>
> >> ---
> >>
> >> 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.
> 
> OK. I agree that clearing after is a bit safer. But in this case we need
> a separate flag or other way to detect that we're already inside the
> 'vhost_net_stop()'.
> 
> What do you think about that old patch:
> https://lists.gnu.org/archive/html/qemu-devel/2016-03/msg06732.html  ?
> 
> It implements the same thing but introduces additional flag. It even could
> be still applicable.

I see. I think the issue is with how host set status works.
It only sets the status afterwards.

So here's the plan:
- set status to set the new status first, and
  pass the old status as parameter
- fix all callbacks

-- 
MST