Re: [Qemu-devel] [PATCH] vhost-user: fix qemu crash caused by failed backend

[Marc-André Lureau]

On Tue, Jan 15, 2019 at 7:59 AM Michael S. Tsirkin <address@hidden> wrote:
>
> On Tue, Oct 02, 2018 at 01:54:25PM +0400, Marc-André Lureau wrote:
> > Hi
> >
> > On Thu, Sep 27, 2018 at 7:37 PM Liang Li <address@hidden> wrote:
> > >
> > > During live migration, when stopping vhost-user device, 'vhost_dev_stop'
> > > will be called, 'vhost_dev_stop' will call a batch of 'vhost_user_read'
> > > and 'vhost_user_write'. If a previous 'vhost_user_read' or 
> > > 'vhost_user_write'
> > > failed because the vhost user backend failed, the 'CHR_EVENT_CLOSED' event
> > > will be triggerd, followed by the call chain 
> > > chr_closed_bh()->vhost_user_stop()->
> > > vhost_net_cleanup()->vhost_dev_cleanup()
> > >
> > > vhost_dev_cleanup will clear vhost_dev struct, so the later 
> > > 'vhost_user_read'
> > > or 'vhost_user_read' will reference null pointer and cause qemu crash
> >
> > Do you have a backtrace to help understand the issue?
> > thanks
>
> Marc-André, Maxime any input on this patch?

It looks like this may be reproducible today. It would be great to
have a test or an easy way to reproduce it on master. It could be
based on existing vhost-user-test migration test.

> I agree flags like break_down don't exactly look elegant ...

Indeed, and its state is somehow managed by both vhost-user and
vhost-net. I wonder if vhost_dev_cleanup() shouldn't set it, instead
of vhost_net_mark_break_down().

>
>
> > >
> > > Signed-off-by: Liang Li <address@hidden>
> > > ---
> > >  hw/net/vhost_net.c        |  6 ++++++
> > >  hw/virtio/vhost-user.c    | 15 +++++++++++++--
> > >  include/hw/virtio/vhost.h |  1 +
> > >  include/net/vhost_net.h   |  1 +
> > >  net/vhost-user.c          |  3 +++
> > >  5 files changed, 24 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> > > index e037db6..77994e9 100644
> > > --- a/hw/net/vhost_net.c
> > > +++ b/hw/net/vhost_net.c
> > > @@ -113,6 +113,11 @@ uint64_t vhost_net_get_features(struct vhost_net 
> > > *net, uint64_t features)
> > >              features);
> > >  }
> > >
> > > +void vhost_net_mark_break_down(struct vhost_net *net)
> > > +{
> > > +    net->dev.break_down = true;
> > > +}
> > > +
> > >  void vhost_net_ack_features(struct vhost_net *net, uint64_t features)
> > >  {
> > >      net->dev.acked_features = net->dev.backend_features;
> > > @@ -156,6 +161,7 @@ struct vhost_net *vhost_net_init(VhostNetOptions 
> > > *options)
> > >      net->dev.max_queues = 1;
> > >      net->dev.nvqs = 2;
> > >      net->dev.vqs = net->vqs;
> > > +    net->dev.break_down = false;
> > >
> > >      if (backend_kernel) {
> > >          r = vhost_net_get_fd(options->net_backend);
> > > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > > index b041343..1394719 100644
> > > --- a/hw/virtio/vhost-user.c
> > > +++ b/hw/virtio/vhost-user.c
> > > @@ -213,14 +213,20 @@ static bool ioeventfd_enabled(void)
> > >  static int vhost_user_read(struct vhost_dev *dev, VhostUserMsg *msg)
> > >  {
> > >      struct vhost_user *u = dev->opaque;
> > > -    CharBackend *chr = u->user->chr;
> > > +    CharBackend *chr;
> > >      uint8_t *p = (uint8_t *) msg;
> > >      int r, size = VHOST_USER_HDR_SIZE;
> > >
> > > +    if (dev->break_down) {
> > > +        goto fail;
> > > +    }
> > > +
> > > +    chr = u->user->chr;
> > >      r = qemu_chr_fe_read_all(chr, p, size);
> > >      if (r != size) {
> > >          error_report("Failed to read msg header. Read %d instead of %d."
> > >                       " Original request %d.", r, size, msg->hdr.request);
> > > +        dev->break_down = true;
> > >          goto fail;
> > >      }
> > >
> > > @@ -299,9 +305,12 @@ static int vhost_user_write(struct vhost_dev *dev, 
> > > VhostUserMsg *msg,
> > >                              int *fds, int fd_num)
> > >  {
> > >      struct vhost_user *u = dev->opaque;
> > > -    CharBackend *chr = u->user->chr;
> > > +    CharBackend *chr;
> > >      int ret, size = VHOST_USER_HDR_SIZE + msg->hdr.size;
> > >
> > > +    if (dev->break_down) {
> > > +        return -1;
> > > +    }
> > >      /*
> > >       * For non-vring specific requests, like VHOST_USER_SET_MEM_TABLE,
> > >       * we just need send it once in the first time. For later such
> > > @@ -312,6 +321,7 @@ static int vhost_user_write(struct vhost_dev *dev, 
> > > VhostUserMsg *msg,
> > >          return 0;
> > >      }
> > >
> > > +    chr = u->user->chr;
> > >      if (qemu_chr_fe_set_msgfds(chr, fds, fd_num) < 0) {
> > >          error_report("Failed to set msg fds.");
> > >          return -1;
> > > @@ -319,6 +329,7 @@ static int vhost_user_write(struct vhost_dev *dev, 
> > > VhostUserMsg *msg,
> > >
> > >      ret = qemu_chr_fe_write_all(chr, (const uint8_t *) msg, size);
> > >      if (ret != size) {
> > > +        dev->break_down = true;
> > >          error_report("Failed to write msg."
> > >                       " Wrote %d instead of %d.", ret, size);
> > >          return -1;
> > > diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> > > index a7f449f..86d0dc5 100644
> > > --- a/include/hw/virtio/vhost.h
> > > +++ b/include/hw/virtio/vhost.h
> > > @@ -74,6 +74,7 @@ struct vhost_dev {
> > >      bool started;
> > >      bool log_enabled;
> > >      uint64_t log_size;
> > > +    bool break_down;
> > >      Error *migration_blocker;
> > >      const VhostOps *vhost_ops;
> > >      void *opaque;
> > > diff --git a/include/net/vhost_net.h b/include/net/vhost_net.h
> > > index 77e4739..06f2c08 100644
> > > --- a/include/net/vhost_net.h
> > > +++ b/include/net/vhost_net.h
> > > @@ -27,6 +27,7 @@ void vhost_net_cleanup(VHostNetState *net);
> > >
> > >  uint64_t vhost_net_get_features(VHostNetState *net, uint64_t features);
> > >  void vhost_net_ack_features(VHostNetState *net, uint64_t features);
> > > +void vhost_net_mark_break_down(VHostNetState *net);
> > >
> > >  bool vhost_net_virtqueue_pending(VHostNetState *net, int n);
> > >  void vhost_net_virtqueue_mask(VHostNetState *net, VirtIODevice *dev,
> > > diff --git a/net/vhost-user.c b/net/vhost-user.c
> > > index a39f9c9..b99e20b 100644
> > > --- a/net/vhost-user.c
> > > +++ b/net/vhost-user.c
> > > @@ -270,6 +270,9 @@ static void net_vhost_user_event(void *opaque, int 
> > > event)
> > >          if (s->watch) {
> > >              AioContext *ctx = qemu_get_current_aio_context();
> > >
> > > +            if (s->vhost_net) {
> > > +                vhost_net_mark_break_down(s->vhost_net);
> > > +            }
> > >              g_source_remove(s->watch);
> > >              s->watch = 0;
> > >              qemu_chr_fe_set_handlers(&s->chr, NULL, NULL, NULL, NULL,
> > > --
> > > 1.8.3.1
> > >
> > >
> >
> >
> > --
> > Marc-André Lureau



-- 
Marc-André Lureau