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Subject: Re: [PATCH v3 2/6] vhost-user: introduce shared vhost-user state
On Thu, May 24, 2018 at 07:21:01AM +0800, Tiwei Bie wrote:
> On Wed, May 23, 2018 at 06:43:29PM +0300, Michael S. Tsirkin wrote:
> > On Wed, May 23, 2018 at 06:36:05PM +0300, Michael S. Tsirkin wrote:
> > > On Wed, May 23, 2018 at 04:44:51PM +0300, Michael S. Tsirkin wrote:
> > > > On Thu, Apr 12, 2018 at 11:12:28PM +0800, Tiwei Bie wrote:
> > > > > When multi queue is enabled e.g. for a virtio-net device,
> > > > > each queue pair will have a vhost_dev, and the only thing
> > > > > shared between vhost devs currently is the chardev. This
> > > > > patch introduces a vhost-user state structure which will
> > > > > be shared by all vhost devs of the same virtio device.
> > > > >
> > > > > Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> > > >
> > > > Unfortunately this patch seems to cause crashes.
> > > > To reproduce, simply run
> > > > make check-qtest-x86_64
> > > >
> > > > Sorry that it took me a while to find - it triggers 90% of runs but not
> > > > 100% which complicates bisection somewhat.
>
> It's my fault to not notice this bug before.
> I'm very sorry. Thank you so much for finding
> the root cause!
>
> > > >
> > > > > ---
> > > > > backends/cryptodev-vhost-user.c | 20 ++++++++++++++++++-
> > > > > hw/block/vhost-user-blk.c | 22 +++++++++++++++++++-
> > > > > hw/scsi/vhost-user-scsi.c | 20 ++++++++++++++++++-
> > > > > hw/virtio/Makefile.objs | 2 +-
> > > > > hw/virtio/vhost-stub.c | 10 ++++++++++
> > > > > hw/virtio/vhost-user.c | 31 +++++++++++++++++++---------
> > > > > include/hw/virtio/vhost-user-blk.h | 2 ++
> > > > > include/hw/virtio/vhost-user-scsi.h | 2 ++
> > > > > include/hw/virtio/vhost-user.h | 20 +++++++++++++++++++
> > > > > net/vhost-user.c | 40 ++++++++++++++++++++++++++++++-------
> > > > > 10 files changed, 149 insertions(+), 20 deletions(-)
> > > > > create mode 100644 include/hw/virtio/vhost-user.h
> [...]
> > > > > qemu_chr_fe_set_handlers(&s->chr, NULL, NULL,
> > > > > net_vhost_user_event, NULL, nc0->name, NULL,
> > > > > @@ -319,6 +336,15 @@ static int net_vhost_user_init(NetClientState *peer, const char *device,
> > > > > assert(s->vhost_net);
> > > > >
> > > > > return 0;
> > > > > +
> > > > > +err:
> > > > > + if (user) {
> > > > > + vhost_user_cleanup(user);
> > > > > + g_free(user);
> > > > > + s->vhost_user = NULL;
> > > > > + }
> > > > > +
> > > > > + return -1;
> > > > > }
> > > > >
> > > > > static Chardev *net_vhost_claim_chardev(
> > > > > --
> > > > > 2.11.0
> > >
> > > So far I figured out that commenting the free of
> > > the structure removes the crash, so we seem to
> > > be dealing with a use-after free here.
> > > I suspect that in a MQ situation, one queue gets
> > > closed and attempts to free the structure
> > > while others still use it.
> > >
> > > diff --git a/net/vhost-user.c b/net/vhost-user.c
> > > index 525a061..6a1573b 100644
> > > --- a/net/vhost-user.c
> > > +++ b/net/vhost-user.c
> > > @@ -157,8 +157,8 @@ static void net_vhost_user_cleanup(NetClientState *nc)
> > > s->vhost_net = NULL;
> > > }
> > > if (s->vhost_user) {
> > > - vhost_user_cleanup(s->vhost_user);
> > > - g_free(s->vhost_user);
> > > + //vhost_user_cleanup(s->vhost_user);
> > > + //g_free(s->vhost_user);
> > > s->vhost_user = NULL;
> > > }
> > > if (nc->queue_index == 0) {
> > > @@ -339,8 +339,8 @@ static int net_vhost_user_init(NetClientState *peer, const char *device,
> > >
> > > err:
> > > if (user) {
> > > - vhost_user_cleanup(user);
> > > - g_free(user);
> > > + //vhost_user_cleanup(user);
> > > + //g_free(user);
> > > s->vhost_user = NULL;
> > > }
> > >
> >
> >
> > So the following at least gets rid of the crashes.
> > I am not sure it does not leak memory though,
> > and not sure there aren't any configurations where
> > the 1st queue gets cleaned up first.
> >
> > Thoughts?
>
> Thank you so much for catching it and fixing
> it! I'll keep your SoB there. Thank you so
> much! I do appreciate it!
>
> You are right. This structure is freed multiple
> times when multi-queue is enabled.
After a deeper digging, I got your point now..
It could be a use-after-free instead of a double
free.. As it's safe to deinit the char which is
shared by all queue pairs when cleanup the 1st
queue pair, it should be safe to free vhost-user
structure there too.
>
> I think it's safe to let the first queue pair
> free the vhost-user structure, because it won't
> be touched by other queue pairs during cleanup.
>
> Best regards,
> Tiwei Bie
>
>
> >
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> >
> > ---
> >
> > diff --git a/net/vhost-user.c b/net/vhost-user.c
> > index 525a061..7549d25 100644
> > --- a/net/vhost-user.c
> > +++ b/net/vhost-user.c
> > @@ -156,19 +156,20 @@ static void net_vhost_user_cleanup(NetClientState *nc)
> > g_free(s->vhost_net);
> > s->vhost_net = NULL;
> > }
> > - if (s->vhost_user) {
> > - vhost_user_cleanup(s->vhost_user);
> > - g_free(s->vhost_user);
> > - s->vhost_user = NULL;
> > - }
> > if (nc->queue_index == 0) {
> > if (s->watch) {
> > g_source_remove(s->watch);
> > s->watch = 0;
> > }
> > qemu_chr_fe_deinit(&s->chr, true);
> > + if (s->vhost_user) {
> > + vhost_user_cleanup(s->vhost_user);
> > + g_free(s->vhost_user);
> > + }
> > }
> >
> > + s->vhost_user = NULL;
Maybe we should move above line, like:
if (nc->queue_index == 0) {
if (s->watch) {
g_source_remove(s->watch);
s->watch = 0;
}
qemu_chr_fe_deinit(&s->chr, true);
+ if (s->vhost_user) {
+ vhost_user_cleanup(s->vhost_user);
+ g_free(s->vhost_user);
+ s->vhost_user = NULL;
+ }
}
otherwise s->vhost_user may not be freed.
> > +
> > qemu_purge_queued_packets(nc);
> > }
> >
> > @@ -341,7 +342,6 @@ err:
> > if (user) {
> > vhost_user_cleanup(user);
> > g_free(user);
> > - s->vhost_user = NULL;
I don't get why cannot zero it in this case.
> > }
> >
> > return -1;
Best regards,
Tiwei Bie
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