Re: [PATCH v2] chardev: avoid use-after-free when client disconnect

[Daniel P . Berrangé]

On Wed, Jul 20, 2022 at 08:55:46AM +0000, Wangjing(Hogan) wrote:
> 
> > On Wed, Jul 20, 2022 at 11:36:07AM +0400, Marc-André Lureau wrote:
> > > Hi
> > > 
> > > On Wed, Jul 20, 2022 at 11:13 AM Hogan Wang via 
> > > <qemu-devel@nongnu.org>
> > > wrote:
> > > 
> > > > IOWatchPoll object did not hold the @ioc and @src objects reference, 
> > > > then io_watch_poll_prepare execute in IO thread, if IOWatchPoll 
> > > > removed by mian thread, then io_watch_poll_prepare access @ioc or
> > > >
> > > 
> > > mian->main
> > > 
> > > 
> > > > @src concurrently lead to coredump.
> > > >
> > > > In IO thread monitor scene, the IO thread used to accept client, 
> > > > receive qmp request and handle hung-up event. Main thread used to 
> > > > handle qmp request and send response, it will remove IOWatchPoll and 
> > > > free @ioc when send response fail, then cause use-after-free
> > > >
> > > 
> > > I wonder if we are misusing GSources in that case, by removing sources 
> > > from different threads.. Could you be more specific about the code 
> > > path that leads to that?
> > 
> > It is permitted, but unfortunately every version of glib prior to 2.64 has 
> > a race condition that means you'll periodically get a use-after-free and a 
> > crash:
> > 
> >   https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1358
> > 
> > Libvirt worked around this problem by not calling 'g_source_unref'
> > directly, but instead have a helper that uses g_idle_add to delay the unref 
> > such that its guaranteed to happen inside the main event loop thread.
> > 
> > So I'd like to know what version of glib Hogan is using 
> 
> I am using glib2-2.62.5 in test environment, so it's looks like a glib2 known 
> issue.

Hmm, actually the fix should have been backported into the 2.62.5
release according to this

  https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1361

With regards,
Daniel
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