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Re: [Qemu-devel] chardev's and fd's in monitors

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] chardev's and fd's in monitors
Date: Thu, 20 Oct 2016 09:34:36 +0100
User-agent: Mutt/1.7.0 (2016-08-17)

On Wed, Oct 19, 2016 at 10:51:07PM +0200, Paolo Bonzini wrote:
> On 19/10/2016 19:01, Dr. David Alan Gilbert wrote:
> > * Paolo Bonzini (address@hidden) wrote:
> >>
> >>
> >> On 18/10/2016 16:01, Daniel P. Berrange wrote:
> >>>>>
> >>>>> I already use error_report's in places in migration threads of various
> >>>>> types; I'm not sure if that's a problem.
> >>> Unless those places are protected by the big qemu lock, that sounds
> >>> not good. error_report calls into error_vprintf which checks the
> >>> 'cur_mon' global "Monitor" pointer. This variable is updated at
> >>> runtime - eg in qmp_human_monitor_command(), monitor_qmp_read(),
> >>> monitor_read(), etc. So if migration threads outside the BQL are
> >>> calling error_report() that could well cause problems. If you
> >>> are lucky messages will merely end up going to stderr instead of
> >>> the monitor, but in worst case I wouldn't be surprised if there
> >>> is a crash possibility in some race conditions.
> >>
> >> Writes to chardevs *are* thread-safe (assuming qio_channel_create_watch
> >> is thread-safe; it seems to be).
> > 
> > Hmm that's useful (although it doesn't solve error_report because 
> > error_vprintf
> > is racy itself).
> How is it racy?  Because of the case where cur_mon changes under the
> feet of error_vprintf?  I guess that can be ignored for now (just a TODO
> comment will do).

Yes, the usage of cur_mon is unsafe as there's a TOCTOU race in
there that I believe can result in a NULL pointer crash.

eg these two methods:

static inline bool monitor_is_qmp(const Monitor *mon)
    return (mon->flags & MONITOR_USE_CONTROL);

bool monitor_cur_is_qmp(void)
    return cur_mon && monitor_is_qmp(cur_mon);

In using error_vprintf() from a non-main thread and not holding
the big QEMU lock, then 'cur_mon' can change between time we
check that its non-NULL, and when accessing cur_mon->flags.

Admittedly its a rare race - you could have to have an error
being reported by background migration, concurrently with a
monitor command being invoked, but that will hit someone
eventually unless I'm missing some synchronization somewhere.

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