Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held

[Julien Grall <julien@xxxxxxx>]

Hi,

On 04/12/2020 12:01, Jan Beulich wrote:
> On 04.12.2020 12:51, Julien Grall wrote:
> > On 04/12/2020 11:48, Jan Beulich wrote:
> > > On 04.12.2020 12:28, Julien Grall wrote:
> > > > Hi Jan,
> > > >
> > > > On 03/12/2020 10:09, Jan Beulich wrote:
> > > > > On 02.12.2020 22:10, Julien Grall wrote:
> > > > > > On 23/11/2020 13:30, Jan Beulich wrote:
> > > > > > > While there don't look to be any problems with this right now, the lock
> > > > > > > order implications from holding the lock can be very difficult to follow
> > > > > > > (and may be easy to violate unknowingly). The present callbacks don't
> > > > > > > (and no such callback should) have any need for the lock to be held.
> > > > > > >
> > > > > > > However, vm_event_disable() frees the structures used by respective
> > > > > > > callbacks and isn't otherwise synchronized with invocations of these
> > > > > > > callbacks, so maintain a count of in-progress calls, for evtchn_close()
> > > > > > > to wait to drop to zero before freeing the port (and dropping the lock).
> > > > > > AFAICT, this callback is not the only place where the synchronization is
> > > > > > missing in the VM event code.
> > > > > >
> > > > > > For instance, vm_event_put_request() can also race against
> > > > > > vm_event_disable().
> > > > > >
> > > > > > So shouldn't we handle this issue properly in VM event?
> > > > > I suppose that's a question to the VM event folks rather than me?
> > > > Yes. From my understanding of Tamas's e-mail, they are relying on the
> > > > monitoring software to do the right thing.
> > > >
> > > > I will refrain to comment on this approach. However, given the race is
> > > > much wider than the event channel, I would recommend to not add more
> > > > code in the event channel to deal with such problem.
> > > >
> > > > Instead, this should be fixed in the VM event code when someone has time
> > > > to harden the subsystem.
> > > Are effectively saying I should now undo the addition of the
> > > refcounting, which was added in response to feedback from you?
> > Please point out where I made the request to use the refcounting...
> You didn't ask for this directly, sure, but ...
>
> > I pointed out there was an issue with the VM event code.
> ... this has ultimately led to the decision to use refcounting
> (iirc there was one alternative that I had proposed, besides
> the option of doing nothing).
One other option that was discussed (maybe only on security@xxxxxxx) is 
to move the spinlock outside of the structure so it is always allocated.
> > This was latter
> > analysed as a wider issue. The VM event folks doesn't seem to be very
> > concerned on the race, so I don't see the reason to try to fix it in the
> > event channel code.
> And you won't need the refcount for vpl011 then?
I don't believe we need it for the vpl011 as the spin lock protecting 
the code should always be allocated. The problem today is the lock is 
initialized too late.
> I can certainly
> drop it again, but it feels odd to go back to an earlier version
> under the circumstances ...
The code introduced doesn't look necessary outside of the VM event code.
So I think it would be wrong to merge it if it is just papering over a 
bigger problem.
Cheers,



> Jan
--
Julien Grall