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

[Julien Grall <julien@xxxxxxx>]

On 07/12/2020 08:02, Jan Beulich wrote:
> On 04.12.2020 16:09, Julien Grall wrote:
> > 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:
> > > > > > On 03/12/2020 10:09, Jan Beulich wrote:
> > > > > > > On 02.12.2020 22:10, Julien Grall wrote:
> > > > > > > > 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.
> Oh, right - forgot about that one, because that's nothing I would
> ever have taken on actually carrying out.
>
> > > > 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.
> So to translate this to a clear course of action: You want me to
> go back to the earlier version by dropping the refcounting again?
Yes.

> (I don't view this as "papering over" btw, but a tiny step towards
> a solution.)
This is implying that the refcounting is part of the actual solution. I 
think you can solve it directly in the VM event code without touching 
the event channel code.
Furthermore, I see no point to add code in the common code if the 
maintainers of the affected subsystem think there code is safe (I don't 
believe it is).
Cheers,

--
Julien Grall