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

From: Julien Grall <julien@xen.org>
To: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Ian Jackson <iwj@xenproject.org>, Wei Liu <wl@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Tamas K Lengyel <lengyelt@ainfosec.com>,
	Petre Ovidiu PIRCALABU <ppircalabu@bitdefender.com>,
	Alexandru Isaila <aisaila@bitdefender.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held
Date: Mon, 7 Dec 2020 17:22:34 +0000	[thread overview]
Message-ID: <90ace303-b0a9-7d83-098d-ec01c3b308ad@xen.org> (raw)
In-Reply-To: <381cbc5b-29e8-d84d-0b7c-e84de82bc1a4@suse.com>

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@xen.org) 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