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

From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>
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 09:02:22 +0100	[thread overview]
Message-ID: <381cbc5b-29e8-d84d-0b7c-e84de82bc1a4@suse.com> (raw)
In-Reply-To: <2edfc711-d8d9-4854-94a2-2d9e4d9902ec@xen.org>

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?
(I don't view this as "papering over" btw, but a tiny step towards
a solution.)

Jan