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>,
	Tamas K Lengyel <tamas.k.lengyel@gmail.com>
Subject: Re: [PATCH v3 5/5] evtchn: don't call Xen consumer callback with per-channel lock held
Date: Mon, 7 Dec 2020 16:28:08 +0100	[thread overview]
Message-ID: <fd384990-376e-40f4-f0b8-1a889b3a0c51@suse.com> (raw)
In-Reply-To: <CABfawhkWQiOhLL8f3NzoWbeuag-f+YOOK0i_LJzZq5Yvoh=oHQ@mail.gmail.com>

On 04.12.2020 20:15, Tamas K Lengyel wrote:
> On Fri, Dec 4, 2020 at 10:29 AM Julien Grall <julien@xen.org> wrote:
>> On 04/12/2020 15:21, Tamas K Lengyel wrote:
>>> On Fri, Dec 4, 2020 at 6:29 AM Julien Grall <julien@xen.org> wrote:
>>>> 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.
>>>
>>> I double-checked and the disable route is actually more robust, we
>>> don't just rely on the toolstack doing the right thing. The domain
>>> gets paused before any calls to vm_event_disable. So I don't think
>>> there is really a race-condition here.
>>
>> The code will *only* pause the monitored domain. I can see two issues:
>>     1) The toolstack is still sending event while destroy is happening.
>> This is the race discussed here.
>>     2) The implement of vm_event_put_request() suggests that it can be
>> called with not-current domain.
>>
>> I don't see how just pausing the monitored domain is enough here.
> 
> Requests only get generated by the monitored domain. So if the domain
> is not running you won't get more of them. The toolstack can only send
> replies.

Julien,

does this change your view on the refcounting added by the patch
at the root of this sub-thread?

Jan