Re: [PATCH] Revert "evtchn: refuse EVTCHNOP_status for Xen-bound event channels"

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.04.2024 13:10, Daniel P. Smith wrote:
> On 4/3/24 02:16, Jan Beulich wrote:
>> On 02.04.2024 19:06, Andrew Cooper wrote:
>>> The commit makes a claim without any kind of justification.
>>
>> Well, what does "have no business" leave open?
> 
> Why does it not have any business? Why should a domain that creates an 
> event channel not be able to inquire about its status?

Event channels we talk about here are created via
alloc_unbound_xen_event_channel(). IOW it's not any domain creating them.
Once connected, the respective domain is of course fine to query its end
of the channel.

>>> The claim is false, and the commit broke lsevtchn in dom0.
>>
>> Or alternatively lsevtchn was doing something that was never meant to work
>> (from Xen's perspective).
> 
> Again, you have not said why this is a problem. What concern does it 
> create? Does it open the door for access elevation, resource 
> deprivation, or some other malicious behaviors?

It exposes information that perhaps better wouldn't be exposed. Imo if
Xen owned resource state is of interest, it would want exposing via
hypfs.

>>>   It is also quite
>>> obvious from XSM_TARGET that it has broken device model stubdoms too.
>>
>> Why would that be "obvious"? What business would a stubdom have to look at
>> Xen's side of an evtchn?
> 
> Again, you have not expressed why it shouldn't be able to do so.

See above - not its resource, nor its guest's.

>>> Whether to return information about a xen-owned evtchn is a matter of 
>>> policy,
>>> and it's not acceptable to short circuit the XSM on the matter.
>>
>> I can certainly accept this as one possible view point. As in so many cases
>> I'm afraid I dislike you putting it as if it was the only possible one.
> 
> In fact, this commit is in violation of the XSM. It hard-codes a 
> resource access check outside XSM, thus breaking the fine-grained access 
> control of FLASK.

Perhaps; see below and see the question raised in the subsequent reply
to the patch.

>> In summary: The supposed justification you claim is missing in the original
>> change is imo also missing here then: What business would any entity in the
>> system have to look at Xen's side of an event channel? Back at the time, 3
>> people agreed that it's "none".
> 
> As stated, you provided no reason or justification for "has no business" 
> and by face value is an opinion that a few people agreed with. As for 
> why, there could be a myriad number of reasons a domain may want to 
> check the status of an interface it has with the hypervisor. From just 
> logging its state for debug to throttling attempts at sending an event. 
> So why, from a security/access control decision, does this access have 
> to absolutely blocked, even from FLASK?

I didn't say it absolutely needs to be blocked. I'm okay to become
convinced otherwise. But in the description complaining about lack of
reasons in the 3-4 year old change, just to then again not provide any
reasons looks "interesting" to me. (And no, just to take that example,
lsevtchn not working anymore on such channels is not on its own a
reason. As indicated, it may well be that conceptually it was never
supposed to be able to have access to this information. The latest not
anymore when hypfs was introduced.)

Jan