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Re: [Xen-devel] [PATCH v3 2/2] xsm: hide detailed Xen version from unprivileged guests


  • To: Julien Grall <julien@xxxxxxx>, Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
  • From: George Dunlap <george.dunlap@xxxxxxxxxx>
  • Date: Thu, 23 Jan 2020 15:02:00 +0000
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  • Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Doug Goldstein <cardoe@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxx, Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
  • Delivery-date: Thu, 23 Jan 2020 15:02:27 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 1/23/20 2:52 PM, Julien Grall wrote:
> Hi,
> 
> On 23/01/2020 14:45, George Dunlap wrote:
>> On 1/23/20 2:42 PM, Julien Grall wrote:
>>> Hi,
>>>
>>> On 23/01/2020 11:32, Sergey Dyasli wrote:
>>>> On 22/01/2020 11:25, Julien Grall wrote:
>>>>>
>>>>>
>>>>> On 22/01/2020 11:19, Sergey Dyasli wrote:
>>>>>> On 22/01/2020 10:14, Julien Grall wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 22/01/2020 10:01, Sergey Dyasli wrote:
>>>>>>>> On 20/01/2020 10:01, Jan Beulich wrote:
>>>>>>>>> On 17.01.2020 17:44, Sergey Dyasli wrote:
>>>>>>>>>> v2 --> v3:
>>>>>>>>>> - Remove hvmloader filtering
>>>>>>>>>
>>>>>>>>> Why? Seeing the prior discussion, how about adding
>>>>>>>>> XENVER_denied to
>>>>>>>>> return the "denied" string, allowing components which want to
>>>>>>>>> filter
>>>>>>>>> to know exactly what to look for? And then re-add the filtering
>>>>>>>>> you
>>>>>>>>> had? (The help text of the config option should then perhaps be
>>>>>>>>> extended to make very clear that the chosen string should not
>>>>>>>>> match
>>>>>>>>> anything that could potentially be returned by any of the XENVER_
>>>>>>>>> sub-ops.)
>>>>>>>>
>>>>>>>> I had the following reasoning:
>>>>>>>>
>>>>>>>> 1. Most real-world users would set CONFIG_XSM_DENIED_STRING=""
>>>>>>>> anyway.
>>>>>>>>
>>>>>>>> 2. Filtering in DMI tables is not a complete solution, since denied
>>>>>>>> string leaks elsewhere through the hypercall (PV guests, sysfs,
>>>>>>>> driver
>>>>>>>> logs) as Andrew has pointed out in the previous discussion.
>>>>>>>>
>>>>>>>> On the other hand, SMBios filtering slightly improves the
>>>>>>>> situation for
>>>>>>>> HVM domains, so I can return it if maintainers find it worthy.
>>>>>>>
>>>>>>> While I am not a maintainer of this code, my concern is you impose
>>>>>>> the conversion from "denied" to "" to all the users (include those
>>>>>>> who wants to keep "denied").
>>>>>>
>>>>>> This is not what's happening here: the default is still "<denied>"
>>>>>> (as
>>>>>> per patch 1); but patch 2 makes XENVER_extraversion,
>>>>>> XENVER_compile_info
>>>>>> and XENVER_changeset to return "<denied>" instead of the real values
>>>>>> which causes the UI / logs issues.
>>>>>
>>>>> I was referring the SMBios filtering... I don't think doing a blank
>>>>> filtering is the right thing to do in the hvmloader for the reason
>>>>> explained above.
>>>>
>>>> Apologies for misunderstanding the context. But I disagree about
>>>> hvmloader. Returning "denied" from xen_version hypercall to guests is
>>>> one thing, but hvmloader and SMBios tables are parts of the hypervisor
>>>> and putting "denied" there is simply a terrible user experience.
>>>
>>> I am not going to comment on the user experience because this is up to
>>> another bikeshedding. The question is which string will you use? Why an
>>> empty string over something different?
>>>
>>> However, if an admin has control over the "deny" string, why would he
>>> ever want to filter it in hvmloader? Why can't he just replace the one
>>> in Kconfig?
>>
>> Most admins don't compile their own version of Xen...
> 
> Those admins are also unlikely going to build there own hvmloader, right?
> 
> Therefore, they will have to accept whatever string is reported by
> HVMLoader (or Xen). As you already allow Xen to configure it, why would
> that be a problem to change the one in Kconfig? Why do you need to fix
> it up in hvmloader as well?

Right, the idea was perhaps as upstream, we should modify hvmloader to
*always* replace "<denied>" with "".  (Or potentially with a more benign
string, like "hypervisor build unknown".)

 -George

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