Re: [Xen-devel] [PATCH v2] xsm: hide detailed Xen version from unprivileged guests

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.01.2020 16:28, George Dunlap wrote:
> On 1/10/20 11:02 AM, Andrew Cooper wrote:
>> On 10/01/2020 10:37, Sergey Dyasli wrote:
>>> Hide the following information that can help identify the running Xen
>>> binary version: XENVER_extraversion, XENVER_compile_info, XENVER_changeset.
>>> Add explicit cases for XENVER_commandline and XENVER_build_id as well.
>>>
>>> Introduce xsm_filter_denied() to hvmloader to remove "<denied>" string
>>> from guest's DMI tables that otherwise would be shown in tools like
>>> dmidecode.
>>>
>>> Signed-off-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
>>> ---
>>> v1 --> v2:
>>> - Added xsm_filter_denied() to hvmloader instead of modifying xen_deny()
>>> - Made behaviour the same for both Release and Debug builds
>>> - XENVER_capabilities is no longer hided
>>>
>>> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> CC: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
>>> CC: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
>>> CC: Jan Beulich <jbeulich@xxxxxxxx>
>>> CC: Julien Grall <julien@xxxxxxx>
>>> CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>> CC: Wei Liu <wl@xxxxxxx>
>>> CC: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
>>
>> I realise there are arguments over how to fix this, but we (the Xen
>> community) have already f*cked up once here, and this is doing so a
>> second time.
>>
>> Nack.
>>
>> Fixing it anywhere other than Xen is simply not appropriate.

(replying here, because the original mail doesn't seem to have
made it into my inbox)

I've said so to Sergey already on v1: The "fix" you want needs to
be at or closer to the presentation layer. From Xen's perspective
the request for information was _indeed_ denied.

>> The reason for this (which ought to be obvious, but I guess only to
>> those who actually do customer support) is basic human physiology. 
>> "denied" means something has gone wrong.  It scares people, and causes
>> them to seek help to change fix whatever is broken.
> 
> This seems like a reasonable argument that "<denied>" causes issues.
> But that doesn't change the fact that "" also causes issues.
> 
> What about changing the string to "<build-id hidden>" or something like
> that?  That makes it more clear what would have been in that place, and
> "hidden" is a lot less scary than "denied".

I could live with this. But (judging from the picture that was
provided earlier on) it would still require filtering at or close
to the presentation layer, and by changing the prior <denied> to
different and varying strings may make that job harder (albeit
perhaps they could look for any <...>).

Jan

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