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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>, Jan Beulich <jbeulich@xxxxxxxx>
  • From: George Dunlap <george.dunlap@xxxxxxxxxx>
  • Date: Wed, 22 Jan 2020 10:57:38 +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: Wed, 22 Jan 2020 10:57:51 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 1/22/20 10:14 AM, 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").
> 
> If you were doing any filtering in hvmloader, then it would be best if
> this is configurable. But this is a bit pointless if you already allow
> the user to configure the string at the hypervisor level :).

So there are two things we're concerned about:
- Some people don't want to scare users with a "<denied>" string
- Some people don't want to "silently fail" with a "" string

The fact is, in *both cases*, this is a UI problem.  EVERY caller of
this interface should figure out independently what a graceful way of
handling failure is for their target UI.  Any caller who does not think
carefully about what to do in the failure case is buggy -- which
includes every single caller today.  The CONFIG_XSM_DENIED_STRING is a
gross hack fallback for buggy UIs.

Now, I don't like to tell other people to do work, and I certainly don't
plan on fixing hvmloader at the moment, because it's low-priority for
me.  But I do think that having hvmloader detect failure and explicitly
make a sensible decision is the right thing to do, regardless of the
availability of CONFIG_XSM_DENIED_STRING to work around buggy callers.

 -George

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