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Re: [Xen-devel] Ping: [PATCH] x86: improve vCPU selection in pagetable_dying()


  • To: Jan Beulich <JBeulich@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>
  • From: George Dunlap <george.dunlap@xxxxxxxxxx>
  • Date: Wed, 3 Oct 2018 17:56:06 +0100
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  • Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Wed, 03 Oct 2018 16:56:19 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 09/26/2018 08:04 AM, Jan Beulich wrote:
>>>> On 25.09.18 at 18:22, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 18/09/18 13:44, Jan Beulich wrote:
>>>>>> On 10.09.18 at 16:02, <JBeulich@xxxxxxxx> wrote:
>>>> Rather than unconditionally using vCPU 0, use the current vCPU if the
>>>> subject domain is the current one.
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>> What improvement is this intended to bring?
> 
> I've come across this quite a while ago when investigating possibly
> dangerous uses of d->vcpu[], well before your series to improve the
> situation there. I generally consider it wrong to hard code use of
> d->vcpu[0] whenever it can be avoided.
> 
>> Shadows are per-domain, and the gmfn in question is passed in by the
>> caller.  AFACIT, it is a logical bug that that the callback takes a vcpu
>> rather than a domain in the first place.
> 
> Did you look at the 3-level variant of sh_pagetable_dying()? It very
> clearly reads the given vCPU's CR3.

Yes; and so the current implementation which unconditionally passes vcpu
0 is clearly a bug.

> Looking at things again (in particular
> the comment ahead of pagetable_dying()) I now actually wonder why
> HVMOP_pagetable_dying is permitted to be called by other than a domain
> for itself. There's no use of it in the tool stack. Disallowing the unused
> case would mean the fast-path logic in sh_pagetable_dying() could
> become the only valid/implemented case. Tim?

Not so -- a guest could still call pagetable_dying() on the top level PT
of a process not currently running.

I would be totally in favor of limiting this call to the guest itself,
however -- that would simplify the logic even more.

  -George

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