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Re: [Xen-devel] [PATCH V6 1/4] x86/mm: Add array_index_nospec to guest provided index values


  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: George Dunlap <george.dunlap@xxxxxxxxxx>
  • Date: Fri, 27 Dec 2019 10:52:55 +0000
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  • Cc: Petre Ovidiu PIRCALABU <ppircalabu@xxxxxxxxxxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>, Tamas K Lengyel <tamas@xxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Razvan COJOCARU <rcojocaru@xxxxxxxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Fri, 27 Dec 2019 10:53:38 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 12/27/19 7:59 AM, Jan Beulich wrote:
> On 23.12.2019 19:08, George Dunlap wrote:
>> What about the attached series of patches (compile-tested only)?
> 
> This ...
> 
>> +#define nospec_clip(index, size)                 \
>> +    ({                                           \
>> +        bool clipped = (index >= size);          \
>> +        index = array_index_nospec(index, size); \
>> +        clipped;                                 \
>> +    })
> 
> ... in particular may misguide people on its use: If the clipped
> "index" gets stored in a register, all is going to be fine (afaict),
> but if it ends up in memory, there's be new (mis-)speculation
> opportunities.

That makes sense; but in that case code like this:

> +    idx = array_index_nospec(idx, MAX_ALTP2M);
> +

...could end up stored on the stack and re-read, couldn't it?  I mean
yes, it's *very likely* going to stay in a register, but there's no way
to actually guarantee it, is there?

 -George

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