Re: [Xen-devel] [PATCH] x86/PV: properly populate descriptor tables

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.10.15 at 15:58, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 26/10/15 14:55, Andrew Cooper wrote:
>> On 26/10/15 14:43, David Vrabel wrote:
>>> On 23/09/15 16:34, Jan Beulich wrote:
>>>> Us extending the GDT limit past the Xen descriptors so far meant that
>>>> guests (including user mode programs) accessing any descriptor table
>>>> slot above the original OS'es limit but below the first Xen descriptor
>>>> caused a #PF, converted to a #GP in our #PF handler. Which is quite
>>>> different from the native behavior, where some of such accesses (LAR
>>>> and LSL) don't fault. Mimic that behavior by mapping a blank page into
>>>> unused slots.
>>>>
>>>> While not strictly required, treat the LDT the same for consistency.
>>> This change causes a 32-bit userspace process running in a 32-bit PV
>>> guest to segfault.
>>>
>>> The process is a Go program and it is using the modify_ldt() system call
>>> (which is successful) but loading %gs with the new descriptor causes a
>>> fault.  Even a minimal (empty main()) go program faults.
>> D'uh - its obvious now you point it out.
>>
>> By filling the shadow ldt slots as present, zero entries, we break their
>> demand-faulting.
>>
>> We can't be safe to incorrect faults from LAR/LSL, *and* perform demand
>> faulting of the LDT.
> 
> Wait.  Yes we can.  I am talking nonsense.
> 
> Hunk 2 should be reverted, and the demand fault handler should populate
> a zero entry rather than passing #GP back to the guest.

Considering this

"While not strictly required, treat the LDT the same for consistency."

in the changelog, simply reverting the LDT part would seem
sufficient to me (albeit that's more than just hunk 2 afaics).

Jan


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