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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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