Re: [Xen-devel] [PATCH v2 1/3] x86/ldt: Make modify_ldt synchronous

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Tue, Jul 21, 2015 at 4:38 PM, Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
> On 21/07/2015 22:53, Boris Ostrovsky wrote:
>> On 07/21/2015 03:59 PM, Andy Lutomirski wrote:
>>> --- a/arch/x86/include/asm/mmu_context.h
>>> +++ b/arch/x86/include/asm/mmu_context.h
>>> @@ -34,6 +34,44 @@ static inline void load_mm_cr4(struct mm_struct
>>> *mm) {}
>>>   #endif
>>>     /*
>>> + * ldt_structs can be allocated, used, and freed, but they are never
>>> + * modified while live.
>>> + */
>>> +struct ldt_struct {
>>> +    int size;
>>> +    int __pad;    /* keep the descriptors naturally aligned. */
>>> +    struct desc_struct entries[];
>>> +};
>>
>>
>>
>> This breaks Xen which expects LDT to be page-aligned. Not sure why.
>>
>> Jan, Andrew?
>
> PV guests are not permitted to have writeable mappings to the frames
> making up the GDT and LDT, so it cannot make unaudited changes to
> loadable descriptors.  In particular, for a 32bit PV guest, it is only
> the segment limit which protects Xen from the ring1 guest kernel.
>
> A lot of this code hasn't been touched in years, and it certainly
> predates me.  The alignment requirement appears to come from the virtual
> region Xen uses to map the guests GDT and LDT.  Strict alignment is
> required for the GDT so Xen's descriptors starting at 0xe0xx are
> correct, but the LDT alignment seems to be a side effect of similar
> codepaths.
>
> For an LDT smaller than 8192 entries, I can't see any specific reason
> for enforcing alignment, other than "that's the way it has always been".
>
> However, the guest would still have to relinquish write access to all
> frames which make up the LDT, which looks to be a bit of an issue given
> the snippet above.

Does the LDT itself need to be aligned or just the address passed to
paravirt_alloc_ldt?

>
> I think I have a solution, but I doubt it is going to be very popular.
>
> * Make a new paravirt hook for allocation of ldt_struct, so the paravirt
> backend can choose an alignment if needed
> * Make absolutely certain that __pad has the value 0 (so size and __pad
> combined don't look like a present descriptor)
> * Never hand selector 0x0008 to unsuspecting users.

Yuck.

>
> This will allow ldt_struct itself to be page aligned, and for the size
> field to sit across the base/limit field of what would logically be
> selector 0x0008  There would be some issues accessing size.  To load
> frames as an LDT, a guest must drop all refs to the page so that its
> type may be changed from writeable to segdesc.  After that, an
> update_descriptor hypercall can be used to change size, and I believe
> the guest may subsequently recreate read-only mappings to the frames in
> question (although frankly it is getting late so you will want to double
> check all of this).
>
> Anyhow, this looks like an issue which should be fixed up with slightly
> more PVOps, rather than enforcing a Xen view of the world on native Linux.
>

I could presumably make the allocation the other way around so the
size is at the end.  I could even use two separate allocations if
needed.

--Andy

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