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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/07/2015 01:07, Andy Lutomirski wrote:
> 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?

The address which Xen receives needs to be aligned.

It looks like xen_alloc_ldt() blindly assumes that the desc_struct *ldt
it is passed is page aligned, and passes it straight through.

>
>> 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.

I actually meant 0x0004, but yes.  Very much 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.

I suspect two separate allocations would be the better solution, as it
means that the size field doesn't need to be subject to funny page
permissions.

~Andrew

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