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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Tue, Jul 21, 2015 at 5:21 PM, Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
> 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.

xen_alloc_ldt is just fiddling with protection though, I think.  Isn't
it xen_set_ldt that's the meat?  We could easily pass xen_alloc_ldt a
pointer to the ldt_struct.

>
>>
>>> 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.

True.  OTOH we never write to the size field after allocating the thing.

--Andy

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