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

On Tue, Jul 21, 2015 at 5:49 PM, Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
> On 22/07/2015 01:28, Andy Lutomirski wrote:
>> 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.
> So it is.  It is the linear_addr in xen_set_ldt() which Xen currently
> audits to be page aligned.
>>>>> 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.
> Right, but even reading it is going to cause problems if one of the
> paravirt ops can't re-establish ro mappings.

Does paravirt_alloc_ldt completely deny access or does it just set it RO?


> ~Andrew

Andy Lutomirski
AMA Capital Management, LLC

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