Re: [Xen-devel] Mapping active GDT

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 03/13/2017 01:30 PM, Thomas Garnier wrote:
> On Mon, Mar 13, 2017 at 6:09 AM, Boris Ostrovsky
> <boris.ostrovsky@xxxxxxxxxx> wrote:
>> On 03/11/2017 08:06 AM, Andrew Cooper wrote:
>>> On 11/03/2017 03:58, Boris Ostrovsky wrote:
>>>> On 03/10/2017 09:39 PM, Boris Ostrovsky wrote:
>>>>> I am looking into GDT remap series [0] which crashes PV guests and it
>>>>> seems that the problem lies in the fact that we cannot establish new
>>>>> mapping to an already existing GDT.
>>>>>
>>>>> The mapping is created by
>>>>>
>>>>> +static inline void setup_fixmap_gdt(int cpu)
>>>>> +{
>>>>> +       __set_fixmap(get_cpu_gdt_ro_index(cpu),
>>>>> +                    __pa(get_cpu_gdt_rw(cpu)), PAGE_KERNEL);
>>>>> +}
>>>>>
>>>>> with get_cpu_gdt_rw(cpu) being the current GDT pointer. This results in
>>>>>
>>>>> (XEN) mm.c:2570:d94v0 Bad type (saw 5400000000000001 != exp
>>>>> 7000000000000000) for mfn 1538fb (pfn 3e809)
>>>>> (XEN) mm.c:1022:d94v0 Could not get page type PGT_writable_page
>>>>> (XEN) mm.c:1074:d94v0 Error getting mfn 1538fb (pfn 3e809) from L1 entry
>>>>> 80000001538fb063 for l1e_owner=94, pg_owner=94
>>> The problem here is that you are trying to create a writeable mapping to
>>> the GDT frame.
>>>
>>> Allowing the guest writeable access would be a security hole, as it is
>>> trivial to escalate privilege under those circumstances.
>> Right, and I saw the same failure symptoms (hypervisor errors) even when
>> I replaced PAGE_KERNEL with PAGE_KERNEL_RO.
>>
>> And then I realized that I was on a branch with other unrelated changes,
>> so I switched the branch and then things started to work. Thus my mea
>> culpa below.
>>
> So we are good? :) (Meaning I still need to push the fix for 32 bit
> but that's it).
>


No, it will need a few small changes. I am actually finishing the test
run (in the next hour or so) and will reply on the Linux thread.

-boris

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