Re: [Xen-devel] [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 30/07/15 19:30, Andy Lutomirski wrote:
> On Wed, Jul 29, 2015 at 5:29 PM, Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 30/07/2015 00:13, Andy Lutomirski wrote:
>>> On Wed, Jul 29, 2015 at 4:02 PM, Andrew Cooper
>>> <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> On 29/07/2015 23:49, Boris Ostrovsky wrote:
>>>>> On 07/29/2015 06:46 PM, David Vrabel wrote:
>>>>>> On 29/07/2015 23:11, Andrew Cooper wrote:
>>>>>>> On 29/07/2015 23:05, Andy Lutomirski wrote:
>>>>>>>> On Wed, Jul 29, 2015 at 2:37 PM, Andrew Cooper
>>>>>>>> <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>>>>>> On 29/07/2015 22:26, Andy Lutomirski wrote:
>>>>>>>>>> On Wed, Jul 29, 2015 at 2:23 PM, Boris Ostrovsky
>>>>>>>>>> <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>>>>>>>> On 07/29/2015 03:03 PM, Andrew Cooper wrote:
>>>>>>>>>>>> On 29/07/15 15:43, Boris Ostrovsky wrote:
>>>>>>>>>>>>> FYI, I have got a repro now and am investigating.
>>>>>>>>>>>> Good and bad news.  This bug has nothing to do with LDTs
>>>>>>>>>>>> themselves.
>>>>>>>>>>>>
>>>>>>>>>>>> I have worked out what is going on, but this:
>>>>>>>>>>>>
>>>>>>>>>>>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
>>>>>>>>>>>> index 5abeaac..7e1a82e 100644
>>>>>>>>>>>> --- a/arch/x86/xen/enlighten.c
>>>>>>>>>>>> +++ b/arch/x86/xen/enlighten.c
>>>>>>>>>>>> @@ -493,6 +493,7 @@ static void set_aliased_prot(void *v,
>>>>>>>>>>>> pgprot_t prot)
>>>>>>>>>>>>             pte = pfn_pte(pfn, prot);
>>>>>>>>>>>>    +       (void)*(volatile int*)v;
>>>>>>>>>>>>           if (HYPERVISOR_update_va_mapping((unsigned long)v,
>>>>>>>>>>>> pte, 0)) {
>>>>>>>>>>>>                   pr_err("set_aliased_prot va update failed w/
>>>>>>>>>>>> lazy mode
>>>>>>>>>>>> %u\n", paravirt_get_lazy_mode());
>>>>>>>>>>>>                   BUG();
>>>>>>>>>>>>
>>>>>>>>>>>> Is perhaps not the fix we are looking for, and every use of
>>>>>>>>>>>> HYPERVISOR_update_va_mapping() is susceptible to the same problem.
>>>>>>>>>>> I think in most cases we know that page is mapped so hopefully
>>>>>>>>>>> this is the
>>>>>>>>>>> only site that we need to be careful about.
>>>>>>>>>> Is there any chance we can get some kind of quick-and-dirty fix that
>>>>>>>>>> can go to x86/urgent in the next few days even if a clean fix isn't
>>>>>>>>>> available yet?
>>>>>>>>> Quick and dirty?
>>>>>>>>>
>>>>>>>>> Reading from v is the most obvious and quick way, for areas where
>>>>>>>>> we are
>>>>>>>>> certain v exists, is kernel memory and is expected to have a backing
>>>>>>>>> page.  I don't know offhand how many of current
>>>>>>>>> HYPERVISOR_update_va_mapping() callsites this applies to.
>>>>>>>> __get_user((char *)v, tmp), perhaps, unless there's something better
>>>>>>>> in the wings.  Keep in mind that we need this for -stable, and it's
>>>>>>>> likely to get backported quite quickly due to CVE-2015-5157.
>>>>>>> Hmm - something like that tucked inside HYPERVISOR_update_va_mapping()
>>>>>>> would probably work, and certainly be minimal hassle for -stable.
>>>>>>>
>>>>>>> Altering the hypercall used is certainly not something to backport, nor
>>>>>>> are we sure it is a viable fix at this time.
>>>>>> Changing this one use of update_va_mapping to use mmu_update_normal_pt
>>>>>> is the correct fix to unblock this LDT series.  I see no reason why this
>>>>>> cannot be backported.
>>>>> To properly fix it should include batching and that is not something
>>>>> that I think we should target for stable.
>>>> Batching is absolutely not necessary to alter update_va_mapping to
>>>> mmu_update_normal_pt.  After all, update_va_mapping isn't batched.
>>>>
>>>> However this isn't the first issue issue we have had lazy mmu faulting,
>>>> and I doubt it is the last.  There are not many callsites of
>>>> update_va_mapping - I will audit them tomorrow and see if any similar
>>>> issues are lurking elsewhere.
>>> One thing I should add: nothing flushes old aliases in xen_alloc_ldt,
>>> yet I haven't been able to get xen_alloc_ldt to fail or subsequent LDT
>>> access to fault.  Is this something we should be worried about?
>> Yes.  update_va_mapping() will function perfectly well taking one RW
>> mapping to RO even if there is a second RW mapping.  In such a case, the
>> next LDT access will fault.
> Which is a problem because that alias might still exist, and also
> because Linux really doesn't expect that fault.
>
>> On closer inspection, Xen is rather unhelpful with the fault.  Xen's
>> lazy #PF will be bounced back to the guest with cr2 adjusted to appear
>> in the range passed to set_ldt().  The error code however will be
>> unmodified (and limited only by not-user and not-reserved), so will
>> appear as a non-present read or write supervisor access to an address
>> which the kernel has a valid read mapping of.
> More yuck.
>
> I think I'm just going to stick an unconditional vm_flush_aliases in 
> alloc_ldt.
>
>> Therefore, set_ldt() needs to be confident that there are no writeable
>> mappings to the frames used to make up the LDT.  It could proactively
>> fault them in by accessing one descriptor in each page inside the limit,
>> but by the time a fault is received it is probably too late to work out
>> where the other mapping is which prevented the typechange (or indeed,
>> whether Xen objected to one of the descriptors instead).
> This seems like overkill.
>
> I'm still a bit confused, though: the failure is in xen_free_ldt.  How
> do we make it all the way to xen_free_ldt without the vmapped page
> existing in the guest's page tables?  After all, we had to survive
> xen_alloc_ldt first, and ISTM that should fail in exactly the same
> way.

(Summarising part of a discussion which has just occurred on IRC)

I presume that xen_free_ldt() is called while in the context of an mm
which doesn't have the particular area of the vmalloc() space faulted in.

This is (I presume) why reading 'v' (which occasionally causes a
pagefault to occur) fixes the issue.

~Andrew

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