Re: [patch] kvm: make cr3 loading more robust

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Avi Kivity wrote:
> Ingo Molnar wrote:
>> * Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
>>
>>  
>>>> rmap_write_protect() has a BUG_ON() if a physical address is not 
>>>> found the the memslot. But this is a possible scenario if a buggy 
>>>> guest OS loads an invalid or corrupted cr3 value. So exit more 
>>>> gracefully.
>>>>       
>>> I think a better solution is to detect the invalid cr3 in new_cr3(). 
>>> That way the entire chain of error returns is avoided.
>>>     
>>
>> i'm wondering what the right semantics would be though. Kill the VM 
>> context?
>>
>> on a real CPU an invalid cr3 does nothing explicitly (besides being a 
>> sure way to get a triple fault) - physical memory is always 
>> accessible, non-present physical memory at most generates an #MCE, 
>> but is typically just returning 0xff, right? So perhaps keep a 
>> non-mapped page filled with 0xff and map non-existent RAM to that? 
>> But then this 'RAM' needs to be made non-writable.
>
> That's a good solution.  I don't see why it has to be made 
> non-writable -- it has undefined content, and any old value will do.  
> We have (or maybe had) something like that somewhere.

It probably solves other cases as well, and simplifies error handling 
elsewhere.

-- 
error compiling committee.c: too many arguments to function


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