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

Avi Kivity
2007-01-03T12:00:07Z

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.

> Looks a bit messy. Cleanest would be to kill the
> VM context (without injecting any fault into the guest), but that would
> then require an error return chain from set_cr3() ...
>

set_cr3() is fairly close to the top. I'm mostly worried about keeping
the innards of the mmu clean.

An alternative is to add a flag to the vcpu which would be examined on
entry (vcpu->triple_faulted).