Re: [Xen-devel] [PATCH for-4.5] x86/HVM: Partial revert of 28b4baacd5

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 25.11.14 at 11:58, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 25/11/14 10:46, Andrew Cooper wrote:
>> On 25/11/14 10:42, Jan Beulich wrote:
>>>>>> On 25.11.14 at 11:08, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> A failed vmentry is overwhelmingly likely to be caused by corrupt VMCS 
> state.
>>>> As a result, injecting a fault and retrying the the vmentry is likely to 
>>>> fail
>>>> in the same way.
>>> That's not all that unlikely - remember that the change was prompted
>>> by the XSA-110 fix. There CS pieces being in a bad state would get
>>> corrected by the exception injection.
>>>
>>>> One other alternative, which I would pursue if we were not already in -rc2
>>>> would be to add some extra logic to detect repeated vmentry failure and 
>>>> allow
>>>> one attempt to shoot userspace before giving up and crashing the domain.
>>> That's not even needed afaict (and if it really is, it can't be all that
>>> difficult/intrusive): Did you observe what you attempt to fix here in
>>> practice, or is this just from theoretical considerations? I ask because
>>> I don't think it can actually happen, as the second time we get here
>>> the guest ought to be in kernel mode (due to the exception injection)
>>> and hence would get crashed anyway.
>> Only from theoretical considerations.  A bad CS (and possibly SS) would
>> be fixed by this, but there are many others which wouldn't
> 
> Actually, as Tim correctly points out, a bad CS/SS won't be fixed by
> this without emulating the event injection.  Per the XSA-106 followup,
> we only ever emulate enough of event injection to cover the dpl checks
> on software events for older generation SVM.  We never actually emulate
> the context switch itself.

Which suggests that rather than doing the partial revert as you
propose we might better extend the check to become "kernel mode
or event injection pending".

Jan


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