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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/11/14 11:31, Jan Beulich wrote:
>>>> 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".

At that point, it is safer just to unconditionally crash on a repeated
vmentry failure, rather than gain a list of conditions which we hope
wont leave us spinning in a loop.

~Andrew

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