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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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

~Andrew

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