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[Xen-devel] Ping: Re: [PATCH 2/2] x86: don't allow clearing of TF_kernel_mode for other than 64-bit PV

>>> On 31.05.17 at 13:54,  wrote:
>>>> On 31.05.17 at 13:08, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 31/05/17 08:15, Jan Beulich wrote:
> >> The flag is really only meant for those, both HVM and 32-bit PV tell
> >> kernel from user mode based on CPL/RPL. Remove the all-question-marks
> >> comment and let's be on the safe side here and also suppress clearing
> >> for 32-bit PV (this isn't a fast path after all).
> >>
> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > 
> > Wouldn't it just be safer to disallow starting a 64bit PV guest in user
> > mode?
> > 
> > No real kernel would do such a thing, and keeping the corner case around
> > is bad from an attack-surface point of view.
> If it really was "starting a guest", I would probably agree. But we're
> talking about starting a vCPU, and I could see uses for this (not the
> least in XTF). After all the operation allows for enough state to be
> set up such that further initialization inside the guest may not be
> necessary.

Any opinion here, or change of opinion on the original patch?


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