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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.07.17 at 16:56,  wrote:
>>>> 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?

I'd really like to get this off my list.

Jan


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