Re: [Xen-devel] [PATCH 2/3] x86/altp2m: Add a hvmop for setting the suppress #VE bit

[Adrian Pop <apop@xxxxxxxxxxxxxxx>]

On Tue, Jun 06, 2017 at 07:08:43AM -0600, Jan Beulich wrote:
> >>> On 06.06.17 at 15:00, <apop@xxxxxxxxxxxxxxx> wrote:
> > On Mon, May 29, 2017 at 08:38:33AM -0600, Jan Beulich wrote:
> >> >>> On 18.05.17 at 17:07, <apop@xxxxxxxxxxxxxxx> wrote:
> >> > +
> >> > +    if ( !cpu_has_vmx )
> >> > +        return -EOPNOTSUPP;
> >> 
> >> Is this enough? Wouldn't it be better to signal the caller whenever
> >> hardware (or even software) isn't going to honor the request?
> > 
> > Well, the caller checks the return value.  The libxc function
> > xc_altp2m_set_suppress_ve for instance will return a negative in this
> > case.
> 
> The question wasn't what the caller does but whether checking just
> cpu_has_vmx is enough. What if you're running on a machine with
> VMX but no #VE support?

Oh, all right.  I misinterpreted it.  Yes, at least using something like
cpu_has_vmx_virt_exceptions instead of cpu_has_vmx would definitely be
more appropriate in this case.  Do you think there should be a more
thorough check?

> >> And then there are two general questions: Without a libxc layer
> >> function, how is one supposed to use this new sub-op? Is it
> >> really intended to permit a guest to call this for itself?
> >  
> > Well, the sub-op could be used from a Linux kernel module if libxc is
> > not available if struct xen_hvm_altp2m_op and struct
> > xen_hvm_altp2m_set_suppress_ve are defined.
> > 
> > Our use case, though, involves either Dom0 or a "privileged" DomU
> > altering the suppress #VE bit for the target guest.
> 
> This doesn't really answer the question: What are the security
> implications if a guest can invoke this on itself?

Indeed it would be desirable that the guest doesn't use this hvmop on
itself.  It's also less than ideal that a DomU can call this on other
DomUs.

After some talks it turns out that restricting this hvmop to a
privileged domain solves this issue and still works for our use case.
What do you think?

> (FTR I think my first question was kind of pointless, as patch 3
> looks like it does introduce a libxc function; I simply didn't realize
> that back then, because I'd generally have expected the
> consumer of the hypercall to be introduce together with the
> producer.)

I can merge these two patches for v2 if that's what you want.

> Jan

Thank you!

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