Re: [PATCH] xen/arm64: Hide FEAT_SME

[Julien Grall <julien@xxxxxxx>]

Hi,

On 16/08/2024 08:15, Michal Orzel wrote:
 > On 14/08/2024 23:00, Julien Grall wrote:
> > Newer hardware may support FEAT_SME. Xen doesn't have any knowledge but
> > it will still expose the feature to the VM. If the OS is trying to use
> > SME, then it will crash.
> >
> > Solve by hiding FEAT_SME.
> >
> > Signed-off-by: Julien Grall <julien@xxxxxxx>
> Acked-by: Michal Orzel <michal.orzel@xxxxxxx>
>
> > ---
> >
> > The current approach used to create the domain cpuinfo is to hide
> > (i.e. a denylist) what we know Xen is not supporting. The drawback
> > with this approach is for newly introduced feature, Xen will expose it
> > by default.
> >
> > If a kernel is trying to use it then it will crash. I can't really
> > make my mind whether it would be better to expose only what we support
> > (i.e. use an allowlist).
> >
> > AFAICT, there is no security concerns with the current approach because
> > ID_* registers are not a way to tell the kernel which features are
> > supported. A guest kernel could still try to access the new registers.
> I agree with the security aspect but the part of the sentence in the middle is 
> a bit misleading.
Indeed. It was poorly worded. I was meant to say what you wrote below :).

> ID_ registers *are* a way of informing the kernel about implemented PE 
> features. It's just that
> the kernel could still access the features. That said, it should be considered 
> an incorrect behavior
> and definitely not something we should worry about.
Cheers,

--
Julien Grall