Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation

Peter Zijlstra
2020-04-07T20:51:24Z

On Tue, Apr 07, 2020 at 01:27:45PM -0700, Nadav Amit wrote:
> > On Apr 7, 2020, at 12:38 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Tue, Apr 07, 2020 at 11:55:21AM -0700, Nadav Amit wrote:
> >>> On Apr 7, 2020, at 4:02 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >>>
> >>> Since we now have infrastructure to analyze module text, disallow
> >>> modules that write to CRn and DRn registers.
> >>
> >> Assuming the kernel is built without CONFIG_PARAVIRT, what is the right way
> >> for out-of-tree modules to write to CRs? Let’s say CR2?
> >
> > Most of them there is no real justification for ever writing to. CR2 I
> > suppose we can have an exception for given a sane rationale for why
> > you'd need to rewrite the fault address.
>
> For the same reason that KVM writes to CR2 - to restore CR2 before entering
> a guest, since CR2 not architecturally loaded from the VMCS. I suspect there
> are additional use-cases which are not covered by the kernel interfaces.

So I'm not much of a virt guy (clearly), and *groan*, that's horrible.
I'll go make an exception for CR2.