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

Nadav Amit
2020-04-07T20:27:51Z

> 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.