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

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Date:   Tue, 7 Apr 2020 14:22:11 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, hch@...radead.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        mingo <mingo@...hat.com>, bp <bp@...en8.de>, hpa@...or.com,
        x86 <x86@...nel.org>, "Kenneth R. Crudup" <kenny@...ix.com>,
        Jessica Yu <jeyu@...nel.org>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Tony Luck <tony.luck@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        jannh@...gle.com, keescook@...omium.org, David.Laight@...lab.com,
        Doug Covelli <dcovelli@...are.com>, mhiramat@...nel.org
Subject: Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation

> On Apr 7, 2020, at 1:50 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Tue, Apr 07, 2020 at 01:27:45PM -0700, Nadav Amit wrote:
>>> On Apr 7, 2020, at 12:38 PM, Peter Zijlstra <peterz@...radead.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@...radead.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.

Clearly you are not a virt guy if you think that this is the horrible part
in x86 virtualization ;-)

Anyhow, I do not think it is the only use-case which is not covered by your
patches (even considering CRs/DRs alone). For example, there is no kernel
function to turn on CR4.VMXE, which is required to run hypervisors on x86.

I think a thorough analysis of existing software is needed to figure out
which use-cases are valid, and to exclude them during module scanning or to
provide alternative kernel interfaces to enable them. This may require a
transition phase in which module scanning would only issue warnings and
would not prevent the module from being loaded.