linux-kernel - Re: [PATCH 3/4] x86,module: Detect VMX vs SLD conflicts

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Date:   Tue, 7 Apr 2020 16:44:57 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        hch@...radead.org, sean.j.christopherson@...el.com,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kenny@...ix.com, jeyu@...nel.org, rasmus.villemoes@...vas.dk,
        fenghua.yu@...el.com, xiaoyao.li@...el.com, nadav.amit@...il.com,
        thellstrom@...are.com, tony.luck@...el.com, rostedt@...dmis.org,
        jannh@...gle.com, keescook@...omium.org, David.Laight@...lab.com,
        dcovelli@...are.com, mhiramat@...nel.org
Subject: Re: [PATCH 3/4] x86,module: Detect VMX vs SLD conflicts

On 07/04/20 16:35, Greg KH wrote:
> On Tue, Apr 07, 2020 at 01:02:39PM +0200, Peter Zijlstra wrote:
>> It turns out that with Split-Lock-Detect enabled (default) any VMX
>> hypervisor needs at least a little modification in order to not blindly
>> inject the #AC into the guest without the guest being ready for it.
>>
>> Since there is no telling which module implements a hypervisor, scan
>> all out-of-tree modules' text and look for VMX instructions and refuse
>> to load it when SLD is enabled (default) and the module isn't marked
>> 'sld_safe'.
>>
>> Hypervisors, which have been modified and are known to work correctly,
>> can add:
>>
>>   MODULE_INFO(sld_safe, "Y");
>>
>> to explicitly tell the module loader they're good.
> 
> What's to keep any out-of-tree module from adding this same module info
> "flag" and just lie about it?  Isn't that what you are trying to catch
> here, or is it a case of, "if you lie, your code will break" as well?

It's the latter.  Basically it's doing _the users_ of out-of-tree
modules a favor by avoiding crashes of their virtual machines;
developers need to fix them anyway.

Paolo