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Subject: Re: [PATCH 3/4] x86,module: Detect VMX vs SLD conflicts
To:     Greg KH <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, linux-kernel@vger.kernel.org,
        hch@infradead.org, sean.j.christopherson@intel.com,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org,
        kenny@panix.com, jeyu@kernel.org, rasmus.villemoes@prevas.dk,
        fenghua.yu@intel.com, xiaoyao.li@intel.com, nadav.amit@gmail.com,
        thellstrom@vmware.com, tony.luck@intel.com, rostedt@goodmis.org,
        jannh@google.com, keescook@chromium.org, David.Laight@aculab.com,
        dcovelli@vmware.com, mhiramat@kernel.org
References: <20200407110236.930134290@infradead.org>
 <20200407111007.352324393@infradead.org> <20200407143543.GB876345@kroah.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <527b5c72-96cb-a6cc-5301-449d86015ec9@redhat.com>
Date:   Tue, 7 Apr 2020 16:44:57 +0200
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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