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

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Date:   Tue, 7 Apr 2020 16:55:56 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, 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 Tue, Apr 07, 2020 at 04:44:57PM +0200, Paolo Bonzini wrote:
> 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.

Ok, seems kind of a heavy hammer, but oh well...

thanks for the explanation.

greg k-h