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Date:   Tue, 7 Apr 2020 17:44:59 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     tglx@linutronix.de, linux-kernel@vger.kernel.org,
        hch@infradead.org, sean.j.christopherson@intel.com,
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        pbonzini@redhat.com, fenghua.yu@intel.com, xiaoyao.li@intel.com,
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        rostedt@goodmis.org, jannh@google.com, keescook@chromium.org,
        David.Laight@aculab.com, dcovelli@vmware.com, mhiramat@kernel.org
Subject: Re: [PATCH 3/4] x86,module: Detect VMX vs SLD conflicts
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On Tue, Apr 07, 2020 at 05:24:12PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 07, 2020 at 04:35:43PM +0200, 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?
> 
> If they lie they get to keep both pieces.
> 
> The thing I worry about is them lying about "intree", is there anything
> that avoids that?

Yeah, the build system should be enforcing that.  I haven't looked in a
while if that really is able to be "faked", but no distro would do that
so it shouldn't be an issue except for custom systems.

greg k-h