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Date:   Tue, 7 Apr 2020 22:50:42 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nadav Amit <nadav.amit@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, hch@infradead.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        mingo <mingo@redhat.com>, bp <bp@alien8.de>, hpa@zytor.com,
        x86 <x86@kernel.org>, "Kenneth R. Crudup" <kenny@panix.com>,
        Jessica Yu <jeyu@kernel.org>,
        Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Xiaoyao Li <xiaoyao.li@intel.com>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Tony Luck <tony.luck@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        jannh@google.com, keescook@chromium.org, David.Laight@aculab.com,
        Doug Covelli <dcovelli@vmware.com>, mhiramat@kernel.org
Subject: Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation
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On Tue, Apr 07, 2020 at 01:27:45PM -0700, Nadav Amit wrote:
> > On Apr 7, 2020, at 12:38 PM, Peter Zijlstra <peterz@infradead.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@infradead.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.