linux-kernel - Re: [patch 1/2] x86,module: Detect VMX modules and disable Split-Lock-Detect

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Date:   Fri, 3 Apr 2020 09:25:55 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jessica Yu <jeyu@...nel.org>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Kenneth R. Crudup" <kenny@...ix.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Nadav Amit <namit@...are.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Tony Luck <tony.luck@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        jannh@...gle.com, keescook@...omium.org
Subject: Re: [patch 1/2] x86,module: Detect VMX modules and disable
 Split-Lock-Detect

On Fri, Apr 03, 2020 at 06:12:05PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 03, 2020 at 09:01:56AM -0700, Sean Christopherson wrote:
> > On Fri, Apr 03, 2020 at 05:21:58PM +0200, Peter Zijlstra wrote:
> > > On Fri, Apr 03, 2020 at 04:35:00PM +0200, Jessica Yu wrote:
> 
> > > > I wonder if it would make sense then to limit the text scans to just
> > > > out-of-tree modules (i.e., missing the intree modinfo flag)?
> > > 
> > > It would; didn't know there was one.
> > 
> > Rather than scanning modules at all, what about hooking native_write_cr4()
> > to kill SLD if CR4.VMXE is toggled on and the caller didn't increment a
> > "sld safe" counter?
> 
> And then you're hoping that the module uses that and not:
> 
>   asm volatile ("mov %0, cr4" :: "r" (val));
> 
> I think I feel safer with the scanning to be fair. Also with the intree
> hint on, we can extend the scanning for out-of-tree modules for more
> dodgy crap we really don't want modules to do, like for example the
> above.

Ya, that's the big uknown.  But wouldn't they'd already be broken in the
sense that they'd corrupt the CR4 shadow?  E.g. setting VMXE without
updating cpu_tlbstate.cr4 would result in future in-kernel writes to CR4
attempting to clear CR4.VMXE post-VMXON, which would #GP.