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 18:12:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
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 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.