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 10:21:29 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Jessica Yu <jeyu@...nel.org>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86 <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>,
        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" <jannh@...gle.com>,
        "keescook@...omium.org" <keescook@...omium.org>
Subject: Re: [patch 1/2] x86,module: Detect VMX modules and disable
 Split-Lock-Detect

On Fri, Apr 03, 2020 at 04:48:35PM +0000, Nadav Amit wrote:
> > On Apr 3, 2020, at 9:40 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > On Fri, Apr 03, 2020 at 09:25:55AM -0700, Sean Christopherson wrote:
> >> 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.
> > 
> > Sadly the CR4 shadow is exported, so they can actually fix that up :/
> 
> I do not think that Sean’s idea would work for VMware.

Well phooey.