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

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Message-ID: <7b83b632e41c48698ab892b07673f42d@AcuMS.aculab.com>
Date:   Fri, 3 Apr 2020 16:16:38 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        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" <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" <jannh@...gle.com>,
        "keescook@...omium.org" <keescook@...omium.org>
Subject: RE: [patch 1/2] x86,module: Detect VMX modules and disable
 Split-Lock-Detect

From: Peter Zijlstra
> Sent: 03 April 2020 17:12
> 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.

Could you do the scanning in the last phase of the module build
that has to be done against the target kernel headers and with the
target kernel build infrastructure?

	David

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