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

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Message-ID: <20200403160156.GA2701@linux.intel.com>
Date:   Fri, 3 Apr 2020 09:01:56 -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 05:21:58PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 03, 2020 at 04:35:00PM +0200, Jessica Yu wrote:
> > +++ Rasmus Villemoes [03/04/20 01:42 +0200]:
> > > On 02/04/2020 14.32, Thomas Gleixner wrote:
> > > > From: Peter Zijlstra <peterz@...radead.org>
> > > > 
> > > > It turns out that with Split-Lock-Detect enabled (default) any VMX
> > > > hypervisor needs at least a little modification in order to not blindly
> > > > inject the #AC into the guest without the guest being ready for it.
> > > > 
> > > > Since there is no telling which module implements a hypervisor, scan the
> > > > module text and look for the VMLAUNCH instruction. If found, the module is
> > > > assumed to be a hypervisor of some sort and SLD is disabled.
> > > 
> > > How long does that scan take/add to module load time? Would it make
> > > sense to exempt in-tree modules?
> > > 
> > > Rasmus
> > 
> > I second Rasmus's question. It seems rather unfortunate that we have
> > to do this text scan for every module load on x86, when it doesn't
> > apply to the majority of them, and only to a handful of out-of-tree
> > hypervisor modules (assuming kvm is taken care of already).
> > 
> > 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?

Partially tested patch incoming...