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

[Peter Zijlstra]

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 :/