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

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