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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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