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

[Nadav Amit]

> On Apr 3, 2020, at 9:40 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> 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 :/

I do not think that Seanâs idea would work for VMware.