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

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Message-ID: <FF9F1233-1312-4B98-A476-0C20D92200E3@vmware.com>
Date:   Fri, 3 Apr 2020 16:48:35 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Jessica Yu <jeyu@...nel.org>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86 <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>,
        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" <jannh@...gle.com>,
        "keescook@...omium.org" <keescook@...omium.org>
Subject: Re: [patch 1/2] x86,module: Detect VMX modules and disable
 Split-Lock-Detect

> On Apr 3, 2020, at 9:40 AM, Peter Zijlstra <peterz@...radead.org> 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 Sean’s idea would work for VMware.