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

[Thomas Gleixner]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> On Fri, Apr 03, 2020 at 04:35:00PM +0200, Jessica Yu wrote:
>> +++ Rasmus Villemoes [03/04/20 01:42 +0200]:
>> > On 02/04/2020 14.32, Thomas Gleixner wrote:
>> > > From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> > > 
>> > > It turns out that with Split-Lock-Detect enabled (default) any VMX
>> > > hypervisor needs at least a little modification in order to not blindly
>> > > inject the #AC into the guest without the guest being ready for it.
>> > > 
>> > > Since there is no telling which module implements a hypervisor, scan the
>> > > module text and look for the VMLAUNCH instruction. If found, the module is
>> > > assumed to be a hypervisor of some sort and SLD is disabled.
>> > 
>> > How long does that scan take/add to module load time? Would it make
>> > sense to exempt in-tree modules?
>> > 
>> > Rasmus
>> 
>> I second Rasmus's question. It seems rather unfortunate that we have
>> to do this text scan for every module load on x86, when it doesn't
>> apply to the majority of them, and only to a handful of out-of-tree
>> hypervisor modules (assuming kvm is taken care of already).
>> 
>> 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.

But that still would not make it complete.

I was staring at virtualbox today after Jann pointed out that this
sucker does complete backwards things.

  The kernel driver does not contain any VM* instructions at all.

The actual hypervisor code is built as a separate binary and somehow
loaded into the kernel with their own magic fixup of relocations and
function linking. This "design" probably comes from the original
virtualbox implementation which circumvented GPL that way.

TBH, I don't care if we wreckage virtualbox simply because that thing is
already a complete and utter trainwreck violating taste and common sense
in any possible way. Just for illustration:

  - It installs preempt notifiers and the first thing in the callback
    function is to issue 'stac()'!

  - There is quite some other horrible code in there which fiddles in
    the guts of the kernel just because it can.

  - Conditionals in release code which check stuff like
    VBOX_WITH_TEXT_MODMEM_HACK, VBOX_WITH_EFLAGS_AC_SET_IN_VBOXDRV,
    VBOX_WITH_NON_PROD_HACK_FOR_PERF_STACKS along with the most absurd
    hacks ever.

If you feel the need to look yourself, please use your eyecancer
protection gear.

Can someone at Oracle please make sure, that this monstrosity gets shred
in pieces?

Enough vented, but that still does not solve the SLD problem in any
sensible way.

Thanks,

        tglx