Re: [Qemu-devel] [visorchipset] invalid opcode: 0000 [#1] PREEMPT SMP

[Borislav Petkov]

Should we perhaps CC qemu-devel here for an opinion.

Guys, this mail should explain the issue but in case there are
questions, the whole thread starts here:

http://lkml.kernel.org/r/address@hidden

Thanks.

On Sat, Apr 12, 2014 at 01:35:49AM +0800, Jet Chen wrote:
> On 04/12/2014 12:33 AM, H. Peter Anvin wrote:
> > On 04/11/2014 06:51 AM, Romer, Benjamin M wrote:
> >>
> >>> I'm still confused where KVM comes into the picture.  Are you actually
> >>> using KVM (and thus talking about nested virtualization) or are you
> >>> using Qemu in JIT mode and running another hypervisor underneath?
> >>
> >> The test that Fengguang used to find the problem was running the linux
> >> kernel directly using KVM. When the kernel was run with "-cpu Haswell,
> >> +smep,+smap" set, the vmcall failed with invalid op, but when the kernel
> >> is run with "-cpu qemu64", the vmcall causes a vmexit, as it should.
> > 
> > As far as I know, Fengguang's test doesn't use KVM at all, it runs Qemu
> > as a JIT.  Completely different thing.  In that case Qemu probably
> > should *not* set the hypervisor bit.  However, the only thing that the
> > hypervisor bit means is that you can look for specific hypervisor APIs
> > in CPUID level 0x40000000+.
> > 
> >> My point is, the vmcall was made because the hypervisor bit was set. If
> >> this bit had been turned off, as it would be on a real processor, the
> >> vmcall wouldn't have happened.
> > 
> > And my point is that that is a bug.  In the driver.  A very serious one.
> >  You cannot call VMCALL until you know *which* hypervisor API(s) you
> > have available, period.
> > 
> >>> The hypervisor bit is a complete red herring. If the guest CPU is
> >>> running in VT-x mode, then VMCALL should VMEXIT inside the guest
> >>> (invoking the guest root VT-x), 
> >>
> >> The CPU is running in VT-X. That was my point, the kernel is running in
> >> the KVM guest, and KVM is setting the CPU feature bits such that bit 31
> >> is enabled.
> > 
> > Which it is because it wants to export the KVM hypercall interface.
> > However, keying VMCALL *only* on the HYPERVISOR bit is wrong in the extreme.
> > 
> >> I don't think it's a red herring because the kernel uses this bit
> >> elsewhere - it is reported as X86_FEATURE_HYPERVISOR in the CPU
> >> features, and can be checked with the cpu_has_hypervisor macro (which
> >> was not used by the original author of the code in the driver, but
> >> should have been). VMWare and KVM support in the kernel also check for
> >> this bit before checking their hypervisor leaves for an ID. If it's not
> >> properly set it affects more than just the s-Par drivers.
> >>
> >>> but the fact still remains that you
> >>> should never, ever, invoke VMCALL unless you know what hypervisor you
> >>> have underneath.
> >>
> >> From the standpoint of the s-Par drivers, yes, I agree (as I already
> >> said). However, VMCALL is not a privileged instruction, so anyone could
> >> use it from user space and go right past the OS straight to the
> >> hypervisor. IMHO, making it *lethal* to the guest is a bad idea, since
> >> any user could hard-stop the guest with a couple of lines of C.
> > 
> > Typically the hypervisor wants to generate a #UD inside of the guest for
> > that case.  The guest OS will intercept it and SIGILL the user space
> > process.
> > 
> >     -hpa
> > 
> 
> Hi Ben,
> 
> I re-tested this case with/without option -enable-kvm.
> 
> qemu-system-x86_64 -cpu Haswell,+smep,+smap                   invalid op
> qemu-system-x86_64 -cpu kvm64                                 invalid op
> qemu-system-x86_64 -cpu Haswell,+smep,+smap -enable-kvm       everything OK
> qemu-system-x86_64 -cpu kvm64 -enable-kvm                     everything OK
> 
> I think this is probably a bug in QEMU.
> Sorry for misleading you. I am not experienced in QEMU usage. I don't realize 
> I need try this case with different options Until read Peter's reply.
> 
> As Peter said, QEMU probably should *not* set the hypervisor bit. But based 
> on my testing, I think KVM works properly in this case.
> 
> Thanks,
> Jet
> 

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
--